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Swift XRT observation of 1A 0535+262

ATEL # 2343; Mark Reynolds and Jon Miller (University of Michigan)
on 10 Dec 2009; 5:49 UT
Password Certification: Mark Reynolds (

Subjects: X-ray, Neutron Stars, Pulsars, Transients

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

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