Optical and NIR observations of XTE J1739-285

ATel #784; M. A. P. Torres, D. Steeghs, M. R. Garcia, J. E. McClintock (CfA), P. G. Jonker (SRON/CfA), D. Kaplan (MIT), J. Casares (IAC), R. Greimel (ING), T. Augusteijn (NOT)
on 30 Mar 2006; 21:13 UT
Credential Certification: Manuel Torres (mtorres@cfa.harvard.edu)

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We report optical and near-infrared (NIR) observations of the field containing the neutron star X-ray transient XTE J1739-285 (IAUC # 7300 , ATel #622). The recently reported Chandra localization (ATel #777) provides a significant improvement over the Swift XRT position reported in ATel #602, facilitating the identification of potential counterparts.

Optical observations were performed on 2005 Aug 30 20:59-21:14 UT and 2005-09-16 20:39-20:55 at the 2.56-m Nordic Optical Telescope (NOT) on La Palma using ALFOSC. On Aug 30 we acquired three 180s R-band images under seeing conditions of 0.8 arcsec (note that these observations were reported in ATel #602 with a wrong observing date). On Sep 16 several R-band images ranging in exposure times between 130 and 200s were obtained with a seeing of 0.8 arcsec. The images were flux calibrated using Landolt standards.

The NIR observations consist of a series of Ks-band images obtained with the 6.5m Magellan Baade telescope on 2005 Sep 13 00:10-01:01 and 2005 Oct 20 23:44-23:56 using the PANIC camera and totaling 900s and 225s on source respectively. The frames were acquired with a variable seeing ranging from 0.5-0.8 arcsec during the night of Sep 13 and 0.4 arcsec during Oct 20. Photometric and astrometric scales were defined using 2MASS sources in the field of view.

No optical counterpart is found in the NOT images down to a 3-sigma detection limit of R > 21.5 (Aug 30) and R > 22.5 (Sep 16). We remark here that the Aug 30 images were obtained at the peak of X-ray activity in 2005 (ATel #592) whereas the Sep 16 images were acquired after this event. Taking into account the 0.5 Crab flux reported during type I X-ray bursts (ATel #622) which must be below or equal to the Eddington Luminosity, we derive a (rough) upper limit on the distance to XTE J1739-285 of 12 kpc. This translates into an upper limit R-band absolute magnitude of M_R > 3.2 during the peak of X-ray activity and this suggests an intrinsically faint transient. Here we have used A_R=2.9 mag as expected from the hydrogen column towards the source (NH=7.4e21 cm**-2: Dickey & Lockman 1990).

Two NIR sources are visible inside the Chandra error circle for XTE J1739-285. The sources have approximate magnitudes of Ks = 16.9 +/- 0.2 and 17.8 +/- 0.3. We could not establish significant variability in either of these two sources. Using the above constraint on the distance and reddening (A_K=0.4), we derive M_K > 1.1 and 2.0 for the absolute magnitude in the K-band. However, the positional coincidence of both sources is insufficient to establish any of them as the counterpart to XTE J1739-285; a K5V (M2V) field star at 2 kpc would have an apparent magnitude of 16.9 (17.8) and 21.2 (22.9) at the K- and R-bands respectively.

Further NIR photometry to search for variability during quiescence and/or outburst is required to confirm/reject any of the above candidates.

Finding Chart