1A 0535+262

Swift XRT spectrum of the Be X-ray pulsar binary 1A 0535+262 taken 2009 Dec 08. The exposure time is 1ks in windowed timing mode.
The best fit model consists of a blackbody accretion disc plus blackbody plus a narrow Gaussian line at 6.4 keV, pha(diskbb+bb+gaus).

N_H	0.46 (fixed)
T_diskbb	0.46 +/- 0.03 keV
norm_diskbb	(5 +/- 2)e4
T_bb	1.75 +/- 0.05 keV
norm_bb	2.68 +/- 0.09
E_line	6.4 +/- 0.06 keV
EW	150 +/- 50 eV

Astronomers Telegram (#2343)

Swift XRT observation of 1A 0535+262

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.