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Optical/UV/X-ray observations of the gamma-ray binary candidate PSR J2032+4127/MT91 213

ATel #10851; Kwan-Lok Li (Michigan State University), P. H. Thomas Tam (Sun Yat-sen University), Jumpei Takata (Huazhong University of Science and Technology), and Albert K. H. Kong (National Tsing Hua University, Taiwan)
on 15 Oct 2017; 17:33 UT
Credential Certification: K. L. Li (lilirayhk@gmail.com)

Subjects: Optical, Ultra-Violet, X-ray, Variables, Pulsar

Referred to by ATel #: 10993

PSR J2032+4127/MT91 213 (J2032 hereafter) is a pulsar and Be star binary (Lyne et al. 2015), of which the periastron was predicted to be around November 2017 (Ho et al. 2017). Since 2016, Swift has been monitoring J2032 in X-rays and UV on weekly timescales and an X-ray source with an increasing flux has been seen (Ho et al. 2017; Li et al. 2017). Furthermore, a TeV enhancement in September 2017 was detected by VERITAS and MAGIC (ATel #10810). All these observations suggest a strong interaction between the pulsar wind and the stellar wind (and possibly the disk around the periastron passage) as the pulsar approaches. Takata et al. (2017) predicted that the X-ray/TeV flux will continue to increase in the next few weeks until a sharp flux drop at the periastron passage.

From the recent Swift/XRT observations (last data used: MJD 58040), the X-ray flux of J2032 is still steadily increasing. With the last ten XRT observations (MJD 58018 to 58040, over which the X-ray flux is relatively stable), the X-ray source can be well described by an absorbed power-law with the best-fit parameters of hydrogen column density = (9+/-2)E21 cm^-2, photon index = 1.5+/-0.2, and unabsorbed flux (0.3-10 keV) = 5E-12 erg/cm^2/s (reduced chi-squared = 62.63/51). Apparently, the photon index is much harder than that measured by NuSTAR (2.7+/-0.2) and XMM-Newton (1.9+/-0.1) in September 2017 and November 2017, respectively (Li et al. 2017).

In optical and UV bands, the Be star was clearly detected by UVOT. Interestingly, a recent strong UV brightening is seen (most obvious in the U band with an amplitude of 0.2-0.3 mag), while the variability is not clearly shown in the ASAS-SN V-band light curve obtained from the ASAS-SN Sky Patrol (last data used: MJD 58037). Given the strong intrinsic variability of the Be star, it is now unclear whether the UV brightening is associated with the X-ray/TeV activity. In case they are associated, the brightening is likely caused by the shock heating on the Be disk matter and/or stellar wind.

We also observed the Be star with the FRODOSpec spectrograph on the 2-m Liverpool Telescope. The H-alpha equivalent width (EW) are -13.4 A, -12.4 A, and -9.6 A (A means Angstrom here) on 2017 July 25, August 14, and October 06, respectively, indicating that the Be disk is evolving. It is worth mentioning that the strongest H-alpha EW known of the Be star was -13.2 A on 2009 June 13 and the EW was rapidly changing between -3.3 A and -10.1 A in 2016 (Ho et al. 2017; Li et al. 2017).

More UV and optical observations to understand the UV variability and the change in the H-alpha EW are strongly encouraged.

We acknowledge the use of ASAS-SN Sky Patrol public all-sky light curve interface (Kochanek et al. 2017). ASAS-SN is funded in part by the Gordon and Betty Moore Foundation through grant GBMF5490 to the Ohio State University, NSF grant AST-1515927, the Mt. Cuba Astronomical Foundation, the Center for Cosmology and AstroParticle Physics (CCAPP) at OSU, and the Chinese Academy of Sciences South America Center for Astronomy (CASSACA).

We thanks the Swift team for approving and carrying out our monitoring campaign. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester.

Optical/UV/X-ray light curves of PSR J2032+4127/MT91 213