Swift observations and refined location for Swift J0549.7-6812

ATel #5293; H. A. Krimm (CRESST/GSFC/USRA), S. T. Holland (STScI) and J. A. Kennea (PSU)
on 16 Aug 2013; 21:15 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Hans A. Krimm (Hans.Krimm@nasa.gov)

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A 117-second Swift target of opportunity observation was performed (as part of a planned longer observation) starting at 2013 August 16 04:47:08.426 . The XRT observed in Photon Counting mode and a UVOT-enhanced position was determined at:

XRT position (J2000):
RA): 05 50 06.54 (87.5273 deg)
Dec: -68 14 55.1 (-68.2486 deg)
90% Error radius: 2.1" (stat + syst)

A single optical source within the XRT error circle is detected in the UVOT b band with a magnitude b = 15.13 +/- 0.04 mag (1-sigma uncertainty). The UVOT position is:

UVOT position (J2000):
RA: 05 50 06.46 (87.52692 deg)
Dec: -68 14 56.6 (-68.24906 deg)
90% Error radius: 0".42 (stat + syst)

This position is consistent with a bright catalog star, listed as USNO-B1.0 0217-0105057 and 2MASS 05500646-6814559. The best position found is from the UCAC2 Catalogue (Zacharias, 2004) :

Optical catalog position (J2000):
RA: 05 50 06.449 (87.5268724 deg)
Dec: -68 14 56.04 (-68.2489012 deg)
Quoted statistical errors are 28 mas in RA and 15 mas in declination.

The magnitudes of the catalog star in quiescence are:
R = 14.96 +/- 0.19 (GSC 2.2 Catalog; STScI, 2001)
B = 16.31 +/- 0.53 (GSC 2.2)
J = 14.846 +/- 0.038 (2MASS All-Sky Catalog of Point Sources; Cutri+ 2003)
H = 14.767 +/- 0.082 (2MASS)
K = 14.444 +/- 0.100 (2MASS)

An X-ray source consistent with Swift J0549.7-6812 is found in the ROSAT All-Sky Survey Faint Source Catalog (Voges, 2000). This is 1RXS J055007.0-681451, with a reported count rate of 1.91 X 10^-2 ct/sec.

ROSAT position (J2000):
RA: 05 50 07.0 (87.52917 deg)
Dec: -68 14 51.5 (-68.24764 deg)
Position error is 8 arc seconds.

The XRT data are well fitted by an absorbed power-law model (Cstat = 94.67 for 121 dof) with the following parameters:

N_H = 1.00 (15.5, -10.0) x 10^21 cm-2
Gamma = 1.2 +/- 0.4
Flux (0.3-10 keV; unabsorbed) = 2.1 +/- 0.4 x 10^-10 erg/s/cm-2
There is no evidence of any lines or other deviations from a smooth spectrum. The spectral fitting was carried out using data and analysis based on Evans et al. (2009, MNRAS, 397, 1177). The corresponding XRT (0.3-10 keV) count rate is ~4 ct/s. Using WebPIMMS to convert from the historical ROSAT count rate to the XRT band predicts a count rate of ~0.02 ct/sec. Thus Swift J0549.7-6812 is currently at least ~100 times brighter than it is in quiescence.

Given the positional coincidences between the XRT, ROSAT, UVOT and optical sources, we propose that 2MASS 05500646-6814559 is the optical counterpart to Swift J0549.7-6812 and that 1RXS J055007.0-681451 is the quiescent X-ray counterpart. With this association, we find that the optical source is only about 2 standard deviations brighter at the current epoch than in the archival observations. This is inconsistent with a black hole binary, for which the optical brightness often increases by ~5 magnitudes due to emission from the jet and accretion disk. The optical brightness at the distance of the LMC, combined with the X-ray spectrum, suggests a HMXB/NS system in the hard state. An alternative explanation is a LMXB/BHC, but this would require a very faint optical source (b > ~19.9) since no second UVOT source is found in the XRT error circle.

We note that the behavior of Swift J0549.7-6812 at this time is similar to that of another Swift-discovered source, the LMC HXMB/NS Swift J0513.4-6547 (ATel #2011). The quiescent counterpart is B=15.3 and the UVOT detected it at b=14.99. The XRT photon index was hard at the peak of the outburst. For this source, RXTE/PCA data, which clearly showed a pulse period of ~27 seconds.

The current results do not allow us to conclusively determine the nature of the source. Further Swift observations have been approved and will be carried out in XRT Windowed Timing mode, which will allow the search for pulsations.

Swift/BAT Hard X-ray Transient light curve for Swift J0549.7-6812