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Pulsed Radio Emission from PSR J1119-6127 disappeared

ATel #9286; M. Burgay (INAF-OACagliari), A. Possenti (INAF-OACagliari), M. Kerr (CASS-ATNF-Marsfield), P. Esposito (U. Amsterdam), N. Rea (U. Amsterdam, CSIC-IEEC), F. Coti Zelati (U. Amsterdam, U. Insubria, INAF-OAB), G. L. Israel (INAF-OARoma) and S. Johnston (CASS-ATNF-Marsfield) on behalf of a larger collaboration
on 31 Jul 2016; 08:42 UT
Credential Certification: Andrea Possenti (possenti@oa-cagliari.inaf.it)

Subjects: Radio, X-ray, Neutron Star, Soft Gamma-ray Repeater, Transient, Pulsar

Referred to by ATel #: 9321, 9366

We have searched for pulsed radio emission from the direction of the known high magnetic field pulsar PSR J1119-6127 (spin period of 409 ms, magnetic field intensity at surface of about 4 x 10^13 G, spin-down age of about 1700 yr, embedded in the supernova remnant SNR G292.2-0.5), which showed bursts similar to those typical of the Soft Gamma Repeaters (SGR) at 13:02:07.91 UT on 27 July 2016 (Younes et al,GCN Circular #19735) and at 01:27:51 UT on 28 July 2016 (Kennea et al,Atel #9274). The occurrence of a glitch has also been reported (Archibald et al, Atel#9284) on the basis of Fermi-LAT and Swift-XRT data. Two series of observations were performed at the Parkes radio telescope on 29 July 2016 (starting at UT 04:59:00) and 30 July 2016 (starting at UT 01:08:18) using the H-OH receiver and the DFB back-end at a central frequency of 1465 MHz and 512 MHz of bandwidth (sometimes reduced to 256 MHz for mitigating the effects of RFI). During both days, data were collected in two modes. [i] folding-mode data were taken at the expected topocentric spin period and dispersion measure of the pulsar: in particular on 29 July, two integrations were done with 30 s sub-integration time and having about 8 min and 6 min duration, respectively; on 30 July 2x20 min-long observations were collected with 10 s sub-integration time. [ii] several search-mode observations (of duration ranging from 20 min to 60 min) were also acquired with a sampling time of 256 us at 8 bits (on 29 July) and at 4 bits (on 30 July). Calibration data were also collected. No hint of regular pulsation appears in any of the observations with typical peak S/N ratios in the fold-mode observations of 2-4. A search for single pulses of width up to 56 ms also did not produce any detection above a S/N threshold of 6. In order to extract an upper limit for the pulsed flux density, a profile likelihood was calculated by varying the PSR J1119-6127 pulse amplitude and phase. Interpreting the likelihood as a probability distribution for the flux density and applying a uniform prior, the interval containing 0 and 95% of the posterior distribution was calculated to estimate an upper limit. For both the days, the best upper limits for the calibrated flux density are 90 uJy. Similar limits result from alternative calculations. At the used frequency band, PSR J1119-6127 typically shows a flux density of about 1 mJy with small fluctuations of few percent at most. During an observation taken at UT 01:15:49 on 24 July 2016 with the same setup as the folding mode observations reported here, the flux density was 972+\-36 uJy. That implies a reduction of the pulsed radio flux of more than a factor 10 in last 5 days and strongly suggests a correlation among that and the occurrence of the SGR-like burst. Interestingly, Swift data in WT mode taken among UT 17:21:16 of 28 July 2016 and UT 03:11:09 of 29 July 2016, for about 9.6 ks, display strong pulsations at 0.4098626(1) s in the entire 1-10 keV band (see a figure at http://pulsar.oa-cagliari.inaf.it/pulsar/plot/J1119_Swift.pdf ), including the hard portion of the band (see also Antonopoulou et al,Atel#9282). That is in contrast with any previous observation of PSR J1119-6127, which showed X-ray pulsations in the soft band only. The 1-10 keV spectrum can be fit by a power-law or a blackbody model. The latter provides a better fit, with a reduced chi-squared of 0.86 for 140 degrees of freedom versus 1.01 for 140 dof, and the derived absorption is in better agreement with previous measures. The best-fitting parameters for the blackbody model are nH=(1.0+/-0.1)x10^22 cm^-2 and kT=(0.87+/-0.02) keV, and the unabsorbed flux in the 1-10 keV band is (2.3+/-0.1)x10^-11 erg/cm^2/s, corresponding to an isotropic luminosity of about 2 x 10^35 erg/s at the nominal distance of 8.4 kpc. Monitoring in gamma-ray, X-ray and radio are ongoing for following the evolution of the burst and/or the re-activation of the radio pulsations.