Swift XRT observation of 1A 0535+262
ATel #2343; Mark Reynolds and Jon Miller (University of Michigan)
on 10 Dec 2009; 05:49 UT
Credential Certification: Mark Reynolds (markrey@umich.edu)
Subjects: X-ray, Neutron Star, Transient, Pulsar
Referred to by ATel #: 2970
The current giant outburst of the Be X-ray pulsar binary 1A 0535+262
(ATel #2324, #2336, #2337) was observed by Swift/XRT in windowed
timing mode for 1ks on 2009-12-08.
As the raw spectrum is piled-up with a count rate of 250 -- 300 ct/s, a
circle with a radius of 10 pixels centered at the source position was
excluded during spectral extraction. A background spectrum was extracted
from a neighbouring region.
Modeling the spectrum as the sum of emission from an accretion disc plus
blackbody modified by interstellar absorption, pha(diskbb+bb), reveals an
unabsorbed 0.6 - 10 keV flux of (2.15 +\- 0.05)e-7 erg/s/cm^2. We note that
this is approximately 0.6 L_Edd for a 1.4 M_sun neutron star at 2 kpc.
The best fitting spectral parameters are as follows:
N_H -- 0.46e22 cm^-2 (fixed)
T_bb -- 1.75 +\- 0.05 keV
norm_bb -- 2.68 +\- 0.09
T_diskbb -- 0.46 +\- 0.03 keV
norm_diskbb -- (5 +\- 2)e4
Addition of a narrow Gaussian line (E_line = 6.40 +\- 0.06 keV; EW = 150 +\-
50 eV) is found to significantly improve the fit. Formally the line is
required at greater than the 5-sigma confidence level as measured by an
F-test. We note that replacing this narrow Gaussian with a relativistic line
(xspec - diskline) provides a statistically equivalent fit.
All errors are quoted at the 90% confidence level.
1A 0535+262 XRT spectrum