Enhanced X-ray emission of the binary millisecond pulsar J1023+0038

ATel #5515; A. K.H. Kong (National Tsing Hua University, Taiwan)
on 25 Oct 2013; 21:49 UT
Credential Certification: Albert Kong (akong@phys.nthu.edu.tw)

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Following the report of the disappearance of the radio pulsation and enhanced gamma-ray emission of the binary millisecond pulsar PSR J1023+0038 (ATel #5513), we search for the available Swift X-ray Telescope (XRT) observations in public archive. PSR J1023+0038 has been observed on 2013 June 10, 12 and October 18. The exposure times for the first two observations are less than 2 ks while the last observation has an exposure time of ~10 ks. The X-ray counterpart of PSR J1023+0038 was detected in all observations and the source is significantly brighter in the last observation.

We performed an X-ray spectral analysis for all three datasets. Since the number of counts is small for the first two observations, we combined the two datasets for spectral analysis. Following Archibald et al. (2010) and Tam et al. (2010), the spectrum can be fit with an absorbed power-law model with the absorption fixed at the Galactic value (4e20 cm^-2). The resulting photon index is 1.4 (+1.1,-0.9; 90% confidence). The 0.3-10 keV luminosity is 1e32 erg/s assuming a distance of 1.3 kpc. This is consistent with the XMM observations taken in quiescence (Archibald et al. 2010; Tam et al. 2010). For the 10ks XRT observation taken on 2013 Oct 18, the spectrum can also be fit with an absorbed power-law model with best-fit parameters N_H=(3.1+/-2)e20 cm^-2 and photon index=1.68+/-0.09. The 0.3-10 keV luminosity is 2.4e33 erg/s. Therefore the X-ray luminosity is at least a factor of 20 higher than all previous quiescent values. The spectrum is also significantly softer in terms of the photon index.

If the radio pulsation of PSR J1023+0038 disappeared sometime near 2013 June 19-23 (ATel #5513), the first two XRT observations were taken in the "radio-loud" state and the X-ray emission should be relatively faint as a radio millisecond pulsar. On the other hand, when the X-ray source has become much brighter in mid-October, recent optical spectroscopy shows strong double peaked Ha emission (ATel #5514), indicating an accretion disk. This strongly suggests that the enhanced X-ray activity is from the accretion disk and the system is now switching to a low-mass X-ray binary.