X-Ray and UV/Optical Variability of the Missing Link Binary Pulsar PSR J1023+0038

ATel #5516; A. Patruno (Univ. Leiden/ASTRON), A. Archibald (ASTRON), S. Bogdanov (Columbia Univ.), V. Kaspi (McGill Univ.), S. Tendulkar (Caltech), C. Bassa (ASTRON), G. Janssen (ASTRON), B. Stappers (Univ. Manchester), A. Lyne (Univ. Manchester).
on 25 Oct 2013; 23:15 UT
Credential Certification: Alessandro Patruno (patruno@strw.leidenuniv.nl)

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Following the reported state change of the missing link binary PSR J1023+0038 (ATel #5513), we analysed the Swift observation started on October 18, 2013 at 05:11:50 UT with a total on-source exposure time of about 10 ks. The X-ray and UV/optical counterparts were observed with the XRT instrument in PC mode (time resolution of 2.5 s) and with the UVOT telescope with the U and W1 filters.

A preliminary X-ray analysis reported in ATel #5515 shows an increased X-ray flux level 20 times higher than previously reported in quiescence (see e.g., Archibald et al. 2010). By looking at the XRT lightcurve we determine a time-averaged source count rate of 0.22 ct/s in the 0.5-10 keV band with strong variability observed on time-scales of few seconds. The flickering changes the count rate between a maximum of about 1 ct/s down to a level compatible with the background emission level. By using the source distance of 1.3 kpc, this corresponds to a source luminosity changing from a maximum of about 10^34 erg/s down to values of the order of 10^32 erg/s, with the change happening in less than few tens of seconds. The lower limit on the luminosity is comparable with the quiescent 0.5-10 keV X-ray flux measured with XMM-Newton in 2008 (9x10^31 erg/s, see Archibald et al. 2010 ApJ, 722, 88, Bogdanov et al. 2011, ApJ, 742, 97). The short time-scale of the variability indicates that its origin is probably not related to the orbital phase (orbital period 0.2 days).

The UVOT images show a clear UV/optical counterpart in both the optical U and ultra-violet UW1 filters with magnitudes of 16.04 and 16.32, respectively (Vega system; no correction for extinction is applied). The combined statistical and systematic errors are 0.02 mag (U) and 0.05 mag (UW1). The total exposure times are approximately 9200 (U filter) and 650 seconds (UW1 filter). A Swift/UVOT observation taken on June 12, 2013 shows a marginally detected UW1 counterpart with magnitude of approximately 20, demonstrating that the source has substantially brightened not only in X-rays but also in the UV part of the spectrum. This is consistent with what observed and reported in the V band (ATel #5514).