Swift J053321.3-684121 - A candidate HMXB in the LMC

ATel #3993; R. Sturm, F. Haberl, W. Pietsch (all MPE), S. Immler (NASA/CRESST/GSFC), A. Udalski (Warsaw University Observatory)
on 22 Mar 2012; 08:53 UT
Credential Certification: Richard Sturm (rsturm@mpe.mpg.de)

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During the Swift/UVOT survey of the LMC (PI: S. Immler), we found a serendipitous hard XRT source. The 2 ks enduring observation (00045431001) was performed on 2012-03-13 from 07:21 to 23:40 (UT). The source was found at high off-axis angle (~10') at 05 33 21.3 -68 41 21 (J2000, uncertainty 5.5", 90% confidence) with a net count rate of 0.0090±0.0021 cts s^-1. In a 2.9 ks Swift follow-up observation (00032310001) on 2012-03-20 from 09:18 to 11:23 the source was observed on-axis. We detect the source at 05 33 21.3 -68 41 21 (uncertainty 4.6", 90% confidence) with a net count rate of 0.0168±0.0025.

With an angular separation of 3.5", the source correlates with Sk -68 122, a blue supergiant in the LMC (V=12.78, B-V=-0.09), classified as B3 I (Rousseau et al. 1978, AASS, 31, 243). The source is saturated in OGLE III, but strong variability in the I band can be excluded.

To estimate X-ray spectral parameters, we analysed the spectrum using C statistics. We assumed an absorbed power-law spectrum with Galactic foreground absorption fixed to 6 × 10²⁰ cm^-2 and an additional absorbing column density for the LMC with 0.5 solar abundances for elements heavier than helium. We obtain an absorption of (1.2-7.5) × 10²² cm^-2, a photon index of 0.8-2.8 and fluxes of (5.4±2.3) 10^-13 erg cm^-2s^-1 and (8.1±2.1) 10^-13 erg cm^-2s^-1 in the 0.3-5.0 keV band for the first and second observation respectively. For a distance of 50 kpc, this corresponds to 1.6 10³⁵ erg s^-1 and 2.4 10³⁵ erg s^-1. Within the second observation, we find a flux increase by a factor of 2.3±0.2. This field in the LMC was not observed with XMM-Newton or Chandra. We find only a marginally detected (2.1 sigma) ROSAT source (1RXH J053322.5-684125, 8" distance).

The indicative time variability, relatively hard and absorbed spectrum, and the possible optical counterpart point to an identification of the source as a new high mass X-ray binary in the LMC. Identification as a background AGN would be possible from the X-ray spectra but less likely due to the short term X-ray variability. Deeper observations are needed to confirm the HMXB identification.

We thank the Swift team for scheduling the ToO observation.