Search for the NIR and Optical counterpart to IGRJ17407-2808 in quiescence

ATel #3688; S. Greiss (Warwick), D. Steeghs (Warwick/CfA), T. Maccarone (Southampton), R. I. Hynes (LSU), C. T. Britt (LSU), P. G. Jonker (SRON/RU/CfA), M. A.P. Torres (SRON/CfA), N. Masetti (INAF/IASF, Bologna), A. Rojas (PUC Santiago), C. Heinke (Alberta), R. Kaur (Alberta), T. Bird (Southampton), the GBS consortium, the VVV consortium
on 18 Oct 2011; 13:18 UT
Credential Certification: Danny Steeghs (dsteeghs@cfa.harvard.edu)

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In order to search for the counterpart of the fast and hard X-ray transient IGR J17407-2808 (Atel #3685), we investigated recent near-infrared (NIR) data of the Galactic Bulge region obtained as part of the VVV survey (Minniti et al. 2010, New Astronomy, Volume 15, 433). The observations took place while the source was in quiescence. The data have been acquired using the 4-m VISTA telescope at Paranal observatory (Chile), using a camera with pixel sampling of 0.34 arcsec.

We analysed JHKs images obtained on the 7th of April 2010, at airmass 1.12 and under average seeing conditions of 0.85 arcsec. Total on source exposure times were 24s for the J-band images and 8s for both H- and Ks-band frames.

We detect a NIR source 0.67 arcsec from the Chandra position reported in Heinke et al (2009, ApJ, 701, 1627) (RA = 17:40:42.05, Dec = -28:07:24.6), which has the following magnitudes: J = 16.89 +/- 0.14, H = 15.95 +/- 0.17 and Ks = 15.46 +/- 0.17. The coordinates of this source are RA = 17:40:42.01, Dec = -28:07:25.0 with an astrometric fit RMS of 0.1 arcsec. The match is within the Chandra error circle and is the closest detected NIR source consistent with that position. The next closest NIR match is 2.22 arcsec away from the Chandra X-ray position, which appears inconsistent.

The colours of this match indicate moderate amounts of reddening towards the counterpart candidate. Extinction values in this region obtained using red clump giants with VVV data (Gonzalez et al. 2011, arXiv:1107.5496) are: A_J = 1.75 mag, A_H = 1.00 mag and A_Ks = 0.58 mag, when using Nishiyama et al. (2009, ApJ, 696, 1407) extinction law towards the Galactic Center. This corresponds to E(B-V)=2.27. The de-reddened magnitudes of the nearest counterpart to IGRJ17407-2808 are then : J = 15.14 +/- 0.14, H = 14.95 +/- 0.17 and Ks = 14.88 +/- 0.17.

The quiescent X-ray counterpart was also detected by the Chandra Galactic Bulge Survey (GBS; Jonker et al. 2011, ApJS, 194, 18) as source number 1102. As part of a program to identify variable counterparts to GBS sources, this field was observed with Mosaic-II on the Blanco 4-m telescope at Cerro Tololo Interamerican Observatory repeatedly between 2010 July 9-16. We identify the same star found in the VVV data at an approximate magnitude of 22.0 +/- 0.5 in SDSS r. We find no variability in the candidate optical counterpart with r.m.s. greater than 10% on timescales of hours to a week. Using the E(B-V) value quoted above, we find an A_r of 5.8 mag and a de-reddened r-band magnitude of 16.2 +/ 0.5 mag.

The de-reddened optical and NIR magnitudes and colours are more or less consistent with a late type F dwarf. The absolute magnitude of such a star places it at a distance of ~3.8 kpc. The absolute magnitudes of a giant or a supergiant at Galactic Bulge distances would lead to much brighter apparent magnitudes than the ones observed. Therefore, given the magnitudes and colours of the nearby optical and NIR counterpart candidate, we can safely conclude that this source is not consistent with a supergiant fast X-ray transient (SFXT). We note that the counterpart candidate could be a foreground object not associated with the transient, making the actual quiescent counterpart much fainter. This is not surprising given the unusual nature of the hard X-ray flares which are atypical for either a SFXT or a standard LMXB transient.