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GROND optical/NIR and Swift/XRT observation of IGR J17091-3624

ATel #8795; J. Greiner, J. Bolmer (both MPE Garching), P. Gandhi, D. Altamirano, P. A. Charles, J. M. Court (all Southampton Univ.), D. A. Kann (TLS Tautenburg),D. J. Walton (Caltech)
on 9 Mar 2016; 19:33 UT
Distributed as an Instant Email Notice Transients
Credential Certification: J. Greiner (jcg@mpe.mpg.de)

Subjects: Infra-Red, Optical, X-ray, Binary, Black Hole, Transient

Referred to by ATel #: 8821, 8834

The black hole transient IGR J17091-3624, similar in X-ray behaviour to GRS 1915+105 (Greiner et al. 1996, ApJ 473, L107; Altamirano et al. 2011, ApJ 742, 17; Altamirano & Belloni 2012, ApJ 747, 4), has been recently reported to be in outburst based on an increased Swift/BAT rate and subsequent Swift/XRT ToO (Miller et al. 2016, ATel #8742; Grinberg et al. 2016, ATel #8761 and Barthelmy et al. 2016, GCN #19155 -- Swift/BAT Trigger 677981).

We observed the field of IGR J17091-3624 simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPG telescope at ESO La Silla Observatory (Chile) for 2 hrs, at a mean seeing of 1".3 and at an airmass of 1.1. This is contemporaneous with a NuSTAR and XMM-Newton observation on the same day. At a mid-time of 09:20 UT on 2016-03-07 we measure for the counterpart candidate C2 of Chaty et al. (2008, A&A 484, 783) the following AB (Vega) magnitudes:

g' = 22.5 (22.5) +- 0.1 mag,  
r' = 20.2 (20.0) +- 0.1 mag, 
i' = 19.2 (18.8) +- 0.1 mag, 
z' = 18.5 (18.0) +- 0.1 mag, 
J  = 17.4 (16.5) +- 0.1 mag, 
H  = 17.1 (15.7) +- 0.1 mag, and 
Ks = 17.0 (15.2) +- 0.1 mag 
The given magnitudes are derived based on calibrating the images against GROND zeropoints (in g'r'i'z') and 2MASS field stars (JHKs) and are not corrected for the Galactic foreground extinction corresponding to a reddening of E(B-V)= 1.9 mag in the direction of IGR J17091-3624 (Schlafly & Finkbeiner 2011, ApJ 737, 103). We caution that the object is not spatially resolved from the nearby northern star (C1 in Chaty et al., around 1".2 distance), so the photometry has an additional systematic uncertainty.

Compared to earlier optical/NIR imaging (Chaty et al.; Torres et al. 2011, ATel #3150), the object is about 1.5 mag brighter in all bands. C2 itself is a blend of at least two stars. In the Ks-band, the source is currently also 2 mag brighter than the blended component at 0."4 distance.

Another Swift/XRT WT-mode observation was performed on March 7th, 2016. The 0.8-10 keV spectrum is well fit with an absorbed power-law model (similarly to the previous observations, see Grinberg et al., ATEL #8761). The absorption is Nh=(1.21+-0.06)e22 cm^-2, and the PL index 1.83+-0.06. This is consistent with the continuous softening observed in the previous observations (ATEL #8761). We calculate an unabsorbed flux of 8.22e-10 erg/cm2/s (0.5-10 keV). We thank the Swift team for the organization of the pointed Swift observations.

Multi-wavelength follow-up observations (and particularly optical/NIR spectroscopic observations) are strongly encouraged.