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Swift detection of pulsations from PSR J1119-6127 promptly after its July 27th 2016 burst

ATel #9282; D. Antonopoulou (University of Southampton), G. Vasilopoulos (MPE Garching), C. M. Espinoza (Universidad de Santiago de Chile)
on 30 Jul 2016; 22:09 UT
Credential Certification: Georgios Vasilopoulos (gevas@mpe.mpg.de)

Subjects: X-ray, Neutron Star, Pulsar

Referred to by ATel #: 9284, 9286, 9321

PSR J1119-6127 is a rotationally-powered (RP) pulsar whose pulsations are detected in radio, X-rays and gamma-rays. It is a high magnetic field neutron star, with an inferred dipole field strength of about 4⋅1013 G. On July 27 it exhibited a strong X-ray burst, detected by Swift/BAT (ATel #9274) and Fermi/GBM (GCN Circular #19736). This is the first radio (and second high-magnetic field) RP pulsar to display magnetar-like activity. In magnetars, such bursts are often accompanied by pulse profile changes, as well as timing events (e.g. glitches).

In quiescence, the pulsar's X-ray emission consists of thermal and non-thermal components, with a high pulsed fraction in the soft band (<2.5 KeV). The profile is single-peaked and phase-aligned with its radio counterpart. Pre-burst observations with XMM-Newton show no pulsations in the 2.5-8 KeV band (see e.g. Ng et al., ApJ 761, 65N (2012), which give a pulsed fraction <10% for this energy band).

Following the announcement of the Swift/BAT detection, we requested a Swift/XRT ToO (obsid: 00034632001) to search for pulse profile changes during the first day of the event. Swift/XRT performed seven snapshots of the target within 37 ks with a total exposure of ~10 Ks.

Preliminary analysis of the new Swift/XRT data revealed a single-peaked pulse profile with a spin period of 0.4098627(3) s, which is consistent with the known rotational period of the pulsar. We also report the appearance of strong pulsations in the 2.5-10 KeV energy band, with a very high pulsed fraction of ~60% (Fmax/Fmin ~ 4) in this range. The X-ray spectrum (0.7-10 keV) is well fitted by an absorbed power-law model with photon index of 2.8±0.2 and an absorption of 4.4(5)⋅1022 cm-2, yielding an unabsorbed flux of 7⋅10-11 erg cm-2 s-1 in the 0.7-10. keV energy band, which is more than two orders of magnitude higher than the pre-burst one (~2⋅10-13 erg cm-2 s-1 for 0.5-8 KeV; Ng et al. 2012). Finally, no significant spectral variation was found in the on off pulse phase. Detailed results and discussion of this unique source will be presented elsewhere.

We thank the Swift team for accepting and carefully scheduling the target of opportunity observations, and we acknowledge the use of public data from the Swift data archive.