Swift Observations of Swift J1357.2-0933

ATel #3142; H. A. Krimm (CRESST/GSFC/USRA), J. A. Kennea (PSU) and S. T. Holland (CRESST/GSFC/USRA)
on 2 Feb 2011; 20:01 UT
Credential Certification: Hans A. Krimm (Hans.Krimm@nasa.gov)

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The newly discovered X-ray transient source Swift J1357.2-0933 (Krimm et al., ATel #3138) was observed by Swift on 1 Feb 2011 (MJD 55593) and 2 Feb 2011. It was detected as a strong source in both Swift/XRT and UVOT.

The XRT observed in Photon Counting mode on 1 Feb 2011 and a position was determined at:

RA (J2000): 13:57:16.92 = 209.32049 (deg)
Dec (J2000): -09:32:39.3 = -9.54424 (deg)
90% Error radius: 2.1"

The 2 Feb 2011 observation was in Windowed Timing mode and the data are well fitted by an absorbed power-law model (Reduced chi2 = 1.06) with the following parameters:

N_H = 1.2 +/- 0.7 x 10^20 cm-2
Gamma = 1.56 +/- 0.03
Flux (0.3-10 keV) = 4.2 +/- 0.1 x 10^-10 erg/s/cm-2

There is no evidence of any lines or other deviations from a smooth spectrum. Furthermore, the spectral fit is not improved with the addition of a thermal component. This consistency of the spectrum with a pure power law is inconsistent with the dwarf nova interpretation suggested by Rau, Greiner & Filgas (ATel #3140) since dwarf novae spectra typically require a thermal component in addition to a power law (e.g. U Gem: T. Guver et al, MNRAS 372:450, 2006) or a thermal bremsstrahlung model with a Gaussian emission line (e.g. SS Cyg: K. E. McGowan, W. C. Priedhorsky & S. P. Trudolyubov, ApJ 601:1100, 2004). The XRT results are consistent with a low-mass or high-mass X-ray binary interpretation.

The XRT flux is at the same level in the two observations. Similarly the BAT count rate remains steady at 0.008 +/- 0.0009 ct/s/cm-2.

The UVOT observations on 1 Feb 2011 yielded a position of

RA (J2000) = 13:57:16.86 = 209.32026 (deg)
Dec (J2000) = -09:32:38.9 = -9.54414 (deg)
90% Error radius: 0.42",

determined by comparison with the USNO-B1.0 catalog. The UVOT counterpart is 0.89 arc seconds from the center of the XRT error circle. We note that the UVOT position is not formally consistent with that reported by GROND (Rau, Greiner & Filgas, ATel #3140). However, we believe that the discrepancy between these two preliminary positions is due to underestimation of systematic uncertainties and the use of two different reference catalogs. We have full confidence that we are observing the same source.

The preliminary UVOT photometry yields the following measurements:

Start (UTC) Filter Mag Err Sigma Mag_AB
--------------------------------------------------------------
2011-02-01 04:59:01 u 14.86 0.02 246.5 15.88
2011-02-01 06:34:41 u 14.88 0.02 101.0 15.90
2011-02-01 21:09:10 v 16.21 0.04 36.9 16.20
2011-02-01 22:45:36 v 16.18 0.05 27.9 16.17
2011-02-01 21:38:35 b 16.16 0.03 59.1 16.04
2011-02-01 21:31:14 u 14.90 0.03 98.5 15.92
2011-02-01 21:23:53 uvw1 14.35 0.03 86.2 15.88
2011-02-01 21:16:31 uvm2 14.17 0.03 70.8 15.85
2011-02-01 21:01:49 uvw2 14.10 0.02 95.0 15.79
2011-02-01 22:38:16 uvw2 14.10 0.02 95.3 15.79
--------------------------------------------------------------

These magnitudes are not corrected for Galactic foreground reddening of E(B-V)=0.04mag (Schlegel et al. 1998). We note that the u band magnitude is consistent over all three pointings indicating that the source has not yet begun to fade in the UV.

Further Swift observations are planned.