Ultraviolet observations of the transient X-ray sources A0535 + 26 and A0620-00

Abstract

and March 1976 when it was at the X-ray low state. Fifty-eight early-type stars which lie within 10° of HDE 245770 were also observed by the ANS, and these stars were used to derive a localized extinction curve. From the depth of the 2200 Á interstellar absorption feature observed in the ANS flux distribution of HDE 245770 and the shape of the localized extinction curve, an E(B -V) = 0.72 is derived for HDE 245770. This system was observed during its X-ray flare-up (October 1980) and quiescent (November 1981) states by the IUE. The fine error sensor on board the IUE detected an increase in the optical brightness of HDE 245770 during the X-ray outburst. This greatly strengthens the identification of HDE 245770 as the optical counterpart of A0535-I-26. There was no significant change in the absorption-line profiles during the X-ray high and low periods indicating that there was no significant increase in the stellar wind density when the X-ray flare-up occurred. The UV continuum level increased by < 5% when it was near peak X-ray luminosity. The UV spectrum of HDE 245770 indicates that it is a B0 V or a B0 III star with its dereddened continuum slope slightly in favor of being a giant. A0620-00 was observed by the ANS in March 1976 when its X-ray and optical brightness were rapidly decaying. Twenty-five early-type stars were used to establish a localized extinction curve and an E(B -V) = 0.35 was derived by using the ANS data obtained by Wu et al. when the system was near X-ray and optical maximum. The dereddened UV and optical continuum shape for the 1976 March period can be described by a combination of 60% T = 18,000 K blackbody and 40% F v = constant spectra.

What is the closest black hole to the Sun?

“…To estimate the extinction toward A0620-00 Shahbaz et al (1994) use an estimation of the color excess E(B − V) = 0.35 from Wu et al (1983). But Wu and collaborators actually quote their own results obtained a few years earlier: Wu et al (1976).…”

Optical spectroscopy of flares from the black hole X-ray transient A0620-00 in quiescence

“…A third‐order polynomial fit to the continuum of the spectrophotometric standard was used to remove the large‐scale variation of the instrumental response and the slit‐loss corrections were calculated using the comparison star spectra. Finally, we use the colour excess of E ( B − V ) = 0.35 ± 0.02 (Wu et al 1983) and the extinction law of Seaton (1979) to correct the spectra of A0620–00 for interstellar reddening assuming A V / E ( B − V ) = 3.1. This is the starting point for the analysis of the spectra presented hereafter.…”

On the outburst amplitude of the soft X-ray transients

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