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
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