Milky Way or nearby Galaxy home to unusual variable X-ray source
A very unusual stellar object has been found that is either located in our own Galaxy or in M31, the Andromeda Galaxy, the closest normal galaxy to us. Undetected in observations with various X-ray telescopes for many years, the 2008 European Space Agency’s X-ray Multi-Mirror Mission (XMM-Newton) survey of M31 suddenly picked it up as a bright X-ray source. In a 2011 Astronomy & Astrophysics paper, the object was identified as a probable black hole low-mass X-ray binary.
A recent publication in The Astrophysical Journal, with lead author Joseph Callingham from the University of Sydney and two CAASTRO co-authors, presents a detailed re-analysis of the object’s spectral and timing properties that might point towards a rare class of Galactic magnetars. Most intriguingly, the source remained elusive in follow-up observations by the research team, using the Swift X-ray Telescope in 2011, and in archival data at different wavelengths (ultra-violet, optical, near-infrared, and radio at several frequencies).
The X-ray variability and lack of an identifiable counterpart place strong constraints on the nature of this source, ruling out emission-stable objects such as cooling neutron stars. The team re-analysed the 2008 XMM-Newton data and tested a number of different models to best fit the spectrum. They found that while a the X-ray spectrum is similar in shape to that typically seen from black hole binaries, other properties were very unusual for a black hole binary, namely a very low accretion disc temperature and a non-variable Compton component. In addition, the location of the source in the periphery of M31 is hard to explain, as such binary systems are usually located in or near globular clusters or star-forming regions. Investigating, therefore, whether the source could be something else than a black hole binary in M31, an equally good fit was achieved assuming the source is a magnetar – which would also account for the observed flux ratio variability and the lack of counterparts – but would suggest that the source belongs to our Milky Way Galaxy. Yet again, the source would be located in a rather unusual place – a fair way out of the galactic plane.
Callingham et al. also discuss the origin of the object and conclude that – assuming it is a magnetar –it was either born in a supernova from a massive progenitor star in the Galactic plane, travelled out and decreased in magnetic field strength, or that it is a run-away progenitor star that separated from a binary system. To clarify, however, whether the object is a magnetar or a black hole X-ray binary after all, we will need to detect spectral state transitions (typical of black hole binaries) or pulsations (typical of magnetars).
J. R. Callingham, S. A. Farrell, B. M. Gaensler, G. F. Lewis, and M. J. Middleton in ApJ 757-169 on 14 September 2012