Onsala Space Observatory - Report on the activities in 2019 of - Chalmers

Dnr: SEE 2020-0085-D4.1

                        Report on the activities in 2019 of

    Onsala Space Observatory
     The Swedish National Infrastructure for Radio Astronomy

This report presents the activities at Onsala Space Observatory (OSO) during 2019, including
the usage of the instruments for scientific purposes, according to the “särskilda villkor” in the
contract for operation of OSO between the Swedish Research Council (VR) and Chalmers.

      The first image of a black hole: the shadow of the supermassive black hole in the centre
      of the galaxy Messier 87, as imaged by the Event Horizon Telescope. The telescopes
      contributing to the image include ALMA and APEX. Credit: EHT Collaboration.

Onsala, 15th June 2020

John Conway
Director, Onsala Space Observatory

Contents
This report is divided into the following sections. The sections or subsections contain (where
relevant) a reference to the corresponding modules defined in OSO’s March 2017 infrastructure
proposal to VR. A financial account is provided separately to VR.

 1. Operations
 2. Key numbers
 3. Selected scientific highlights
 4. Instrument upgrades and technical R&D
 5. SKA – The Square Kilometre Array
 6. Computers and networks
 7. Frequency protection
 8. Membership of international committees
 9. EU projects
10. Conferences, workshops, schools, etc
11. Education
12. Outreach
13. Changes in organisation
14. Importance to society
15. Importance to industry
Acronyms
Publications 2019
Key numbers (nyckeltal)

1 Operations
During 2019 Onsala Space Observatory (OSO) operated the following facilities:

–   The Onsala 20 m telescope for astronomical Very Long Baseline Interferometry (VLBI),
    geodetic VLBI and single-dish astronomy (the latter is not part of VR funded national
    infrastructure activities but is funded by Chalmers-only sources).
–   The Onsala 25 m telescope for astronomical VLBI
–   The Onsala LOFAR station as part of the International LOFAR Telescope (ILT) and in
    stand-alone mode
–   The Atacama Pathfinder Experiment telescope (APEX) used for mm wave single-dish
    astronomy and mm-VLBI
–   The Nordic ARC node (the Atacama Large Millimeter/sub-millimeter Array Regional
    Centre node for the Nordic, and Baltic, countries)
–   The Onsala Twin Telescope (OTT) for geodetic VLBI
–   The Onsala gravimeter laboratory for absolute and relative gravimetry
–   The Onsala GNSS stations
–   The tide gauges at Onsala
–   Two water vapour radiometers (WVRs) to support space geodesy
–   The Onsala aeronomy station for observations of H2O, CO, and O3 in the middle
    atmosphere (funded by Chalmers only)
–   The Onsala seismometer station
–   The Onsala time & frequency laboratory

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Operations using the above facilities are described in more detail below under Telescopes
(Sect. 1.1), Nordic ARC node (Sect. 1.2), and Geophysical instruments (Sect. 1.3), resp.

1.1 Telescopes
[Modules 3, 4 and 5]

In general, all telescopes have operated according to the 2019 activity plan without any major
problems, details are given below:

Onsala 20 m telescope: The 20 m telescope was used in accordance with the 2019 activity plan
without any major divergence. The telescope participated in 45 geodetic 24-hour campaigns,
13 astronomical single-dish projects, 5 extended astronomical VLBI sessions, 3+1 short
astronomical/geodetic VLBI out-of-session type observations, and 3 teaching/outreach
observations. In addition, a few days were dedicated to coordinated geodetic tests with the
Onsala Twin Telescopes. Maintenance (including technical observations) occurred at normal
levels. Upgrade-targeted technical activities included: improving the pointing model,
evaluating sensor replacements for the subreflector, replacing the telescope control computer,
and completing the migration to new telescope control software (Bifrost). An advanced
mapping capability was added to Bifrost in the autumn of 2019; it is estimated that 5 days of
telescope testing was used for commissioning this function and other Bifrost features.

Onsala 25 m telescope: The Onsala 25 m telescope was used for astronomical VLBI as
planned without any major problems during the year.

Onsala Twin Telescope (OTT): In 2019, The OTT was used for 40 VGOS sessions of
different kinds, for a total observing time of approximately 700 hours. See Sect. 1.3 for details.

APEX: Swedish APEX observations were conducted on 26 days, organized in three runs,
during 2019. To perform service mode observations, which are organized as three observing
shifts per day (afternoon, night, morning), OSO typically sends four observers per run. A panel
upgrade of the telescope surface was carried out in 2019, in addition the SEPIA receiver was
moved from the A-cabin guest receiver position to a facility receiver position, requiring the
installation of new relay optics. Additional setting of the telescope surface panels is scheduled
for 2020 to further improve telescope surface accuracy and hence the high frequency
performance of the telescope.

LOFAR: The Onsala LOFAR station operates in two main modes: International LOFAR
Telescope (ILT) mode and Local mode. In ILT mode, the station is controlled centrally by
ASTRON in the Netherlands. In Local mode the station is controlled by OSO; this observing
time is partially allocated via an Onsala open Call-for-Proposals basis and partially via ILT
Call-for-Proposals that run in Local mode. In both cases the Local mode time is devoted to
pulsar research.

VLBI: Very Long Baseline Interferometry observations were conducted using the Onsala 25 m
and 20 m telescopes, APEX and OTT (Onsala Twin Telescope) as part of international
networks of telescopes for astronomy and geodesy. The astronomical VLBI observations were
scheduled based on recommendations from time allocation committees [for the 20m and 25
telescopes theses TACs are the European VLBI Network TAC and the Global Millimetre VLBI
Array (GMVA) TAC while for APEX observations were scheduled by the Event Horizon

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Telescope consortium). Geodetic VLBI observations using Onsala telescopes (20m and OTT)
were scheduled by the International VLBI Service (IVS)

The usage of the above telescopes was distributed in the following way:

– The Onsala 20 m telescope:       35 days of astronomical VLBI
                                   45 days of geodetic VLBI
                                   90 days of single-dish astronomy

– The Onsala 25 m telescope:       59 days of regular astronomical VLBI
                                   (in addition, observations were carried out on 81 days for
                                   the VLBI-campaign PRECISE, see Sect. 4.3)

– The Onsala Twin Telescope:       40 days of geodetic VLBI (for details, see Sect. 1.3)

– The APEX telescope:              26 days of single-dish astronomy on Swedish time

– The LOFAR station:               209 days (ILT), 154 days (local)

Note that time for “normal” technical service, pointing, etc. are not included in the above
figures. These service activities amounted to about 15 and 13 days on the 20 m and 25 m
telescopes respectively. Test and commissioning observations that are not related to routine
maintenance and repairs are not included.

1.2 Nordic ARC node
[Module 2]

In 2019 the Nordic ALMA Regional Centre (ARC) provided support to the Nordic astronomical
communities in all the many aspects of the ALMA user experience, from proposal preparation
to data reduction and analysis of PI and Archival data.
        Nordic ARC staff provided support to users to submit proposals for the Cycle 7 Call for
Proposals (deadline April 2019) by all possible means, including visits to the major astronomy
institutes in the region. The total number of submitted proposals worldwide in Cycle 7 was
1787, for the first time this number did not supersede the previous cycle figure. 195 proposals
were submitted by Swedish PI/co-Is, 74 of which were awarded time. In the autumn of 2019,
ALMA opened a supplemental Call for Proposal for filler projects for the ALMA Compact
Array (deadline October 2019). 249 proposals were submitted for that call. Swedish PI/co-Is
submitted 25 proposals out of which 10 were granted observing time. Approved projects from
the Nordic countries get assigned staff from the Nordic ARC as contact scientists; their role
being to act as the point of contact between PIs and the Observatory during the full life cycle
of a project.
        In 2019, data reduction and quality assessment of most of PI observations was carried
by the Joint ALMA Observatory (JAO) in Chile or at ESO using the ALMA Science Pipeline.
In contrast Nordic ARC staff mostly contributed to the data reduction effort of non-standard
Nordic projects that were not manageable by the Pipeline, such as Polarization or High-
Frequency projects. In total, the data reduction and quality assessment of 10 projects was
performed by Nordic ARC staff.
        In the past year, the node supported 11 Face-to-Face visits of Swedish researchers for
data reduction of ALMA projects, including archive research. Each project has taken an average
of one week of full-time support, followed by remote support during a longer period until the

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project goals were achieved. This increased support time per project is due to the increased
complexity and computing time requirements (e.g, array combination, large mosaics, etc.) of
projects choosing face-to-face support. As well as supporting individual observations staff also
participated in providing advice to the Nordic led ESO ALMA Development study “High-
Cadence Imaging of the Sun”.
        Besides the regular collaboration among nodes, two specific events during the year
brought together staff from all the different European ARC Network nodes. The Nordic ARC
node manager and deputy manager participated in the European ARC Representatives meeting
in Jodrell Bank (UK) in March. This annual meeting serves as a discussion forum on the status
of the network, next cycle procedures, general ALMA updates and future developments. In
October 2019, EU the ‘ARC All-hands’ meeting gathered together all the European ARC staff.
During this meeting network progress updates and discussions were held over 3 days (see
section 10). NordicARC staff also attended the ALMA Science Conference that took place in
Cagliari (October 2019).
        The Nordic ARC continued the support and maintenance of the Advanced Data Analysis
Tools (https://www.oso.nordic-alma.se/software-tools.php). The modelling engine in
UVMULTIFIT is being used for the development of advanced calibration algorithms within
the EU H2020 RadioNet project’s working package RINGS (Radio Interferometry Next
Generation Software). The Nordic ARC also hosted a 3-day workshop in September on the
topic of Software Tools development for ALMA within the European ARC network.
        During the year two staff members of the Nordic ARC left their positions in the node,
and one of them finished employment at OSO/Chalmers. At the same time, two new staff
members were recruited. As a consequence, the node went through a reorganization and the
temporary lack of human resources stalled any further development of new software tools
during the latter part of the year. Resources were instead devoted to the support and
maintenance of currently existing tools.

1.3 Geophysical instruments
[Module 6]

Geodetic VLBI: Geodetic VLBI observing uses both the 20 m and OTT telescopes.
Observations with the Onsala 20 m telescope use its S/X frequency band receiver system, are
24 h long and form part of the regular IVS sessions in the R1-, RD-, RDV-, T2- and EUR-
series. In total 45 sessions in the IVS program were observed during 2019. All sessions were
recorded with the DBBC2 in vdif-format on the FlexBuff recorder for geodetic VLBI. These
data were then e-transferred to the respective correlator.
        The Onsala twin telescopes (OTT) were used for 25 international broadband VGOS test
(VT) sessions of 24 h duration each, 17 European broadband VGOS (EV) sessions of 4–6 h
duration each, and 3 VGOS-B (VB) sessions of 1 h duration. During all these sessions both the
OE and OW stations were used in parallel and all data were recorded with the corresponding
DBBC3 backends in vdif-format on dedicated FlexBuff recorders. While the international VT-
sessions were observed in networks of up to 6 VGOS partner stations, the EV-sessions were
observed primarily with two other European partner stations, i.e. Yebes and Wettzell. The VB-
sessions involved Onsala and Ishioka in Japan; as station that also joined a a small number of
EV-sessions in the end of 2019. The VT-sessions were correlated at the MIT/Haystack
correlator, the EV-sessions at the Bonn correlator, and the VB-sessions will be correlated
locally at Onsala using software correlation. The goal of the VB-sessions is to determine UT1-
UTC in parallel with simultaneously observed IVS-Intensive sessions with legacy S/X systems.

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GNSS stations: OSO’s primary GNSS station, called ONSA, was operated continuously during
2019. It is a station in the SWEPOS network operated by Lantmäteriet. It is also one of the
fundamental reference sites used in the global IGS network, as well as in the European EUREF
network. An additional station, ONS1, has also delivered data continuously the same networks
network (more details in Sect. 2.2). In addition to ONSA and ONS1, the 6 GNSS stations
installed close to the Onsala twin telescopes were all running continuously during the year. Of
special interest is the experimental station OTT5 which was equipped with antireflecting
material in December 2018 in order to study the possible improvement from decreasing the
impact of unwanted signal multipath propagation (see Fig. 1.1).

Figure 1.1. Some of the GNSS installations at OSO.

Gravimeter laboratory: The main purpose of the gravimeter laboratory at Onsala is to
maintain a gravity reference and calibration facility co-located with space geodetic techniques.
The facility is one component of the Onsala Fundamental Geodetic Station. The laboratory is
furnished with platforms for visiting absolute gravimeters (AGs), which visit on average one to
three times per year. The laboratory’s primary instrument is a superconducting gravimeter
(SCG, model GWR 054). Within international networks the instrument is called OSG054 and
has been operated continuously with very few breaks in recording (less than 10 days) since its
installation in June 2009. In 2019 OSG054 recorded data over 365 days. It missed 445 1-second
data out of 31536000, which means 14 ppm of missing data. In 2019 there was one AG
campaign from 6–9 August by Andreas Engfeld (Lantmäteriet). Service points to note are;
– 2019-08-28: Coldhead change
– 2019-09-05: Problem with Dewar pressure/heater control starting
– 2019-10-05: Problem fixed.
From November 21 to 26, the dewar was refilled in liquid Helium from 80 % to 95 %. The
connector from the SG barometer to the central unit was destroyed during a storm on July 28
and was eventually replaced on October 14. Meanwhile, air pressure data from another
barometer on site was used.

Tide gauges: The Onsala tide gauge station was running uninterrupted for the entire year,
excluding the yearly cleaning of the well, causing a data gap of approximately 2 hours on the
20th of August. The sea level observations are available from the official web site of national
sea level data operated by Swedish Meteorological and Hydrological Institute
(SMHI). Onsala’s other GNSS-based tide gauge was also operated continually over the year
proving observations with a sampling rate of 1 Hz. Data are stored in Receiver INdependent
EXchange (RINEX) format and include multi-GNSS (i.e. GPS, GLONASS, Galielo, Beidou)
code- and carrier-phase observations as well as signal-to-noise ratio (SNR) measurements.

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Water Vapour Radiometers: Onsala’s two water vapour radiometers, Astrid and Konrad,
measure the sky brightness temperatures at 21 GHz and 31 GHz from which the radio wave
propagation delay in the atmosphere is inferred. This data can be used to improve the accuracy
of geodetic VLBI observations. During 2019 Astrid and Konrad operated side by side in a
continuous mode from the 1st of January to the 28th of July, when lightning from a thunder-
storm damaged both instruments. Konrad was repaired and operated normally again from the
2nd of October to the end of the year.

Aeronomy station: The aeronomy station consists of two radiometers: 1) A single sideband
H2O system (water vapour) that measures the sky brightness temperature at 22 GHz, and 2) the
double sideband CO/O3 system (carbon monoxide and ozone) that measures the sky brightness
temperatures at 111 and 115 GHz. Spectra from both radiometer systems are used to retrieve
vertical profiles of the observed molecules in the middle atmosphere. During 2019 the
mirror pointing system of the H2O radiometer was rebuilt. This meant that the instrument could
not be operated during the re-build phase, and thus there were only 114 days of collected H2O
measurements. However, there were 346 days of collected CO/O3 measurements.

Seismometer station: OSO hosts a seismograph station in the Svenska nationella seismiska
nätverket (SNSN) at Uppsala University. We have data access to the local seismometer and
keep a continuous archive of its recordings. The station's waveform files are used in delay
calibration of the superconducting gravimeter and for noise reduction in absolute gravity
measurements. During 2019 disturbing signals were detected that are most likely due a crack
that has developed in the concrete foundation of the seismometer.

Time and frequency laboratory: The time and frequency laboratory hosts a hydrogen maser,
necessary for VLBI observations, but which also contributes to the universal atomic time. OSO
also collaborates with RISE (Research Institutes of Sweden) on a Swedish time-keeping system.
RISE owns a second hydrogen maser and a cesium clock that are also installed at Onsala. These
instruments are used for comparison measurements and provide redundancy of accurate
reference time (and frequency) for the VLBI observations (both astronomy and geodesy) at the
observatory.

2 Key numbers
2.1 Astronomy
[Modules 2, 3, 4, 5]

Detailed key numbers for the astronomy activities are given in tables at the end of this report,
and in a separate excel file. Here we give only a few comments, a summary of the publication
statistics, and some key numbers for single-dish observations with the 20 m telescope.

Users of the astronomy research infrastructure
The ALMA user project/user statistics given at the end of this report and in the associated excel
file are for Proposal Cycle 6; that is for observations covering the period October 2018 –
September 2019. In conjunction with the annual ALMA deadline the Nordic ARC node is very
active in advertising the use of ALMA in Sweden and the Nordic countries and a significant
number of Nordic/Swedish ALMA project proposals are generated by these Nordic ARC
publicity efforts.

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We note that for ALMA, APEX, astronomical VLBI, and LOFAR taken together, about
31 % of the Swedish users (individuals) were from other institutions than Chalmers. The
Swedish non-Chalmers users were affiliated with Stockholm University, Uppsala University,
and the Royal Swedish Academy of Sciences.
        About 28 % of the users in 2019 were women. There is no clear difference in success
rate (fraction of applications for telescope time which are observed) between men and women.
        All users did research in the subject area 103 Physical Sciences.

Number of refereed scientific papers
The associated excel file gives a list for each instrument of papers in refereed journals
published in 2019 (see also the publication list at the end of this report). Conference publications
are not included (except for conference papers by OSO staff on technical R&D). Below are
given summary statistics of papers for each instrument/activity. For each instrument two figures
are given; in most cases the first number is the total number of instrument-related publications
while the second is the number of publications with at least one Swedish author. In contrast for
ALMA the first number is the number of papers with at least one Nordic author.
         The Nordic ARC provides standard support for all projects with Nordic PIs, and further
dedicated support upon request. The level and type of Nordic ARC node support for each
publication is described in the accompanying excel file. Alongside the raw numbers for ALMA
publications below, on the same line, we give the total number of papers with at least one
Nordic/Swedish author that have received dedicated Nordic ARC node support. The Nordic
ARC provides standard support for projects with Nordic PIs, and further dedicated support upon
request. Accordingly, the bulk of publications that received dedicated Nordic ARC support are
lead by first authors with Nordic/Swedish affiliation (and vice-versa nearly all Swedish-led
proposals get dedicated ARC node support). In contrast no Nordic ARC support is generally
given to ALMA publications which have Nordic/Swedish affiliated co-authors but which are
lead by first authors from other countries.
         For APEX, publications based on all partners’ observing time are counted because OSO
contributes to the full APEX operations costs and because Swedish receivers are used by all
partners. The numbers for astronomical VLBI include observations with EVN and GMVA, and
users of JIVE. Publications by OSO staff on technical R&D are also presented. A publication
list is found at the end of this report.

• ALMA                               104/65, Nordic ARC node dedicated support 19/16
                                                                           (Nordic/Swedish)
• APEX                               65/12                                   (total/Swedish)
• Astronomical VLBI                  35/15                                   (total/Swedish)
• LOFAR                              74/11                                   (total/Swedish)
• 20 m telescope, single-dish        4/1                                     (total/Swedish)
• Technical publ. by OSO staff       6

In addition, in 2019 there was 1 publication using astronomical data from the satellite Odin
(now operating mainly in aeronomy mode), and 6 publications using data from the Swedish-
ESO Submillimetre Telescope SEST (closed in 2003).

Onsala 20 m telescope, single-dish observations
Single-dish observations with the 20 m telescope in Onsala are not supported by VR (but by
Chalmers funding) and key numbers for them are therefore not given in the tables at the end of

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this report. We note that in 2019, there were proposals for 14 projects, out of which 13 were
observed. There were 21 female and 28 male users on the observed projects. Of these, 8 were
Swedish (all from Chalmers).

2.2 Geosciences
[Module 6]

Users of the geoscience research infrastructure
The OSO geoscience instruments, including the geodetic VLBI observations as the major
activity, do not have individual scientific users who apply for observing time. Rather the
geoscience instruments make long-term measurements of Earth parameters – which are
thereafter stored in international databases with open access. Since these databases are open
access, it is impossible to acquire detailed insight of user groups in terms of which universities
or other organisations they belong to and the gender distribution of the users. The data and
derived products in global databases such as station positions, Earth orientation/rotation rate
and gravity field are used both by the global geophysics community for scientific purposes and
by civil society for a variety of practical applications including supporting accurate geo-location
services and monitoring of global change.
        As far as we know, all use of the data for scientific purposes was within the subject area
105 Earth and Related Environmental Sciences.

Number of refereed scientific papers
We have identified 17 papers with one or more Swedish authors and 32 papers with non-
Swedish authors published during 2019 where the use of data or services from OSO are
specifically stated. In addition, there are significantly more papers making use of OSO data
products, especially those using GNSS reference data from OSO via IGS/EUREF, that cannot
be identified because the inclusion of the OSO station is not explicitly mentioned. It is also
likely that there are papers published that we simply are not aware of. A publication list is found
at the end of this report. We are not aware of any patents originating directly from our
geoscience activities. No user has been rejected to use OSO geoscience data. This is in any case
not a readily computable statistic since as described above virtually all of the OSO geoscience
data are automatically distributed via open data bases.

Data submissions
Geodesy VLBI:
The geodetic VLBI observations are carried out within the framework of the International VLBI
Service for Geodesy and Astrometry (IVS), http://ivscc.gsfc.nasa.gov/. In total 45 experiments,
each one with a length of 24 h and rather evenly spread over the year, were carried out during
2019 with the Onsala 20 m telescope. Additionally, we have been observing with the Onsala
twin telescopes during several VGOS sessions, both international (25 sessions, 24 h each) and
European (17 sessions, 4–6 h each) ones, plus 3 one-baseline intensive sessions (1 h long).
       Correlated VLBI observations are provided via the IVS data archives and are available
free of charge. The IVS registers its data also under the umbrella of the World Data System
(WDS), which is an Interdisciplinary Body of the International Council for Science (ICSU).
Databases as well as products are supplied to users around the globe with minimum latency in
order to guarantee that operation critical information, in particular Earth orientation parameters
from VLBI observations, are available for satellite operators, space agencies, and other
stakeholders. These databases are fundamental for many scientific disciplines within
Geophysics. Given also that global navigation satellite systems like GPS, would not be operable
without the Earth orientation parameters provided from VLBI measurements, the true value

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chain and the number of users of products emerging from data collected at globally distributed
VLBI sites, like Onsala, has significant economic value to society; given that everybody relying
on GNSS positioning and navigation has in the end use of these data.

GNSS:
The two major GNSS reference stations at OSO, i.e. ONSA and ONS1, are nodal points for the
Swedish permanent GNSS network, SWEPOS, hosted by Lantmäteriet. All data acquired
continuously are openly distributed via the data archives of IGS https://webigs.ign.fr/gdc/en/,
and EUREF http://www.epncb.oma.be/. These archives serve thousands of users every year.
Additionally, GNSS data are captured using six stations around the Onsala twin telescopes,
called OTT1 to OTT6 whose data are then stored at Lantmäteriet. It shall be noted that the
motivation for using GNSS data from OSO is that these stations are co-located with one of the
most accurately determined VLBI stations world-wide. Therefore, indirectly the OSO VLBI
data also contributes to OSO GNSS data quality. Many of the ultimate users of this data are
found in the geophysical research community where GNSS data typically are used with OSO
acting as a reference site in global, regional and local studies. A vast majority of the downloads
that occur from the international databases operated by IGS and EUREF are by universities and
research agencies for studies such as e.g, plate tectonics, crustal deformation, space weather,
sea level, climate, meteorological monitoring, et cetera. During 2019 the GNSS station OSOI
has in addition been operated and has contributed to ESA’s ionospheric monitoring network.
OSO provides, via contributions to the above database, access for the national and international
user communities to robust international reference frame and other geoscience data.

Gravimeter Laboratory:
Gravimeter data with one-second samples and maximum with a two-minutes latency are
publically available, see http://holt.oso.chalmers.se/hgs/SCG/monitor-plot.html. The records
are also submitted to the archive of IGETS (International Geodynamics and Earth Tide Service)
at GeoForschungsZentrum (GFZ) Potsdam (Germany), on a monthly schedule. OSO delivers
1-minute down-sampled data, raw and “corrected”, i.e. cleaned from earthquake signatures.
IGETS is a service under the auspices of the International Association of Geodesy (IAG).
During 2019 one visit with an absolute gravimeter, Lantmäteriet's FG5, took place.

Ocean tide loading service:
Since 2002, OSO provides a computing service for ocean tide loading effects in application to
surface displacements and gravity (http://holt.oso.chalmers.se/loading). Loading-induced
displacements are computed from a range of global ocean tide maps, using 28 sources featuring
8 to 11 tide species each. This service, endorsed by the IERS, has as its main purpose to provide
consistent reduction of ocean tide loading effects to VLBI, GNSS and SLR analysis centres in
their preparation of products that maintain the International Terestrial Reference Frame (ITRF).
Apart from this, the service’s logbook hints at a large number of users peripheral or outside the
ITRF community making use of the OSO ocean loading computing service in their analysis of
GNSS observations.

Tide gauges:
The data from the super tide gauge are transferred to SMHI in near-real time. These are
available to the public through the SMHI web pages.

Aeronomy station:
During 2019 OSO collected about 114 days of atmospheric H2O data derived from its aeronomy
station. These data are being processed to be delivered to the Network for the Detection of

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Atmospheric Composition Change (NDACC; see http://www.ndsc.ncep.noaa.gov). During
2019 OSO has collected 346 days of CO/O3 data.

3 Selected scientific highlights
Below follows a list of scientific highlights selected to illustrate the different instruments and
science areas covered by OSO. In the listed publications Swedish authors are shown
underlined.

Astronomy
Especially highlighted in this section are papers from Swedish astronomers using OSO
telescopes or receiving user support provided at OSO (via for instance by the Nordic ARC
Node). In addition, some interesting international results that make use of OSO telescopes
and/or instrumentation are listed.

3.1 ALMA
[Module 2]

As described in the annual publication list there have been many ALMA results published in
2019 making use of OSO ARC node support; below details are given of two of the highest
significance such results led by Swedish authors. Another ALMA related scientific highlight
involving OSO staff contributions concerns ALMA’s participation in the Event Horizon
Telescope observations of M87’s black hole (see Sect. 3.3).

Stringent limits on the magnetic field strength in the disc of TW Hya

W. H. T. Vlemmings,      B. Lankhaar,     P. Cazzoletti,        C. Ceccobello,      D. Dall’Olio,
E. F. van Dishoeck, S. Facchini, E. M. L. Humphreys,            M. V. Persson,     L. Testi, and
J. P. Williams. Astronomy & Astrophysics, 624, L7 (2019)

Summary: Magnetic fields likely play an important role during the evolution of planet forming
disks. But only very limited information exists on the magnetic field strength in such disks. So
far, most observations have relied on dust polarization, but unfortunately recent observations
have shown that dust self-scattering at (sub-)millimeter wavelengths significantly complicates
the dust observation interpretation. This leaves direct magnetic field measurements using the
Zeeman effect of the CN radical as one of the most promising ways to determine the magnetic
field strength.
        Vlemmings et al. (2019) used some of the first ALMA Zeeman observations to provide
the so-far most stringent limit on the vertical magnetic field component of the proto-planetary
disk around the T Tauri star TW Hya. Stacking a number of CN hyperfine components, a limit
of |Bz|

Figure 3.1. (left) An integrated intensity map of one of the CN hyperfine components in the disk around
TW Hya. The dashed line is the line of nodes and the cross indicates the peak of the CN emission. (right)
The stacked CN total intensity and circular polarization spectrum after azimuthal averaging. The red
line indicates a model for magnetic field strength of 0.8 mG.

Kinematics around the B335 protostar down to AU scales

P. Bjerkeli, J. P. Ramsey, D. Harsono, H. Calcutt, L. E. Kristensen, M. H. D. van der Wiel,
J. K. Jørgensen, S. Muller, and M. V. Persson. Astronomy & Astrophysics, 631, A64 (2019)

Summary: The kinematics and morphological properties of the region around the protostar
B335 are studied in great detail by combining multiple ALMA data sets of different angular
resolutions. B335 is perhaps one of the youngest known protostars and does not yet show
evidence of a rotationally supported accretion disk. To investigate the relationship between
outflow launching and formation of any disk-like structure, a wide range of spatial scales need
to be covered. With dedicated support from the Nordic ARC node in Onsala, Bjerkeli et al.
(2019) managed to produce a high-fidelity 12CO image, covering an impressive range of
structures scaling from 3 au (0.03”) to 700 au (7”). The image was produced by combining data
acquired between 2013 and 2017 and reveals unprecedented details of the environment of B335
(see Fig. 3.2).

                                                   11

Figure 3.2. Moment 0 map of the 12CO emission (colour contours), overlaid on the continuum
(grayscale) in the combined dataset (excerpt from Bjerkeli et al., 2019; their Fig. 5). The emission is
integrated from 2.0 to 6.0 km s-1 with respect to the source velocity, 8.3 km s-1 w.r.t. vLSR. Selected mean
spectra, averaged over circular regions of 10 au radius, are indicated by the coloured points and the
corresponding coloured spectral line profiles.

The X-shaped outflow shows no signs of rotation at distances larger than 30 au from the
protostar, which suggests that most of the 12CO emitting gas originates at the edge of the
surrounding envelope. Since most of the outflow emission is recovered, the combined data cube
reveals the scales over which entrainment takes place.
        Within ~10 au of the protostar, a clear rotation signature is observed in CH3OH and one
line of SO2. Using high-velocity 12CO features in the vicinity of the protostar, the launching
radius is estimated to be less than 0.1 au from the centre of the continuum peak source, but no
rotationally-supported disk can be seen.

                                                    12

3.2 APEX
[Module 3]

HD 101584: Circumstellar characteristics and evolutionary status

H. Olofsson, T. Khouri, M. Maercker, P. Bergman, L. Doan, D. Tafoya, W.H.T. Vlemmings,
E.M.L. Humphreys, M. Lindqvist, L. Nyman, S. Ramstedt. Astronomy & Astrophysics 623,
153 (2019)

Summary: ALMA and APEX synergetic observations of the binary stellar object HD 101584.
The APEX telescope was used to observe some 20 targeted spectral lines using several different
heterodyne receivers. One example of such a line is visualised in Fig. 3.3, where the
complicated nature of HD 101584 is revealed by the complex line profile extending over some
300 km/s. The existence of the rather complex species H2CO and CH3OH indicates that shocks,
possibly paired with dust grains, may be responsible for the observed abundances of these
molecules.

Figure 3.3. The 13CO(2-1) spectral line near 220 GHz as observed towards HD 101584 with APEX and
ALMA (as averaged over the primary beam). Most of the observed flux is captured by ALMA, especially
at the extreme velocities and in the central peak. For the broad feature between -20 km/s and 100 km/s
only about 40% of the flux is recovered with ALMA as indicated by the single-dish APEX observations.

Weak-lensing mass calibration of the Sunyaev-Zel'dovich effect using APEX-SZ galaxy
clusters

A. Nagarajan, F. Pacaud, M. Sommer, M. Klein, K. Basu, F. Bertoldi, A. T. Lee, P. A. R. Ade,
A. N. Bender, D. Ferrusca, N. W. Halverson, C. Horellou, B. R. Johnson, J. Kennedy,
R. Kneissl, K. M. Menten, C. L. Reichardt, C. Tucker, B. Westbrook. Monthly Notices of the
Royal Astronomical Society 488, 1728 (2019)

Summary: The APEX visitor instrument APEX-SZ, a bolometer array working at 150 GHz with
a 23 arcminutes field of view, was designed to study the SZ effect. Here the authors observed
39 galaxy clusters to provide for an accurate mass calibration of the integrated Compton-
ionization. Reliable mass estimates of galaxy clusters are crucial for their use as cosmological
probes.

                                                 13

3.3 Astronomical VLBI
[Module 4]

First Image of a Black Hole

The Event Horizon Telescope Collaboration (from Sweden, J. Conway, M. Lindqvist,
I. Marti-Vidal). Astrophysical Journal Letters 875, Paper I-VI (2019)

Summary: On 10th April 2019, the Event Horizon Telescope (EHT) collaboration, a global
VLBI array operating at a wavelength of 1.3 mm, presented its first results - an image of the
shadow of the supermassive black hole in galaxy M87, see Fig. 3.4. The EHT collaboration
involves more than 200 scientists and engineers from 19 countries. The published image is
dominated by a ring structure of 42 ± 3 μas diameter that is brighter in the south. The brightness
excess is explained as relativistic beaming of material rotating around the black hole, with gas
to the South approaching the observer. The structure has a central brightness depression with a
contrast of >10:1, which matches the expected signature of the black hole shadow. The
observations give an estimated black hole mass of M = 6.5 ± 0.7 × 109 M⊙.
        Further details can be found in a series of six papers in a special issue of The
Astrophysical Journal Letters. The telescopes contributing to this result were ALMA (Chile),
APEX (Chile), the IRAM 30 m telescope (Spain), the James Clerk Maxwell Telescope
(Hawaii), the Submillimeter Array (Hawaii, USA), the Large Millimeter Telescope Alfonso
Serrano (Mexico), and the Submillimeter Telescope (Arizona, USA).
        OSO contributed to the M87 EHT result both via telescope infrastructure, as one of the
three partners to APEX, and via other specific contributions. The latter include sending
observers to APEX to develop and test operational VLBI and developing critical software for
the EHT. These software contributions included an algorithm and code developed by the OSO
Nordic ARC node member Ivan Marti-Vidal to allow for the combination of ALMA data
(which observes with linear polarization) with the other telescopes in the EHT (observing with
circular polarization). Ivan Marti-Vidal also made important contributions to the software
needed to internally calibrate the phased-up ALMA for inclusion in the EHT VLBI array.

Figure 3.4. EHT images of M87 on four different observing nights in 2017. The indicated
resolving beam shown bottom right is 20 μas in diameter. From the EHT collaboration (2019,
Paper IV).

                                               14

Compact radio emission indicates a structured jet produced by a binary neutron star merger
in gravitational wave source GW170817

G. Ghirlanda, O. S. Salafia, Z. Paragi, M. Giroletti, J. Yang, B. Marcote, J. Blanchard, I. Agudo,
T. An, M. G. Bernardini, R. Beswick, M. Branchesi, S. Campana, C. Casadio, E. Chassande-
Mottin, M. Colpi, S. Covino, P. D’Avanzo, V. D’Elia, S. Frey, M. Gawronski, G. Ghisellini,
L. I. Gurvits, P. G. Jonker, H. J. van Langevelde, A. Melandri, J. Moldon, L. Nava, A. Perego,
M. A. Perez-Torres, C. Reynolds, R. Salvaterra, G. Tagliaferri, T. Venturi, S. D. Vergani,
M. Zhang. Science, Volume 363, Issue 6430, 968 (2019)

Summary: The binary neutron star merger event GW170817 was the first source detected
through both gravitational waves and electromagnetic radiation. The early optical observations
located its host galaxy as NGC 4993 and found temporal and spectral properties consistent with
a source powered by the radioactive decay of material ejected during and after the merger. Later
afterglow emission detected at radio wavelengths is theorised to have been produced by either
a narrow relativistic jet or an isotropic outflow interacting with surrounding material.
        Ghirlanda et al. (2019) presented imaging results from global VLBI observations using
32 radio telescopes including the Onsala 25 m radio telescope. The apparent source size,
207 days after the merger, is constrained to be smaller than 2.5 milli-arc seconds at the 90 %
confidence level. This compact structure indicates that GW170817 produced a structured
relativistic jet instead of an isotropic outflow which would have produced a larger apparent size,
Fig. 3.5.

Figure 3.5. The global VLBI imaging and simulation results of GW170817. The inset shows real and
simulated pseudo-colour images (top), obtained in observations as well as based on model image
distributions (bottom) for various scenarios (collimated jet, jet-cocoon with different opening angles).
The background artist's impression represents the jet braking through the dense gas ejecta, which was
produced during the merger of neutron stars that caused the gravitational waves recorded as
GW170817.

                                                  15

A radio structure resolved at the deca-parsec scale in the radio-quiet quasar PDS 456 with
an extremely powerful X-ray outflow

J. Yang, T. An, F. Zheng, W.A. Baan, Z. Paragi, P. Mohan, Z. Zhang, X. Liu. Monthly Notices
of the Royal Astronomical Society 482, 1701 (2019)

Summary: Rapidly accreting active galactic nuclei (AGN) might host radiatively driven mildly
relativistic outflows. Some of these powerful X-ray absorbing wide-aperture outflows are
expected to produce strong internal shocks resulting in a significant non-thermal emission. The
radio-quiet quasar PDS 456 has a bolometric luminosity reaching the accretion Eddington limit
and a relativistic X-ray outflow with a kinetic power high enough to quench the star formation
in its host galaxy. To search for the outflow-driven radio emission in PDS 456, an international
group led by Yang performed very-long-baseline interferometric (VLBI) observations of the
quasar with the European VLBI Network (EVN) including the Onsala 25 m radio telescope.
The EVN imaging results are displayed in Fig. 3.6. These images show two faint radio
components with a projected separation of about 20 pc in the nuclear region. The high-
resolution VLBI structure at the deca-pc scale can be explained as either a poorly collimated
young jet or a bidirectional radio-emitting outflow, launched in the vicinity of a strongly
accreting central engine.

Figure 3.6. The EVN 5GHz image of the optically bright and radio-quiet quasar PDS 456. There are
two faint radio components C1 and C2, found in the nuclear region. The black cross marks the optical
centroid of the GAIA observations.

The population of SNe/SNRs in the starburst galaxy Arp 220. A self-consistent analysis of
20 years of VLBI monitoring

E. Varenius, J. E. Conway, F. Batejat, I. Martí-Vidal, M. A. Pérez-Torres, S. Aalto, A. Alberdi,
C. J. Lonsdale and P. Diamond. Astronomy & Astrophysics 623, A173 (2019)

Summary: The nearby (77 Mpc) starburst galaxy Arp 220 is an excellent laboratory for studies
of extreme astrophysical environments. For 20 years, Very Long Baseline Interferometry
(VLBI) has been used to monitor a population of compact sources in Arp 220. In this study,
                                                16

Varenius et al. (2019) detect 97 sources (see Fig 3.7), thought to be supernovae (SNe) and
supernova remnants (SNRs). SNe and SNRs are thought to be the sites of relativistic particle
acceleration powering star formation induced radio emission in galaxies. Understanding the
origin of the radio emission from starburst galaxies is important for e.g. for interpreting future
SKA surveys of the early universe.
        Varenius et al. (2019) find a relation between source luminosity and size within Arp 220,
with larger sources being less luminous. This is consistent with the starburst in Arp 220 being
a more extreme version of those seen other galaxies such as M82. They also find a source
luminosity function similar to SNRs in normal galaxies. The similarity is notable because the
environment in Arp 220 is extreme compared to normal galaxies. Finally, they find that about
80 % of the total radio emission from Arp 220 cannot be explained simply as the sum of the
compact sources. Colliding SNR shocks and/or additional secondary electron production in the
ISM could possibly explain the additional flux. Future VLBI observations of Arp 220 are
needed to further constrain the population of weaker sources, and hopefully reveal the origin of
all the radio emission.

Figure 3.7. A zoom on the central part of the 6cm VLBI image of the western nucleus in Arp 220
(Varenius et al. 2019). The bright dots are SNe and SNRs. Resolving these sources to measure their size
is only possible with the very fine angular resolution provide by global VLBI.

3.4 LOFAR
[Module 5]

2019 was a major year for LOFAR. In February 25 articles were published in a special issue of
Astronomy & Astrophysics in connection to the LOFAR Sky Survey’s first Data Release (DR1)
of about 2 % (424 square degrees) of the northern sky. This first part of the LOFAR Two-Meter
Sky Survey (LoTSS; Shimwell et al. 2019) covers the so-called HETDEX region, in which
more than 325 000 radio sources (most of them Active Galactic Nuclei, AGN) were identified
and characterized. The sensitivity of LoTSS is such that the number density of detected sources
is about 10 times higher than in previous wide-area radio surveys. C. Horellou (Chalmers) was
involved in a number of studies based on LoTSS DR1 data (on galaxy groups, Nikiel-

                                                  17

Wroczynski et al. 2019; and on nearby star-forming galaxies, Heesen et al. 2019a, Heesen et al.
2019b, Miskolczi et al. 2019). This work was led by the LOFAR Magnetism Key Science
Project (the MKSP, a consortium of about 100 researchers, of which C. Horellou is co-PI), in
collaboration with the Surveys KSP. Another important work was the detection of polarization
in a giant radio galaxy and the use of the Faraday Rotation Measures (RM) to place constraints
on the intergalactic magnetic field (O'Sullivan et al. 2019). Other LOFAR work included a
detailed analysis of AGN in a galaxy cluster (Clarke et al. 2019) and the discovery of a bridge
of radio emission between two massive clusters of galaxies (Govoni et al. 2019, see below and
Fig 3.8).
        As members of the Surveys KSP Chalmers scientists (J. Conway, C. Horellou) also have
early access to data that will be included in the second data release (DR2; to occur in 2021) that
will cover about 27% of the northern sky). Construction of a LOFAR Magnetism Rotation
Measure (RM) grid is ongoing (O'Sullivan et al., in prep); this will be a major resource of
background sources to probe foreground cosmic magnetic fields because of the high RM
precision achievable by LOFAR (better than 1 rad/m2, which is about 100 times better than
what has been achieved so far at GHz frequencies). A new PhD student, Sara Piras, was hired
at Chalmers in September 2019 to work on LOFAR polarization.
        A large LOFAR magnestism related conference was held in the Netherlands in June
2019 (C. Horellou was a member of the Scientific Organizing Committee) and a LOFAR
Surveys internal meeting was held in Italy in December. Conway and Horellou participated in
both meetings. Horellou also participated and helped organize the annual meeting of the
LOFAR Magnetism KSP in Germany in September. Below are given details of the summary
paper (Shimwell) related to the DR1 ‘paper splash’ and of the detection with LOFAR of radio
emsision from a filament of the cosmic web connecting two galaxy clusters.

The LOFAR Two-metre Sky Survey II. First data release

T. W. Shimwell., …, J. E. Conway, …, S. Bourke, …, C. Horellou, …, E. Varenius, et al.
Astronomy & Astrophysics 622, A1 (2019)

Summary: Published article to accompany the first data release of the LOFAR Two-metre Sky
Survey (LoTSS). LoTSS is the most extensive astronomical survey to date of the sky in the
2 meter wavelength band. It contains more than 300 thousand sources over 424 square degrees
and has achieved a sensitivity of better 100 µJy per beam. This survey will be of great legacy
value to the astronomical community. The release has created media attention both at the time
of its release and through the LOFAR Radio Galaxy Zoo (RGZ) project. The RGZ is a citizen
science initiative to get interested citizens to help astronomers identify radio galaxies.

A radio ridge connecting two galaxy clusters in a filament of the cosmic web

F. Govoni, E. Orrù, A. Bonafede, M. Iacobelli, R. Paladino, F. Vazza, M. Murgia,V. Vacca,
G. Giovannini, L. Feretti, F. Loi, G. Bernardi, C. Ferrari, R.F. Pizzo, C. Gheller, S. Manti,
M. Brüggen, G. Brunetti, R. Cassan, F. de Gasperin, T.A. Enßlin, M. Hoeft, C. Horellou,
H. Junklewitz, H.J.A. Röttgering, A.M.M. Scaife, T.W. Shimwell, R.J. van Weeren, M. Wise.
Science 364, 981 (2019)

Summary: Galaxy clusters are the most massive gravitationally bound structures in the
Universe. They grow by accreting smaller structures in a merging process that produces shocks
and turbulence in the intra-cluster gas. A ridge of radio emission connecting the merging galaxy
clusters Abell 0399 and Abell 0401 was observed with the Low-Frequency Array (LOFAR)

                                               18

telescope network at 140 MHz (see Fig 3.8). This emission requires a population of relativistic
electrons and a magnetic field located in a filament between the two galaxy clusters.
Simulations were performed to show that a volume-filling distribution of weak shocks may
reaccelerate a pre-existing population of relativistic particles, producing emission at radio
wavelengths that illuminates the magnetic ridge.

Figure 3.8. LOFAR image of the 1.4°×1.4° region centered on the Abell 0399 - Abell 0401 system
(Govoni et al. 2019) showing the bridge in radio emission between the two clusters. Colour and contours
show the radio emission at 140 MHz with a resolution of 80′′ and rms sensitivity of 1 mJy beam−1. The
beam size and shape is indicated by the inset at the bottom left. Contour levels start at 3 mJy beam−1
and increase by factors of 2. One negative contour (red) is drawn at –3 mJy beam−1. The black cross
(right ascension 02h 59m 38s declination +13° 54′ 55′′, J2000 equinox) indicates the location of a strong
radio source that was removed from the image.

                                                   19

3.5 Onsala 20 m telescope single dish

Observational Signatures of End-dominated Collapse in the S242 Filamentary Structure

L.K. Dewangan, L.E. Pirogov, O.L. Ryabukhina, D. K. Ojha, I. Zinchenko. Astrophysical
Journal 877, 1 (2019)

Summary: The sideband separating capability of the 3 mm receiver was used to simultaneously
observe the important ground state carbon monoxide isotopologue lines near 110 GHz, as well
as the carbon sulphide 2-1 line near 98 GHz, over large spatial scales covering a filament in the
S235 star forming region, see Fig. 3.9. The comprehensive analysis this enabled showed that
the S242 filament is a very good example of so-called end-dominated collapse.

Figure 3.9. Integrated intensity maps for 13CO(1-0), C18O(1-0), and CS(2-1), resp., in S242.

Detection of Class I Methanol Masers in some IRDCs at 44 GHz with 20-m Onsala Radio
Telescope

N.N. Shakhvorostova, A.V. Alakoz, A.O.H. Olofsson et al. URSI GASS 2020 (abstract for
conference presentation at XXXIII General Assembly and Scientific Symposium of the
International Union of Radio Science URSI)

Summary: A number of new class I methanol maser sources were detected near 44 GHz in a
survey of infrared dark clouds carried out in 2019 with the Onsala 20 m telescope. The project
also began* observing 3 mm band lines in these sources using the sideband separating feature
to cover both the temperature diagnostic line of methyl acteylene at 85 GHz, and carbon
sulphide and several thermal methanol lines near 97 GHz which will aid in estimating the
density in the maser regions. The study of class I methanol masers can reveal the presence of
shock waves in early stages of star formation.
*
  Additional observations were granted and carried out through a DDT proposal in 2020 in order
to finish the high frequency part of the project.

                                                  20

3.6 Geosciences
[Module 6]

Below are listed a selection of papers utilizing data produced by the Onsala geophysical
instruments as part of international networks.

Correcting surface loading at the observation level: impact on global GNSS and VLBI
station networks.

B. Männel, H. Dobslaw, R. Dill, S. Glaser, K. Balidakis, M. Thomas, H. Schuh. Journal of
Geodesy 93, 2003 (2019)

Summary: Time-dependent mass variations of the near-surface geophysical fluids in the
atmosphere, oceans and the continental hydrosphere lead to systematic and significant load-
induced deformations of the Earth’s crust. More than 10 years of observations from about 400
GNSS and 33 VLBI stations were specifically reprocessed to incorporate non-tidal loading
correction models at the observation level. As a result, the coordinate repeatabilities and
residual annual amplitudes decrease by up to 13 mm and 7 mm, respectively, when the loading
models are applied. In addition, the standard deviation of the daily estimated vertical coordinate
is reduced by up to 6.8 mm. Furthermore, the VLBI-based EOP estimates are critically
susceptible to surface loading effects, with root-mean-squared differences reaching of up to
0.2 mas for polar motion, and 10 μs for UT1-UTC.

Evidence of daily hydrological loading in GPS time series over Europe.

A. Springer, M. A. Karegar, J. Kusche, J. Keune, W. Kurtz, S. Kollet. Journal of Geodesy 93,
2145 (2019)

Summary: Loading deformations from atmospheric, oceanic, and hydrological mass changes
mask geophysical processes such as land subsidence and tectonic or volcanic deformation.
While it is known that hydrological loading plays a role at seasonal time scales, the authors
demonstrate in this publication evidence that also fast water storage changes contribute to daily
Global Positioning System (GPS) height time series. Accounting for daily hydrological mass
changes based on a high-resolution hydrological model based on GRACE data reduces the root
mean square scatter of GPS height time series almost by a factor of two when compared to
monthly hydrological mass changes.

The Potential for Unifying Global‐Scale Satellite Measurements of Ground Displacements
using Radio Telescopes.

A. L. Parker, L. McCallum, W. E. Featherstone, J. McCallum, R. Haas. Geophysical Research
Letters 46(21), 11841 (2019)

Summary: The expansion of globally consistent satellite-radar imagery presents new
opportunities to measure Earth-surface displacements on inter-continental scales. Global
applications, including a complete assessment of the land contribution to relative sea-level rise,
first demand new solutions to unify relative satellite-radar observations in a geocentric
reference frame. The international network of Very Long Baseline Interferometry telescopes
provides an existing, yet unexploited, link to unify satellite-radar measurements on a global
scale. Proof-of-concept experiments reveal the suitability of these instruments as high

                                               21

amplitude reflectors for satellite-radar and thus provide direct connections to a globally
consistent reference frame. Automated tracking of radar satellites is easily integrated into
telescope operations alongside ongoing schedules for geodesy and astrometry. Utilizing
existing telescopes in this way completely avoids the need for additional geodetic infrastructure
or ground surveys and is ready to implement immediately across the telescope network as a
first step towards using satellite-radar on a global scale.

3.7 Device physics and Terahertz technology
[Module 8]

The research and development work of OSO’s GARD group supports the development of
future instrumentation for millimetre astronomy (for example future ALMA receiver
upgrades). Example of this work are given below.

Specific capacitance of Nb/Al-AlN/Nb superconducting tunnel junctions

A. Pavolotsky, C. D. López, I. Vrethed Tidekrans, D. Meledin, V. Desmaris, V. Belitsky.
Proceedings 30th International Symposium on Space THz Technology (ISSTT2019),
Gothenburg, Sweden, April 15-17, 2019

Summary: Modern radio astronomy demands broadband receiver systems. For SIS mixers, this
translates into objective to employ superconducting tunnel junctions with a very low RnA and
low specific capacitance. The traditionally used Nb/AlOx/Nb junctions have largely
approached their physical limit of minimizing those parameters. It is commonly recognized,
that it is AlN-barrier junctions, which are needed for further progressing broadband SIS mixer
instrumentation for radio astronomy. In this work, we present the progress in fabrication of high
quality Nb/Al-AlN/Nb superconducting tunnel (SIS) junctions and their characterization in
terms of their DC electric properties and specific capacitance (see Fig. 3.10). GARD has
developed new technology to fabricate such SIS junctions. The specific capacitance of the
studied Nb/Al-AlN/Nb junctions is noticeably lower than that reported for the Nb/AlOx/Nb
junctions. These new junctions will be used in new broadband SIS mixers for short mm-waves
and terahertz frequencies.

                                               22

Figure 3.10. Examples of Nb/Al-AlN/Nb junctions’ current voltage characteristics with RnA ranging
between 3 and 120 Ωµm2. The legend inside each plot panel shows junctions’s size, RnA and Rj/Rn
values.

See also Sect. 4.2 for another highlight from the Group for Advanced Receiver development
(GARD) concerning receiver development for APEX.

4 Instrument upgrades and technical R&D
4.1 ALMA
[Modules 2 & 8]

The ALMA Observatory released in 2018 its development roadmap towards 2030
(https://www.almaobservatory.org/en/publications/the-alma-development-roadmap/),
describing the vision and development of ALMA new capabilities for the coming decade. One
specific project within that time frame, related to building a new correlator, was halted in 2019;
currently alternative designs are currently being explored for this correlator. On the receiver
side Band 1 development (led by East Asia) progressed during 2019 with these receivers
planned for deployment in 2020. Band 2 receivers (lead by Europe) were approved for
prototyping and pre-production. Resources have also been devoted to the project of re-imaging
Cycle 2-4 observations (ARI-L) to expand the products available in the ALMA Science
Archive.
       As part of preparation for ALMA Band 2 deployment, GARD was involved in studies
on the feasibility of such a receiver covering 67–116 GHz (ALMA Band 2+3) with an IF band
of 4–12 GHz. As part of these studies, GARD designed a prototype cold cartridge, which was
used to evaluate optics, amplifiers, orthomode transducer and overall cold cartridge layout. The
                                               23