Discovery of a 5.72-d period in the supergiant fast X-ray transient AX J1845.0-0433
AX J1845.0-0433, also known as IGR J18450-0435, is a supergiant fast X-ray transient (SFXT) consisting of a neutron star and an O9.5I supergiant companion.
SFXTs are mostly observed during a low X-ray state with Lx<1033-1034 erg s-1 and fast X-ray transient activity can rarely be seen: often lasting less than a day and being characterized by flares with short time-scales of a few tens of minutes with peak luminosities of ~1036 erg s-1.
It is difficult to identify periodicities in these systems, because outbursts from SFXTs are quite rare during long periods of quiescence. Periodic flaring of these sources is likely to represent the orbital period of the binary system consequently giving more indications about their geometry (Bird A. J. et al., 2009, MNRAS, 393, 11).
Using data from IBIS (18 - 60 keV), two different periodicity tests were performed and a positive signal at 5.72 days was found in both cases. Several new outbursts were identified and the derived orbital period was used to test whether the source has more outbursts at a certain position around the orbit compared to another. We found that a significantly higher proportion of the outbursts occurred in the phase range 0.6-0.7 days (assumed position of periastron), as opposed to apastron, suggesting that the orbit is mildly eccentric. Further analysis confirmed an eccentricity e < 0.37.
The figures (from top left to bottom right) show the Lomb-Scargle periodogram, including confidence levels, the phase folded light curve together with the locations of the identified outbursts and a plot of the number of outbursts occurring at a certain phase around the orbit.
Reference:
Discovering a 5.72 Day Period in the Supergiant Fast X-ray Transient AX J1845.0-0433
M. Goossens et al.
http://arxiv.org/pdf/1307.0709v1.pdf, MNRAS accepted, 2013
