The Stellar-mass Black Hole IGR J17091-3624 is in an Active State

ATel #11277; J. M. Miller, M. T. Reynolds (University of Michigan), B. Cenko (NASA/GSFC), J. Kennea (Swift, Pennsylvania State University)
on 8 Feb 2018; 16:59 UT
Credential Certification: Jon Miller (jonmm@umich.edu)

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IGR J17091-3624 is a recurrent transient source, and a strong black hole candidate. The source has shown many of the same X-ray timing behaviors and "states" as GRS 1915+105 (e.g., Altamirano et al. 2011, Court et al. 2017), as well as relativistic disk reflection and strong winds (Reis et al. 2012, Xu et al. 2017; King et al. 2012). However, even close to outburst peak, IGR J17091-3624 is much fainter than GRS 1915+105; its outbursts are difficult to detect in the Neil Gehrels Swift Observatory BAT.

Based on suggestive recent BAT activity, an XRT exposure was obtained on 2018 Feb 02, starting at 17:49:53 UT. A net exposure of 970s was obtained in "photon counting" mode. The source is clearly detected in this exposure: 43 total counts are recorded, with an estimated local background of 4 counts. Assuming an interstellar column density of 8 E+21 cm^-2 and a power-law with an index of Gamma = 2, this corresponds to a flux of approximately 2 E-12 erg/cm2/s (0.3-10 keV). The distance to IGR J17091-3624 is quite uncertain; for a distance of 8.5 kpc, this flux implies a luminosity of L = 3.4 E+34 erg/s; for a distance of 25 kpc (Altamirano et al. 2011), the flux implies a luminosity of L = 3.0 E+35 erg/s.

We note that the XRT image is contaminated by mirror rings induced by the nearby bright X-ray source GX 349+2. Source and background regions avoided these rings.

Further monitoring may be able to determine whether the source is in a "hard state" outburst, or if it is rising toward a full outburst. This source may be of particular appeal for X-ray timing studies. The similarity of its QPO frequencies, power spectra, and flux states to those observed in GRS 1915+105 poses an interesting dilemma: either the two sources have remarkably similar black hole masses and spins, or the observed phenomena and frequencies are set by aspects of the accretion flow but not anchored by gravity.

References

Altamirano, D., et al., 2011, ApJ, 742, L17
Court, J. M. C., et al., 2017, MNRAS, 468, 4748
King, A. L., et al., 2012, ApJ, 746, L20
Reis, R., et al., 2012, ATEL 4382
Xu, Y., et al., 2017, ApJ, 851, 103