A search for X-ray evidence of a compact companion to the unusual Wolf-Rayet star HD 50896 (EZ CMa)

Details A search for X-ray evidence of a compact companion to the unusual Wolf-Rayet star HD 50896 (EZ CMa) A search for X-ray evidence of a compact companion to the unusual Wolf-Rayet star HD 50896 (EZ CMa)

Jan 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998newa....3...37s&link_type=abstract

New Astronomy, vol. 3, no. 1, p. 37-49.

Astronomy and Astrophysics

Astronomy

15

Stars: Individual: Hd 50896 (Ez Cma), Stars: Wolf-Rayet, X-Rays: Stars

Scientific paper

We analyze results of a ~=25 ksec ASCA X-ray observation of the unusual Wolf-Rayet star HD 50896 (= EZ CMa). This WN5 star shows optical and ultraviolet variability at a 3.766 day period, which has been interpreted as a possible signature of a compact companion. Our objective was to search for evidence of hard X-rays ( >= 5 keV) which could be present if the WN5 wind is accreting onto a compact object. The ASCA spectra are dominated by emission below 5 keV and show no significant emission in the harder 5-10 keV range. Weak emission lines are present, and the X-rays arise in an optically thin plasma which spans a range of temperatures from <= 0.4 keV up to at least ~=2 keV. Excess X-ray absorption above the interstellar value is present, but the column density is no larger than N_H ~ 10^22 cm^-2. The absorption-corrected X-ray luminosity L_x (0.5-10 keV)=10^32.85 erg s^-1 gives L_x/L_bol~=10^-6, a value that is typical of WN stars. No X-ray variability was detected.Our main conclusion is that the X-ray properties of HD 50896 are inconsistent with the behavior expected for wind accretion onto a neutron star or black hole companion. Alternative models based on wind shocks can explain most aspects of the X-ray behavior, and we argue that the hotter plasma near ~ 2 keV could be due to the WR wind shocking onto a normal (nondegenerate) companion.

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