Bright mid-infrared detections of MAXI J1836-194 with VISIR on the VLT: brightening of the compact jet

ATel #3689; D. M. Russell (Univ. of Amsterdam), T. Belloni (INAF Brera Obs., Italy), J. C. A. Miller-Jones (ICRAR Curtin), K. O'Brien (UC Santa Barbara), P. Gandhi (ISAS, JAXA, Japan), M. Cadolle Bel (ESA/ESAC, Villanueva de la Canada, Spain), S. Markoff (Univ. of Amsterdam), P. Casella (INAF-OAR, Italy), J. Homan (MIT), E. Gallo (Univ. of Michigan), R. P. Fender (Univ. of Southampton), T. Shahbaz (IAC, Tenerife, Spain), S. Corbel (Univ. Paris Diderot / CEA Saclay)
on 18 Oct 2011; 16:54 UT
Credential Certification: David M. Russell (D.M.Russell@uva.nl)

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The new X-ray transient MAXI J1836-194 is a black hole candidate X-ray binary (BHXB; ATel #3611, #3613, #3628 #3652). Its X-ray spectrum was seen to soften (ATel #3618, #3652) but the X-ray flux has recently decayed and the spectrum has hardened (see link below for the light curve and evolution in the X-ray hardness-intensity diagram).

Here we report infrared detections of MAXI J1836-194 at 2 - 12 μm. The Imager and Spectrometer for mid Infrared (VISIR) instrument on the Very Large Telescope (VLT) observed the field of the source on 2011-09-16, 2011-09-27 and 2011-10-11 UT, for approximately one hour on each date (less than half of the time was on source due to the chop-nod mode and overheads). Observing conditions were clear on all occasions. In the most recent observation, a bright source was detected at a position consistent with the BHXB (ATel #3628) in all four filters. The flux densities were 16 +- 3 mJy, 44 +- 1 mJy, 50 +- 1 mJy and 57 +- 1 mJy in K-band (2.2 μm), PAH1 (8.6 μm), SIV (10.5 μm) and J12.2 (12.0 μm), respectively. The de-reddened spectrum (adopting Av = 0.90 which is derived from the nH quoted in ATel #3613) can be fitted by a power law, F_&nu &prop &nu^{-0.68 +- 0.02}. This is typical of optically thin synchrotron emission (see also ATel #3619), and probably originates in the compact jet which is usually detected in the low/hard state of BHXBs. Inverted-spectrum radio emission at similar flux densities, consistent with a partially self-absorbed jet, has been detected (ATel #3628, #3656).

On 2011-09-27 the flux density at 8.6 - 12.0 μm was lower; 20 - 30 mJy, and on 2011-09-16 the source was not detected at 8.6 - 12.0 μm, implying that its flux density was a few mJy at most. The mid-IR flux has brightened by at least ~1 order of magnitude over the last four weeks as the X-ray spectrum hardened. On 2011-09-16 the RXTE X-ray spectrum was fitted by an absorbed (nH was fixed to 2e21 cm^-2; ATel #3613) power law (PL) with a photon index of 1.93 +- 0.02, plus a disk blackbody. The PL flux was 9.96e-10 erg/cm^2/s (3-25 keV) which provided 77% of the total unabsorbed flux. On 2011-10-10, no disk blackbody was required for the fit; a PL with photon index 1.68 +- 0.01 + a narrow gaussian emission (Fe) line frozen at 6.4 keV described the spectrum well (the total flux was 8.84e-10 erg/cm^2/s; 3-25 keV).

Very few mid-IR detections of transient BHXBs exist in the literature, and MAXI J1836-194 is currently one of the brightest ever detected at 10 microns (only GRO J0422+32, GX 339-4 and Cyg X-1 (a high-mass BHXB) were detected at such bright flux densities; van Paradijs et al. 1994, ApJ, 429, 19; Fender et al. 2000, MNRAS, 312, 853; Corbel & Fender 2002, ApJ, 573, L35; Rahoui et al. 2011, ApJ, 736, 63; Gandhi et al. 2011, ApJ, 740, L13). Observations of this bright BHXB, especially at frequencies from radio to infrared, are therefore encouraged while it evolves in the low/hard state.

Based on observations collected at the European Southern Observatory, Chile, under ESO Programme ID 087.D-0914(A).

X-ray light curve and hardness-intensity diagram of MAXI J1836-194