X-ray brightening of the super-Eddington black hole X-ray binary P13 in NGC 7793

ATel #9068; Roberto Soria (ICRAR-Curtin University; The University of Sydney), Christian Motch (Strasbourg Observatory), Manfred Pakull (Strasbourg Observatory), Fabien Grise' (Strasbourg Observatory)
on 20 May 2016; 10:54 UT
Credential Certification: Roberto Soria (rsoria@physics.usyd.edu.au)

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The ultraluminous X-ray source (ULX) P13 in the nearby galaxy NGC 7793 (d = 3.7 Mpc) has recently brightened up to its highest historical flux. Our Swift/XRT observations from 2016 May 10 and May 15 show a count rate of about 0.1 ct/s, twice as high as the previously recorded peak. The historical lightcurve can be seen here.

Our analysis of the combined May 10-15 XRT data shows that the spectrum is well fitted by a power-law of photon index Gamma = 1.0 +/- 0.2 and intrinsic NH = 2 (-2 +5) 10^{20} cm^{-2}, with an observed 0.3-10 keV flux = 6.5 10^{-12} erg cm{-2} s^{-1} and unabsorbed luminosity = (1.1 +/- 0.1) 10^{40} erg s^{-1}. The power-law fit is plotted here. A statistically equivalent fit with the Comptonization model comptt gives an electron temperature kTe = 2.0 (-0.5 +1.6) keV, optical depth tau = 13 +/- 4, NH = 3 (-3 +6) 10^{20} cm^{-2}, observed flux = 5.8 10^{-12} erg cm{-2} s^{-1} and unabsorbed luminosity = (1.0 +/- 0.1) 10^{40} erg s^{-1}. The spectral properties are very similar to those found in previous bright epochs (Motch et al. 2014), despite the higher apparent luminosity.

P13 is a key source for our understanding of ULXs because it has a strong upper limit to its black hole mass, M < 15 Msun, and a known binary period, P = 63.4 d (Motch et al. 2014, Nature, 514, 198). Its apparent luminosity is now at least 5 x L_{Edd}. One scenario is that the recent increase in the observed X-ray flux of P13 corresponds to a true increase in its emitted luminosity. Alternatively, we have proposed that the secular variability of the X-ray flux is due to the precession of the accretion disk over a ~7 year timescale. In the latter scenario, the disk was sufficiently edge-on to occult our direct view of the X-ray source between 2011 and 2013; instead, in 2016 (mid-way through the precessional cycle) we are seeing the disk more face-on than in previously recorded epochs. If our scenario is correct, we expect that follow-up observations of P13 during the next 2 or 3 years will show an X-ray flux decline leading to a new occultation. The invariant shape of the X-ray continuum over the years (at epochs when the source is not occulted) is consistent with our viewing-angle interpretation.

P13 is also the ULX with the brightest optical counterpart, a B9 supergiant with V ~ 20.5 mag. The optical O-C diagram shows a ~7-year superorbital period (consistent with disk precession) superposed on the average 63.4-day binary period (Motch et al. in prep.). If the observed X-ray flux increase is due to a higher accretion rate and luminosity, the reprocessed optical flux from the X-ray-illuminated face of the donor star should also have increased. Instead, if the X-ray flux changes are due to disk precession (as we propose), the irradiation of the donor star and therefore also the brightness peak in the 63.4-day lightcurve should not increase.

P13 coordinates: 23 57 51.01, -32 37 26.6 (J2000).