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Swift observation of the transitional pulsar PSR J1023+0038

ATel #5534; A. Papitto (ICE; CSIC-IEEC), E. Bozzo, C. Ferrigno (ISDC), N. Rea (ICE CSIC-IEEC, UvA)
on 31 Oct 2013; 20:00 UT
Credential Certification: Alessandro Papitto (papitto@ice.csic.es)

Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar

Referred to by ATel #: 5868, 6162

A change of state change was recently reported for the transitional millisecond pulsar PSR J1023+0038 (Atel #5513, #5514, #5515, #5516). A Swift monitoring of the source started on October, 31 at 13:33 UT (ID 33012; PI Papitto), aimed at mapping the transition of the pulsar to an accreting state, and the possible onset of a bright X-ray outburst.

The first observation of this campaign was performed on October 31 (13:33 - 15:24 UT), for an exposure of 1.6 ks. The XRT was operated in photon counting mode. We detected the source at an average unabsorbed 0.3-10 keV flux of 1.1(1)E-11 erg/cm^2/s, which at a distance of 1.37 kpc from the source (Deller et al. 2012) corresponds to an X-ray luminosity of 2.4E33 erg/s. Such a value is compatible with than that observed on 2013 October 18, and is a factor 20 higher than the flux usually observed during quiescence (~1E32 erg/s). The spectrum is described by an absorbed power low with index 1.7(2) (NH=7(6)E20 cm^-2). The count rate observed by Swift/XRT varies between 0.05 and 0.36 c/s on time scales as short as 20 s, a similar behavior to that reported in Atel #5516.

This observation confirms that PSR J1023+0038 has entered a brighter X-ray state, following the disappearance of radio pulsations in June 2013 (Atel #5513), the brightening of its gamma-ray and optical emission (Atel #5513, #5514), and the appearance of a double peaked Halpha emission line (Atel #5514). All these observations indicate that an accretion disk is forming, as already happened in this system in 2000/2001 (Archibald et al. 2009, Science, 324, 1411). The present X-ray state of PSR J1023+0038 is also remarkably similar to that shown by the transitional millisecond pulsar, IGR J1824-2452, in 2008 (Papitto et al. 2013, Nature, 501, 517). Future observations will assess possible changes of the X-ray properties of the source.