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Chandra and Optical Identification of AX J1841.0-0536

ATel #289; J. P. Halpern, E. V. Gotthelf, D. J. Helfand, S. Gezari (Columbia U.), G. A. Wegner (Dartmouth)
on 10 Jun 2004; 15:35 UT
Credential Certification: Jules Halpern (jules@astro.columbia.edu)

Subjects: Infra-Red, Optical, X-ray, Binary, Neutron Star, Transient, Pulsar

Referred to by ATel #: 341, 982, 983

We observed the field of the transient 4.74 s X-ray pulsar AX J1841.0-0536, suspected to be a Be/X-ray binary [Bamba et al., PASJ, 53, 1179 (2001)], with the Chandra ACIS arrays for 20 ks on 2004 May 12. A bright, variable source was detected off-axis on the I3 CCD at (J2000) 18h41m0s.54, -05°35'46".8. Its spectrum can be fitted with a power law of photon index Γ = 1.35±0.30, column density NH = (6.1 ± 1.0)×1022 cm-2, and flux F(0.5-10 keV) = 4.2×10-12 ergs cm-2 s-1. These parameters are all within the range measured by Bamba et al. for AX J1841.0-0536.

An optical/IR candidate was selected by positional coincidence from the digitized UK Schmidt plates and 2MASS, at coordinates (J2000) 18h41m0s.43, -05°35'46".5. Its magnitudes from the USNO B1.0 and 2MASS catalogs are B=15.91, R=12.78, I=10.90, J= 9.74, H=9.22, K= 8.93. Spectra of this star obtained on the 2.4m Hiltner Telescope of the MDM Observatory on 2004 May 27 and June 9-10 show He I and Balmer absorption typical of a B star, as well as interstellar Na I D absorption and several of the diffuse interstellar bands. In addition, a weak, double-peaked Hα emission line is seen, confirming the Be star hypothesis. The brightness of the optical counterpart and the strengths of its interstellar absorption features indicate that most of the X-ray measured NH must be due to absorbing material intrinsic to the X-ray binary.