ABSTRACT
We present the first results from a long (51 ks) XMM-Newton observation of the Galactic X-ray binary SWIFT J1910.2–0546 in an intermediate state, obtained during its 2012 outburst. A clear, asymmetric iron emission line is observed and physically motivated models are used to fully describe the emission-line profile. Unlike other sources in their intermediate spectral states, the inner accretion disk in SWIFT J1910.2–0546 appears to be truncated, with an inner radius of rin 
rg at a 90% confidence limit. Quasi-periodic oscillations are also found at approximately 4.5 and 6 Hz, which correlates well with the break frequency of the underlying broad-band noise. Assuming that the line emission traces the innermost stable circular orbit, as would generally be expected for an intermediate state, the current observation of SWIFT J1910.2–0546 may offer the best evidence for a possible retrograde stellar mass black hole with a spin parameter a < − 0.32cJ/GM2 (90% confidence). Although this is an intriguing possibility, there are also a number of alternative scenarios which do not require a retrograde spin. For example, the inner accretion disk may be truncated at an unusually high luminosity in this case, potentially suffering frequent evaporation/condensation, or it could instead be persistently evacuated through mass loss in a relativistic jet. Further observations are required to distinguish between these different interpretations.
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Footnotes
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In making the lightcurves, we used background regions centered at RAWX = 4 having a width of 2 pixels as suggested in the XMM-Newton Calibration Technical Note for EPIC-pn fast modes http://xmm2.esac.esa.int/docs/documents/CAL-TN-0083.pdf
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The corresponding values for the power spectrum produced without background are: (i) 0(< 0.7) Hz, (ii)
Hz, (iii) 4.53 ± 0.05(1.1 ± 0.2) Hz, (iv) Hz, and (v) Hz, in excellent agreement with the values presented in the main text. - 9
In making this qualitative figure, we used the Dexter Java application of Demleitner et al. (2001) to estimate the values of QPO and break frequencies from Figure 3 of Wijnands & van der Klis (1999) and superimposed our break frequency, as found from the twice-broken power-law model described above (νb = 1.05 ± 0.27 Hz), together with the first potential harmonic at 4.53 ± 0.07 Hz.
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The emissivity profile under normal Newtonian approximations at r ≫ rg, is expected to be flat in the region directly below the illuminating, isotropic source (the corona), while tending to r−3 when r ≫ h, where the flux received by the disk from the source falls off as the inverse square of the distance with a further factor of 1/r arising from the cosine of the angle projecting the ray normal to the disk plane. See Wilkins & Fabian (2011) for more details on the expected emissivity profiles around black holes.
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Assuming SWIFT J1910.2–0546 is at a distance of 6.9 kpc from the sun, similar to NGC 6712.
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Assuming MBH = 8 M☉, h/r = 0.01 and α = 0.1 results in a viscous time of ∼83 s at 10rg.
