A mass of less than 15 solar masses for the black hole in an ultraluminous X-ray source

Abstract

Most ultraluminous X-ray sources 1 (ULXs) display a typical set of properties not seen in Galactic stellar-mass black holes (BHs): higher luminosity (L x > 3 × 10 39 erg s −1 ), unusually soft X-ray components (kT 0.3 keV) and a characteristic downturn 2,3 in their spectra above ≈ 5 keV. Such puzzling properties have been interpreted either as evidence of intermediate-mass BHs 4,5 , or as emission from stellar-mass BHs accreting above their Eddington limit 6,7 , analogous to some Galactic BHs at peak luminosity 8,9 . Recently, a very soft X-ray spectrum has been observed in a rare and transient stellar-mass BH 10 .Here we show that the X-ray source P13 in the galaxy NGC 7793 11 is in a ≈ 64 day period binary and exhibits all three canonical properties of ULXs. By modelling the strong optical and UV modulations due to X-ray heating of the B9Ia donor star, we constrain the BH mass to less than 15 solar masses. Our results demonstrate that in P13, soft thermal emission and spectral curvature are indeed signatures of supercritical accretion. By analogy, ULXs with similar X-ray spectra and luminosities of up to a few 10 40 erg s −1 can be explained by supercritical accretion onto massive stellar BHs.We organised an X-ray, UV and optical spectrophotometric monitoring programme Stacking Swift low state data reveals the source at L X (0.3-10 keV) = 5 ± 1 × 10 37 erg s −1 (90% confidence level), a factor 100 less than in the previously seen bright X-ray state. Scheduled and serendipitous Chandra and XMM-Newton observations carried out in 2011 and 2012 detected the source at the same low X-ray luminosity and will be reported elsewhere.Optical spectra point at a B9I spectral type (Fig. 1). In addition to high-order Balmer absorption lines, the spectrum exhibits Balmer emission up to at least Hγ as well as and V (λ central = 5500Å; ∆ V = 0.5 mag) light-curves and the behaviour of the V -I colour index point at a strong X-ray heating effect of the supergiant star hemisphere facing the compact companion. An X-ray source with a tenth of the nominal X-ray luminosity would brighten the star by only ≈ 0.1 magnitude in V at maximum light and would have basically no effect in the faint X-ray state. Therefore, we conclude that in 2011 part of the companion star photosphere continues to be illuminated by a luminous X-ray source which is however shielded from our view. The Galactic X-ray binary Her X-1 exhibits similar bright/faint X-ray states 18 as well as periodic phase shifts of photometric maxima 19 . By analogy, we suggest that a tilted precessing accretion disk is at the origin of both the X-ray bright & faint states and of the phase jitter of optical maximum light.In order to constrain the geometry and dynamics of the system we simultaneously fitted the V and UVOT u light-curves using the Eclipsing Light Curve code 20 (ELC). We tested four X-ray luminosity levels ranging from 0.7 up to 2 times a nominal value of 4.2 × 10 39 erg s −1 (derived from the diskbb + comptt fit to the Chandra spectrum extrapolated to the 0.3-20 keV r...

An ultraluminous nascent millisecond pulsar

“…For instance, the recently reported 3.3 Hz and 5 Hz frequencies in the X-ray flux of M82 X-1 (Pasham, Strohmayer & Mushotzky 2014) could in fact turn out to be harmonics of 1.67 Hz, i.e., that source may be a P = 0.6 s pulsar. Another ULX, in NGC 7793, has an upper limit to the mass of the compact object of < 15M⊙, with allowed solutions in the neutron star mass range and phenomenology of state transitions similar to that of Her X-1 (Motch et al 2014).…”

Two Eclipsing Ultraluminous X-Ray Sources in M51

“…Outside the Local Group, NGC 300 X-1 ( »Ĺ 5 10 X 38 erg s −1 ; binary period »33 hr) shows X-ray dips, consistent with occultation from geometrically thick structures in the outer disk, or absorption in the wind of the donor star, but not with true eclipses (Binder et al 2015). A strong candidate for a true eclipse is the sharp dip in the Swift/X-ray Telescope flux recorded once from the ULX P13 in NGC 7793, at an orbital phase consistent with the inferior conjunction of its supergiant donor star (Motch et al 2014). However, there is no further confirmation of that single monitoring data point at subsequent epochs.…”

“…Modeling of the optical light curve from the irradiated donor star in NGC 7793-P13 showed (Motch et al 2014) that the source is viewed at an angle >20°and more likely much higher. Thus, in that case, super-Eddington accretion is the reason for the high luminosity, not a heavier BH or a down-the-funnel view.…”

Revisiting the dynamical case for a massive black hole in IC10 X-1

“…Another class of objects thought to contain stellar-mass BHs are the ULX sources (Liu et al 2013, Motch et al 2014. Their X-ray luminosities (10 39 −10 41 erg s −1 ) exceed the Eddington limit for a NS by 1 − 3 orders of magnitude.…”