X-ray outburst of RX J0520.5-6932, confirmed as a Be/X-ray binary

ATel #4748; G. Vasilopoulos (MPE), P. Maggi (MPE), R. Sturm (MPE), F. Haberl (MPE), W. Pietsch (MPE), A. Udalski (Warsaw University Observatory), M. Sasaki (IAAT), S. Immler (NASA/CRESST/GSFC), and the LMC XMM-Newton survey collaboration.
on 19 Jan 2013; 17:12 UT
Credential Certification: Frank Haberl (fwh@mpe.mpg.de)

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A Swift observation performed on 2013 January 13 as part of the LMC UV survey (PI: S. Immler) detected the high-mass X-ray binary candidate RX J0520.5-6932 in a moderately bright X-ray outburst. Assuming a power-law spectrum with a photon index of 0.9 and absorption of 10²¹ cm^-2, we derive a 0.2-12 keV flux of 1.67 × 10^-12 erg s^-1 cm^-2. This flux is ~25 times higher than measured during a recent XMM-Newton observation (see below). Follow-up observations with Swift/XRT detected the source at a similar flux level, with an increasing trend (details in the Table below).

RX J0520.5-6932 was discovered by Schmidtke et al. (1994) with the ROSAT HRI, and subsequent ROSAT PSPC observations showed the source to be variable (Haberl & Pietsch 1999). The source was later detected by XMM-Newton (2XMM J052029.7-693155) in an observation performed on 2004 January 17, and in a recent observation performed on 2012 December 5, as part of the XMM-Newton survey of the LMC (PI: F. Haberl). The highest flux measured with ROSAT was ~8 × 10^-12 erg s^-1 cm^-2 (0.2-12 keV, using the same spectral model as above), and the two XMM-Newton detections yielded the lowest measured fluxes of 7 × 10^-14 erg s^-1 cm^-2. The high variability (two orders of magnitude) is consistent with a Be/X-ray binary nature.

Schmidtke et al. (1994) proposed an O8e star found within the ROSAT 90% error circle as the optical counterpart. Coe et al. (2001) measured a spectral type of O9Ve (with one spectral subtype uncertainty). After astrometrical correction the XMM-Newton survey observation (with the best statistics) yields a position (J2000) of RA = 05:20:29.8, Dec. = -69:31:55, with a 3σ error radius of 2.9". The proposed optical counterpart is the only bright star consistent with a Be/X-ray binary nature within the improved error circle. Coe et al. (2001) obtained a 24.45 d period from OGLE-II data. A period search using the full light curve from phase II, III, and IV of the OGLE survey resulted in a 24.43 d period, while the average luminosity level was nearly constant for the last 15 years, as shown in the OGLE-IV real time monitoring of X-ray variables.

We thank the Swift team for accepting and scheduling the target of opportunity observations.

 
Date          Exp   XRT count rate     Flux (0.2-12 keV) 
              [s]    [counts s-1]      [10-12 erg s-1 cm-2] 
2013 Jan 13   1657   0.018±0.004       1.67 (1.30 - 2.04) 
2013 Jan 14   1986   0.016±0.003       1.48 (1.21 - 1.76) 
2013 Jan 16   1294   0.022±0.005       2.04 (1.58 - 2.50) 
2013 Jan 17    702   0.030±0.008       2.78 (2.04 - 3.52) 
2013 Jan 18   2355   0.047±0.006       4.35 (3.73 - 4.88)