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