The first variable gamma-ray pulsar: challenging the models of high-energy magnetospheric emission - CERN Indico

The first variable gamma-ray pulsar:
                    challenging
the models of high-energy magnetospheric emission

                                        Massimiliano Razzano
                                       (Università di Pisa & INFN)

                                             Luigi Tibaldo
                                             (SLAC/KIPAC)

                                                On behalf
                                            of the Fermi-LAT
                                             Collaboration

                      The Structure and Signals of Neutron Stars
                              from Birth to Death
                         Firenze, March 24th-28th, 2013

                                        J. Bayer, Uranometria (1603)

Pulsars & gamma rays
             (See also Alice Harding’s talk !)

•147 γ-ray pulsars
     (…and counting)

•3 populations
     •Young, radio-loud
     •Young, radio-quiet
     •Millisecond PSRs

•Why gamma rays?
    •Bulk of EM energy is in gamma rays
    •Can track B structure
    •Beam structure very differenf from radio:
                                                 Second Fermi-LAT pulsar Catalog, 117 PSRs
    probe geometry/populations
                                                               (Abdo+2013)

   But…few "dynamical" information !
   • In radio, rich phenomenology (glitches, mode changes, nulling, etc..)
   • At gamma-ray energies, "just" glitches!
   • Axiom: pulsars are steady in gamma rays

Meanwhile, in the heart of Cygnus…

•      SNR G78.2+2.1 (“Gamma Cygni” SNR)
                              (Lande+2012)
         • Extended source, R~0.6°
         • Distance ~ 1.5 kpc
         • Age ~ 7 kyr

•      VERITAS source VER J2019+409
                                                       52 months, >2 GeV, front events
                           (Aliu+2013)

•      ..and the (radio-quiet) pulsar PSR J2021+4026
                                 (Abdo+2009)

        AGILE team reported variability from 1AGL J2022+4032, positionally
    coincident with the PSR (Nov ’07-Aug ’09), but concluded it was more likely
             due to another source along the line of sight (Chen+2011)

A “sister” of Geminga?
•PSR J2021+4026
   •f ~3.8 Hz (P~265 ms)
   •fdot ~-8x10-13 Hz/s
   •Characteristic age τc ~ 77 kyr
   •ĖSD~ 1035 erg/s
   •B~4x1012 G

•Long search for X-ray counterpart (Brazier+1996):
    •Chandra source S20 (Weisskopf+2011)
    •Pinpointed also with γ-ray timing               Chandra (Weisskopt+2011)
    •X-ray pulsations found by XMM (Lin+2013)

•Like Geminga (and PSR J1836+5925) :
     •Radio-quiet
     •X-ray pulsations
     •Peak lag>0.5
     •Large magnetospheric emission
              at all phases

                                                              XMM (Lin+2013)

Fermi-LAT analysis
•    From August 4, 2008 to December 11, 2012 (~ 52 months)
•    P7REP, ‘source’ class
•    Binned likelihood analysis
        • 14°x14° in Galactic coordinates around PSR J2021+4026
        • 100 MeV < E < 300 GeV
        • Combined likelihood to treat separately events converting in front and
          back of the LAT Tracker
        • P7REP_V15_SOURCE Instrument Response functions
        • The model includes 2PC pulsars, 2FGL(1) sources, flaring and steady.
          Extended sources (Cygnus Cocoon and Gamma Cyg SNR)
•    Added 5 new background sources (fit with log parabola)

•    Variability analysis «à la 2FGL»
    • Maximum likelihood ratio Flux constant vs variable
    • Null hypothesis == constant flux  Probability P
    • P small      Source is variable !

                                                  (1) Second Fermi-LAT Source Catalog (Nolan+2012)

Serendipity
                   P ~ 0.9                           30-day bins, E>1 GeV

                   P ~ 0.52

                   P ~ 10-10

• PSR J2021+3651 (Dragonfly)
          for comparison
 • Comparable flux & spectrum
 • 3.5° away
          …and it is steady !

• Steady increase, pre jump             Pre jump,          Post jump,
                                        ~1167 days         ~423 days
           (Kendall + Chi2 test)

Who is varying?
It’s the pulsar ?
       …look at the timing !

•    60-days time bins
    • neglect timing noise
    • f(t)=f0+f1(t-t0)
    • Limited periodicity search in
      f0,f1 (Z2n test)

•      Decrease in f1 !
•      Simultaneous with flux drop
•      Fixing normalization to the
     average post jump residuals
     pointlike at the PSR position

•    Built 2 timing solutions

Zooming
on the “jump”

  • 30 & 7 day bins

  • Tried smaller binning: but
    count rates too low

  • ~ few days after
    MJD 55850
    (2011 Oct. 16)

Changing light curves
•   Fit with 3 Gaussians, or 2 when bridge emission (BR) not significant
•   BR emission becomes less significant (

Spectral evolution

•   P2 unchanged

•   Marginal decrease in
    cutoff energy for P1

•   Spectral changes not
    uniform in phase:
      Not a source
       along the line of sight

Interpretation: what is not…

•   NOT an instrument-related effect
      • J2021+3651 steady

•   NOT another source along the line of sight:
      • Simultaneous timing-flux change
      • Phase-resolved flux drop not uniform in phase

•   NOT a binary motion effect:
      • Doppler shift needs unrealistc orbit

•   NOT a “typical“ glitch
      • No f0 increase
      • No recovery so far
      • Flux changes

Intermittent pulsars?

•Gamma-ray variability claims
   •Crab (Greisen+1975)
   •Vela (Grenier+1988)
   •EGRET variability claims
       due to systematics (Nolan+2003)

                                                Credits: M. Kramer,

•This jump looks more like a mode change, like those seen in radio pulsars
                               (e.g. PSR B1931+24, Kramer+ 2006)

•In our case, |f1| increases
       γ flux decreases
       In radio, flux jumps can be >> 20%

..and what could be
•   PSR J2021+4026 similar to Geminga and PSR J1836+5925
       • Radio-quiet
       • Bright magnetospheric emission at all phases
       • Large P1-P2 lag
•   OG models:
       • small α and nearly equatorial viewing angles ζ, small fΩ
•   TPC models:
       • small α , large ζ, fΩ larger than OG
       • High-altitude TPC give consistent results with OG
•   Viewing geometry confirmed from X-ray (Geminga, Caraveo+2004)
•   Gamma rays in a narrow equatorial strip
       • Small shift in B morphology or α     change in B torque
            change in f1
      • γ-ray beam moves wrt line of sight    change in flux
•   Gradual on longer term? (e.g. force-free precession)

Conclusions
•   Pulsars were considered THE steady gamma-ray sources…
      …up to now !
      • First firm detection of variability in a gamma-ray pulsar
      • Flux drop simultaneous with timing change
      • Short (~days) timescale
      • A shift in magnetospheric configuration?
      • Hard to compare/confirm AGILE variability claim

•   Opening the time-domain gamma-ray pulsar science
      • Another case of amazing new gamma-ray variability
      • Implications on calibration of variability tests for LAT sources
      • Will continue in case of an extension of Fermi !

                           More detail in Allafort et al., ApJL 777,L2 (2013)