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Swift J1910.2-0546: Further Swift Observations

ATel #4149; J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), S. T. Holland (STScI)
on 4 Jun 2012; 23:16 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

Subjects: X-ray, Transient

Referred to by ATel #: 4198, 4210, 4246

On June 4th, 2012 starting at 11:56UT, Swift took a 1ks target-of-opportunity observation of Swift J1910.2-0546 (ATEL #4139). XRT was configured to use Windowed Timing mode (WT) in order to avoid pile-up from the source which has previously seen to be very bright in X-rays (ATEL #4145).

The WT data show that the source is very bright (~545 XRT count s-1 in the full 0.2-10 keV XRT band), and has a steady flux. No QPOs were detected in the short observation. The source spectrum can be well fitted by an absorbed disk-blackbody plus power-law model (reduced chi2 = 1.4). An absorbed power-law model does not well describe the data (reduced chi2 = 18.4), nor does a absorbed disk-blackbody (reduced chi2 = 2.8) which shows significant residuals above 5 keV, likely due to the necessity of a power-law component. The disk-blackbody plus power-law model spectral fit returns the following parameters:

NH = 0.50 +/- 0.03 x 1021 cm-2
kTin = 0.71 +/- 0.01 keV
Photon Index = 2.4 +/- 0.3

The fitted flux is 1.4 x 10-8 erg/s/cm2 (0.5 - 10 keV, uncorrected for absorption). The flux corrected for absorption is ~2.2 x 10-8 erg/s/cm2 (0.5-10 keV). Assuming an 8 kpc distance, this equates to a 0.5-10 keV luminosity of 1.7 x 1038 erg/s, close to the Eddington luminosity for a neutron star.

We note that this is significantly brighter than reported previously in ATEL #4145. However, those results were based on heavily piled-up data taken in Photon Counting mode, making estimates of flux at these count rates difficult. A more detailed analysis of the piled-up data is necessary to determine if the source has become significantly brighter since the previous observation on June 1st, 2012.

Comparing the fluxes of the two components, projected into the standard RXTE energy band (2-20 keV), we find a Disk fraction of 81.4%, suggesting that if Swift J1910.2-0546 is a Black Hole Candidate LMXB, it is currently in the "Thermal" state as defined by Remillard and McClintock (2006, ARA&A, 44, 49), which is consistent with the absence of QPOs in the light-curve.

We performed UVOT observations in four filters, and find the following magnitudes: b = 16.19 +/- 0.04, u = 15.42 +/- 0.05, uvw1 = 16.30 +/- 0.04 and uvw2 = 17.24 +/- 0.07. All magnitudes quoted are on the Vega system, and are uncorrected for Galactic extinction.

The rate in the BAT 15-50 keV band remains fairly constant since discovery at an average rate of 0.0095 +/- 0.0005 ct/s/cm2 (~40 mCrab).

Further observations of Swift J1910.2-0546 by Swift are planned.