Swift Detection of the Black Hole GS 1354-645 in Outburst

ATel #7612; J. M. Miller, M. T. Reynolds (Univ. of Michigan), J. Kennea (Pennsylvania State Univ.)
on 10 Jun 2015; 13:43 UT
Credential Certification: Jon Miller (jonmm@umich.edu)

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Following indications of an outburst in monitoring observations made with the Swift/BAT and MAXI, we requested a Swift/XRT observation of the dynamically confirmed stellar-mass black hole GS 1354-645 (M > 7.5 +/- 0.7 Msun; Casares et al. 2009). The source was last detected in outburst with RXTE in 1997 (e.g. Brocksopp et al. 2001).

An 0.52 ks observation was obtained with the XRT in photon counting mode, starting on 2015-06-10 at 05:18:17. A strong source is clearly detected at the position of GS 1354-645 in the "quick look" data, indicating an outburst.

Approximately 1200 counts are recorded within a 30 pixel radius, and the light curve extracted from this region shows variability. The source flux was high enough to induce photon pile-up, so an annular region excluding the central 10 pixels was selected for spectral extraction.

Fits to the binned spectrum with a power-law, assuming N_H = 8.0 E+21 cm^-2, give a photon index of 1.4 +/- 0.1 (1 sigma errors) and a 0.5-10.0 keV absorbed flux of 3.0 E-11 erg/s/cm^2, or an unabsorbed flux of 3.9 E-11 erg/s/cm^2. A prior Chandra observation of the source in quiescence measured an unabsorbed flux of 1.5 E-13 erg/s/cm^2 in the same band (Reynolds & Miller 2011), indicating the source is now 260 times brighter than in quiescence.

Pile-up may not be completely eliminated in this simple analysis; the true source flux may be higher, and the true photon index steeper.

The distance to Ginga 1354-645 is not well constrained. Casares et al. (2009) give a lower limit of 25 kpc, and an upper limit of 61 kpc for the distance. The implied luminosity of the source is then 2.9 E+36 (d/25kpc)^2 erg/s.

Additional observations in X-ray, radio, and optical bands are encouraged. If the source is really in a low/hard state, as indicated by its spectrum, variability, and luminosity, radio observations are particularly encouraged.

We thank Neil Gehrels and the Swift team for executing this observation.

Brocksopp, C., et al., 2001, MNRAS, 323, 517
Casares, J., et al., 2009, ApJS, 181, 238
Reynolds, M. T., & Miller, J. M., 2011, ApJ, 734, L17