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The X-ray outburst of RX J0520.5-6932 is reaching the Eddington luminosity

ATel #5760; G. Vasilopoulos, R. Sturm, P. Maggi, F. Haberl (MPE)
on 14 Jan 2014; 14:49 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Pierre Maggi (pmaggi@mpe.mpg.de)

Subjects: Optical, X-ray, Binary, Neutron Star, Transient, Pulsar

Referred to by ATel #: 5856

We report on the evolution of the current X-ray outburst of the LMC Be/X-ray binary pulsar RX J0520.5-6932 (see ATel #5673) from our Swift/XRT monitoring. Since the start of the outburst (2013 Dec 28) the luminosity of the source has continued to rise to a maximum of 1.91×10³⁸erg s^-1 (0.3-10 keV band), which is close to or at the Eddington limit for a neutron star. Given the brightness and duration of the event, we classify it as a Type II outburst.
The current spectrum of RX J0520.5-6932 can be best fitted by a power law with a photon index of 1.03. The luminosity evolution of the source, assuming a distance of 50 kpc, is presented in the table below. Luminosities before 2013 Dec 8 were converted from count rate assuming a power-law spectrum with a photon index ~0.7 (as measured on 2013 Dec 10) and an absorption column density of 6.44×10²⁰cm^-2. All other luminosities were derived from individual spectral fits.

The Be-star companion to RX J0520.5-6932 is also undergoing an outburst (see OGLE-IV XROM light curve): Since 2013 Dec 23 it brightened by ~0.1 mag. We encourage optical spectroscopy to measure the Hα line profile during this extraordinary bright phase.

  
Date             XRT count rate      Luminosity (0.3-10 keV)   
                 [counts s^-1]        [10³⁸ erg s^-1]  
2013 Nov 28      0.034±0.007         0.00854 
2013 Nov 30      0.075±0.010         0.0188 
2013 Dec 02      0.133±0.020         0.034 
2013 Dec 04      0.206±0.015         0.056 
2013 Dec 08      0.471±0.025         0.11 
2013 Dec 10      0.494±0.011         0.09 
2013 Dec 12      0.766±0.027         0.16 
2013 Dec 14      1.17±0.04           0.23 
2013 Dec 16      1.57±0.04           0.32 
2013 Dec 18      3.42±0.06           0.70 
2013 Dec 28      3.42±0.06           0.65  
2013 Dec 30      3.08±0.06           0.57  
2014 Jan 01.0    3.04±0.08           0.62  
2014 Jan 01.4    7.894±0.13          1.58  
2014 Jan 03      8.56±0.11           1.59  
2014 Jan 07      9.47±0.10           1.76  
2014 Jan 09      10.7±0.11           1.91  
2014 Jan 11      9.70±0.11           1.67  
2014 Jan 13      11.0±0.11           1.8 
  
 
We thank the Swift team for accepting and scheduling the target of opportunity observations.