We present the first results from a long ( 51 \thinspace ks ) XMM-Newton observation of the Galactic X-ray binary SWIFT J1910.2–0546 in a 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 { \it r } _ { in } = 9.4 ^ { +1.7 } _ { -1.3 } { \it r } _ { g } at a 90 % confidence limit .
Quasi-periodic oscillations are also found at approximately 4.5 and 6 \thinspace Hz , which correlates well with the break frequency of the underlying broad-band noise .
Assuming that the line emission traces the ISCO , 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 < { -0.32 ~ { } cJ / GM ^ { 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 .