http://arxiv.org/abs/1706.09511
We obtained new optical observations of the X-ray source XMMU J083850.38-282756.8, the previously proposed counterpart of the gamma-ray source 3FGL J0838.8-2829. Time-series photometry in the $r’$ band reveals periodic modulation of $\approx1$ magnitude that is characteristic of the heating of the photosphere of a low-mass companion star by a compact object. The measured orbital period is 5.14817$\pm$0.00012 hr. The shape of the light curve is variable, evidently due to the effects of flaring and asymmetric heating. Spectroscopy reveals a companion of type M1 or later, having a radial velocity amplitude of 315$\pm$17 km/s, with period and phasing consistent with the heating interpretation. The mass function of the compact object is 0.69$\pm$0.11 $M_{\odot}$, which allows a neutron star in a high-inclination orbit. Variable, broad H$\alpha$ emission is seen that is probably associated with a wind from the companion. These properties, as well as the X-ray and gamma-ray luminosities at the inferred distance of <1.7 kpc, are consistent with a redback millisecond pulsar in its non-accreting state. A search for radio pulsations is needed to confirm this interpretation and derive complete system parameters for modeling, although absorption by the ionized wind could hinder such detection.
J. Halpern, J. Strader and M. Li
Fri, 30 Jun 17
36/65
Comments: 6 pages, 4 figures, 3 tables, accepted for publication in ApJ
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