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Swift Detection of Ionized X-ray Absorption in MAXI J1305-704

ATel #4070; J. M. Miller (Michigan), A. Beardmore (Leicester), J. Kennea (Swift/PSU), M. T. Reynolds (Michigan), A. L. King (Michigan), A. C. Fabian (Cambridge), C. S. Reynolds (Maryland), J. Raymond (SAO)
on 26 Apr 2012; 18:38 UT
Credential Certification: Jon Miller (jonmm@umich.edu)

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

Referred to by ATel #: 4071, 4073, 4191

We report on Swift/XRT observations of the new X-ray transient MAXI J1305-704, obtained on 2012 April 19 at 13:15:58 UT (7.5 ksec total integration). The source shows dipping behavior that may indicate a very short orbital period (ATEL #4044, #4060). The source has also recently been detected in radio observations (ATEL #4067).

The XRT light curve of this observation also shows dipping behavior, but the dips are short and spectroscopy of the dip-only spectrum was not attempted. We instead fit the time-averaged spectrum in the 0.5-10.0 keV band using simple continua. An absorbed (N_H = 2 E+21) disk blackbody (kT = 1.03+/-0.01 keV) plus power-law (Gamma = 3.53+/-0.04) model characterizes the flux well (unabsorbed flux = 1.3 E-9 erg/cm^2/s), but does not give an acceptable fit.

The fit fails at low energy, in the 0.5-1.2 keV band in particular, perhaps suggesting complex ionized absorption. This is supported by the detection of an absorption line in the Fe K band. Adding a Gaussian, the line is found at E = 6.67+/-0.02 keV, likely corresponding to He-like Fe XXV. The line is not resolved and it is consistent with zero width. A line flux of -2.1+/-0.3 E-4 ph/cm^2/s is measured, corresponding to an equivalent width of 11 +/- 2 eV.

Applying a grid of XSTAR models developed for IGR J17091-3624 (King et al. 2012), it is possible to account for the Fe K absorption line and some (but not all) of the absorption structure near to 1 keV. This model suggests an ionized column of N = 3-4 E+22 cm^-2, log(xi) = 2.8, and cannot constrain a velocity shift.

For a plot of the absorption spectrum, please click here.

References:

King, A. L., et al., 2012, ApJ, 746, L20