SWIFT J1910.2-0546: A POSSIBLE BLACK HOLE BINARY WITH A RETROGRADE SPIN OR TRUNCATED DISK

• Published in: The Astrophysical journal Vol. 778; no. 2; pp. 1 - 11
• Main Authors: Reis, R C, Reynolds, M T, Miller, J M, Walton, D J, Maitra, D, King, A, Degenaar, N
• Format: Journal Article
• Language: English
• Published: United States 01-12-2013
• Subjects: ACCRETION DISKS; APPROXIMATIONS; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ASYMMETRY; BLACK HOLES; Black holes (astronomy); Circular orbits; Confidence intervals; EMISSION; EVAPORATION; INTERMEDIATE STATE; IRON; LUMINOSITY; MASS TRANSFER; Mathematical models; ORBITS; OSCILLATIONS; PERIODICITY; RELATIVISTIC RANGE; SPIN; STELLAR WINDS; X RADIATION; XMM (spacecraft)
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1088/0004-637X/778/2/155

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 r sub(in) = 9.4 super(+1.7) sub(-1.3)r sub(g) 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/GM super(2) (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.