Optical observations of XTE J1701-462 and SAX J1810.8-2609

ATel #1193; M. A.P. Torres (CfA), P. G. Jonker (SRON/CfA), D. Steeghs (Warwick/CfA)
on 25 Aug 2007; 17:17 UT
Credential Certification: Manuel Torres (mtorres@cfa.harvard.edu)

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We report optical observations of the fields containing the neutron star X-ray transients XTE J1701-462 (Remillard, Lin et al. 2006, ATel #696) and SAX J1810.8-2609 (Ubertini et al. 1998, IAUC # 6838 ).

SAX J1810.8-2609: Following the report of renewed X-ray activity from this source (Parsons et al. 2007, GCN #6706), we have performed optical observations on 2007 Aug 5 21:56-22:16 UT at the 4.2m William Herschel Telescope on La Palma using the Aux Port imager. We have obtained two 300s R- and I-band images under seeing conditions of ~ 0.9 arcsec. Weather conditions were good and the images were flux calibrated using the Landolt standard PG1514+034. An astrometric scale was defined using 2MASS sources in the field of view.

One optical source is visible inside the Chandra error circles for SAX J1810.8-2609 (Jonker, Wijnands & van der Klis 2004,MNRAS,349,94). Its position is R.A(J2000)=18:10:44.52 Dec(J2000)=-26:08:42.6 with a positional uncertainty < 0.2 arcsec. PSF-fitting yields I=19.1 +/- 0.1 and R=19.8 +/- 0.1 for the optical counterpart.

We obtain R= 17.57 +/- 0.04 and I=16.75 +/- 0.06 for the field star B and R = 17.71 +/- 0.04 and I = 17.08 +/- 0.07 for field star C (labels are from Greiner et al. 1999,MNRAS,308,17; see also finding chart below). We note that our absolute calibration for these two field stars does not correspond with the calibration in the R-band presented in Greiner et al (see their Table 3). Note also that the field star A is 2MASS J181044.525-260855.94 with magnitudes K=13.50 +/- 0.07 and J=14.39 +/- 0.06 in disagreement with the magnitudes reported in Table 3. A rigorous calibration of the crowded field of SAX J1810.8-2609 is therefore necessary to remove any ambiguities in the flux measurements.

XTE J1701-462: Following the reported rapid decline of the X-ray flux for this source (Homan et al. 2007, ATel #1165), we used the IMACS imaging spectrograph mounted at the 6.5m Magellan Baade telescope on 2007 Aug 13 23:38 - 23:59 UT to acquire three 240s I-band images under seeing of 0.8 arcsec. The absolute calibration was made with respect to two local standards in the field of Centaurus X-4 (see e.g. McClintock & Remillard,1900,ApJ,350,386). The astrometric scale was defined again with 2MASS sources in the field.

PSF-fitting gives I=22.00 +/- 0.05 for the optical counterpart to XTE J1701-462. This indicates that the object has faded by about 2 magnitudes compared to the optical brightness reported at the beginning of the outburst (I~20; Maitra & Bailyn 2006, ATel #712). We urge more observations to establish the quiescent magnitude of this system.

Finding chart for J1810: http://hea-www.harvard.edu/~mtorres/SAXJ1810.8m2609.html

Finding chart for J1701: http://hea-www.harvard.edu/~mtorres/XTEJ1701m462.html