Swift Observations of the 2017 Outburst of Swift J1357.2-0933

ATel #10314; Gregory R. Sivakoff, Bailey E. Tetarenko, Aarran W. Shaw (U. Alberta), and Arash Bahramian (Michigan State University)
on 23 Apr 2017; 03:02 UT
Credential Certification: Gregory R Sivakoff (sivakoff@ualberta.ca)

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The Catalina Real-Time Transient Survey II (CRTS-II) detected the beginning of an outburst from the short period black hole X-ray binary Swift J1357.2-0933 on 20 Apr 2017 (MJD=57864.33; ATel #10297). The optical detection of a X-ray binary outburst prior to detection by an all-sky X-ray telescope allows for probes of the rising hard-state at lower-than-typical fractions of the Eddington luminosity.

Based on the CRTS-II detection, we requested a series of Swift/XRT+UVOT observations, the first of which we report here. This observation, combined with data from the Swift BAT Transient Monitor (Krimm et al., 2013, ApJS 209,14) show that the outburst has begun to be detected at X-ray energies, and that the source is still in an early phase of its outburst.

A pointed Swift observation was taken on 21 Apr 2017. A bright source consistent with the known location of Swift J1357.2-0933 was detected by both Swift/XRT and Swift/UVOT. Swift/XRT observed in Auto mode on 21 Apr 2017, with Photon Counting (PC) mode data taken over ~1.5 ks (MJD=57864.84638-57864.86236). In an annular extraction region (10-70”), the source exhibited a pile-up corrected count rate of ~0.71 cnt/s (0.3-10 keV) in the PC data; we note that there are bad columns that lower this observed count rate. These data were well fit (chi^2 = 49.8 for 55 dof) by an absorbed power-law model with the following parameters (and 90% confidence intervals):
N_H = (3.0 +3.4/-2.9 ) x 10^20 cm^-2
Gamma = 1.53 +0.15/-0.14
Unabsorbed Flux (0.5-10 keV) = (1.046 +0.087/-0.084) x 10^-10 erg/s/cm^2.

Stacked daily observations of Swift/BAT for MJD=57862-57864 have also detected the source at 0.00434 +/- 0.00095 cnt/s/cm^2 (19.7 +/- 4.4 mCrab; 68% confidence interval). Correcting for a non-Crab spectrum, this corresponds to a flux (15-50 keV) of ~ 2.2 x 10^-10 erg/s/cm^2. Assuming the same spectrum, the Swift/BAT flux is about 1.7 times the Swift/XRT flux. It is unclear if this is an issue of variability or curvature in the spectrum. The Swift/XRT flux is about a factor of 4 less than that immediately reported by Swift/XRT after the source's first detection of outburst in 2011 (ATel #3142); the Swift/BAT flux is about a factor of 2 lower (ATel #3138, #3142). This demonstrates that to this point, Swift J1357-0933 is in an early phase of a rising hard state.

With Swift, we have also obtained UVOT photometry (quoted without extinction corrections) during the XRT observation, where the first and second error values are statistical and systematic 1 sigma uncertainties, respectively:
UVW2 = 14.36 +/- 0.03 +/- 0.03
UVM2 = 14.42 +/- 0.03 +/- 0.03
UVW1 = 14.63 +/- 0.04 +/- 0.03
U = 15.12 +/- 0.04 +/- 0.02
B = 16.54 +/- 0.05 +/- 0.02
V = 16.40 +/- 0.09 +/- 0.01

Further Swift observations are planned. Coordinated multi-wavelength observations are strongly encouraged over this outburst; contact sivakoff@ualberta.ca to be added to a mailing-list for coordination.

We are thankful to Brad Cenko and the Swift team for rapidly approving and scheduling these initial observations.