NIR counterparts in the Swift error circles of the active transients SAX J1750.8-2900 and XTE J1810-189

ATel #1446; M. A.P. Torres (CfA), D. Steeghs (Warwick/CfA), J. Kennea (PSU), G. Roelofs (CfA), P. G. Jonker (SRON/CfA), J. Homan (MIT), J. M.Miller (Michigan)
on 28 Mar 2008; 21:22 UT
Credential Certification: Manuel Torres (mtorres@cfa.harvard.edu)

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We report on Swift TOO follow-up observations of the neutron star transient SAX J1750.8-2900 (Natalucci et al. 1999,ApJ,523,45) as well as infrared observations of the fields containing SAX J1750.8-2900 and the neutron star transient XTE J1810-189 (Markwardt et al. 2008, ATel #1424, #1443).

Swift/XRT observations were performed both in photon counting (PC) and windowed timing (WT) mode during March 24 14:47-15:04 UT and March 25 21:18-21:34 UT. The source was positioned away from any detector hot-columns on the PC data, allowing us to obtain a more accurate localization of this transient. The two observations reveal the source at a position consistent with that reported in Steeghs et al. (2008, ATel #1431). Based on the two independent detections (each with an uncertainty of 3.5"; 90% confidence level), we derived a mean position of:

R.A.(J200) = 17:50:24.33, Dec(J2000) = -29:02:14.3

Fits to the 338s X-ray spectrum acquired in WT mode on March 24 using an absorbed power-law model produced acceptable results. The data yield N_H= (2.3 +/- 0.4)e22 cm^-2, a photon index of 1.2 +/- 0.2 and reduced chi**2 = 0.83. Uncertainties are at the 90% level confidence. For the PC spectra we derive similar values. The N_H value is lower than the H column density of ~6e22 cm^-2 obtained from BeppoSAX X-ray spectra (Natalucci et al. 1999) or the 4.4e22 cm^-2 derived from RXTE/PCA observations (Kaaret et al. 2002,ApJ,575,1018).

From the above fits we derive an observed 2 - 10 keV flux of 5.5e-10 erg/cm^2/s on March 24. Analysis of the 92s WT mode data obtained on March 25 supports a 30% decrease in flux between the two Swift observations. Therefore the source has declined in 10 days from a peak brightness of 230 mcrab in the 2-10 keV (Markwardt & Swank 2008, ATel #1425) to a flux of about ~30 mCrab, a value close to the 10 mcrab low-level activity reported at the end of previous outbursts (see Natalucci et al. 1999, Kaaret et al. 2002). Here we have used 1 crab (2-10 keV) ~ 2e-8 erg/cm^2/s as in Kaaret et al.

We have acquired Ks-band images of the fields containing SAX J1750.8-2900 and XTE J1810-189 on 2008 Mar 18 using the PANIC camera attached to the 6.5m Magellan Baade telescope. Seeing conditions were 0.4"-0.5". An astrometric scale was defined using 2MASS sources in the field of view. We caution that given the stellar densities in these low Galactic latitude fields, the odds of finding a non-related field star or unresolved sources within the Swift X-ray error circle are significant.

Our imaging of SAX J1750.8-2900 reveals several point sources within the 3.5" error circle for the improved X-ray location (see chart). The brightest object is visible in 2MASS frames. 2MASS 1810233-1904136, the proposed infrared counterpart to XTE J1810-189 (Starling et al. 2008, ATel #1441), is resolved in our images into a large number of individual sources that are all consistent with the Swift position (see chart).

We plan to acquire additional imaging of these fields to identify the correct infrared counterparts (if detectable) through their variability as the outburst activity declines.

Finding chart for J1810: http://hea-www.harvard.edu/~mtorres/XTEJ1810m189.html

Finding chart for J1750.8: http://hea-www.harvard.edu/~mtorres/SAXJ1750.8m2900.html