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On the absorption dips in MAXI J1305-704

ATel #4073; E. Kuulkers (ESA/ESAC, Spain), S. Motta, T. M. Belloni (INAF/Osservatorio Astronomico di Brera, Italy)
on 27 Apr 2012; 20:18 UT
Credential Certification: Erik Kuulkers (ekuulker@rssd.esa.int)

Subjects: X-ray, Binary, Black Hole, Neutron Star, Transient, Variables

Referred to by ATel #: 4105

The recently discovered X-ray transient MAXI J1305-704 (ATel #4024) has shown clear absorption dips in Swift/XRT data (ATels #4044, #4060, #4070), pointing to a high inclination of the system (ATel #4071). During the dips spectral hardening is observed (ATel #4044). A strong flux dip lasting ~100 sec was reported from Swift/XRT data taken on April 25, with a rapid ingress and egress, suggestive of an eclipse (ATel #4071).

We investigated all Swift/XRT data taken between April 11 and 25. During the absorption dips in observations from 11-23 April, the hardness (as defined by the ratio of the counts in the 3-10 keV and the counts in the 0.3-3 keV bands) indeed increases. We find that the deeper the dip intensity drops, the harder the spectrum becomes. The behaviour seen during April 25 is similar to that seen during the other dip observations, i.e., during the strong flux dip the spectrum becomes harder, being hardest in the middle of the dip. Comparing the hardness versus the 0.3-10 keV intensity during dipping periods and the strong flux dip period, we find that all the dips are part of the same phenomenon. Moreover (see also the picture of the light curve referred to in ATel #4071), before and after the strong flux dip shallower dips are seen, as well as an overall depression of the flux around the strong flux dip. We, therefore, conclude, that the strong flux dip seen on April 25 is not an eclipse, but part of a longer and deeper dipping activity. One can compare the dipping activity with that seen in, e.g., the high-inclination neutron star X-ray binary EXO 0748-676. For example, in Homan et al. (2003, A&A 412, 799) light curves and hardness curves are shown with comparable energy bands. The dipping activity behaviour of MAXI J1305-704 has similarities to that seen during the dipping activity of EXO 0748-676 in their observation 4. During the eclipse of EXO 0748-676, however, no strong hardness variations are seen.

A possible period of ~1.5 hrs has been suggested, based on the recurrence of the dipping activity in 5 consecutive Swift satellite orbits (ATel #4044). It was, however, noted that this periodicity is close to the Swift satellite orbital period. Moreover, a Lomb-Scargle (LS) period search revealed a strongest peak at ~2.7 hrs. We performed a similar search, as well as a phase dispersion minimization (PDM) search, on all Swift/XRT data currently available. The second highest peak in the LS periodogram is near 1.57 hrs, but this can indeed be attributed to the Swift satellite orbit. No peak is found near ~2.7 hrs. The PDM periodogram does not show conclusive evidence for any of the above reported periods. We conclude that no robust period can be found in the Swift/XRT data, casting doubt on a possible short orbital period of the system.