The orbital inclination of A0620 - 00 measured polarimetrically

Abstract

The mass of the degenerate primary in A0620-00 is inferred from its spectroscopic mass function to be > 3.2 making it an excellent candidate for a black hole. The exact value of the mass depends on the orbital inclination. The inclination of a binary system can be determined from the shape of its Stokes parameter light curves if the linear polarization of the system varies as a function of orbital phase. We observed A0620-00 over one 8-hour binary period with the 4.5-m equivalent MMT. Its polarization in the visible (5000 A-9000 A) is variable with orbital phase. Using the standard theory of Brown, McLean, and Emslie (1978), we derive an orbital inclination of i = 57° (+20°, -50°), where the error is the 90% confidence interval derived by the method of Simmons, . The large uncertainty in the derived value of the inclination is directly related to the statistical precision of the observations, which is in turn limited by the relative faintness of the source, i = 57° corresponds to a mass of the compact primary of 6.6 but the large uncertainty in our measured value of the inclination allows the derived mass of A0620-00 to be as low as 3.8 SJ£ 0 . If 5 337 0 * s taken to be the maximum mass of any degenerate configuration consistent with general relativity except a black hole, then the mass of A0620-00 is still not well-enough determined to conclude that it must be a black hole.

Polarized synchrotron emission in quiescent black hole X-ray transients

“…4b). The quiescent optical (Dolan & Tapia 1989;Dubus et al 2008) and NIR (LIRIS) FLP clearly lie above the relation for interstellar dust, and so must have a different origin.…”

“…Dolan & Tapia (1989) found that the optical (white light) FLP varied with the orbital phase in quiescence, with values of FLP ∼ 1.6-3.2 per cent. Dubus et al (2008) also measured FLP = 3.08 ± 0.04 per cent in V -band during quiescence but did not detect the orbital modulation.…”

“…4, the FLP (panel (a)) and PA (panel (b)) of A0620-00 are shown as a function of frequency from optical (literature data) to NIR (our LIRIS data). The phase-averaged quiescent optical FLP (from Dolan & Tapia 1989;Dubus et al 2008) are shown, with the dotted lines with arrows indicating the range of FLP values measured over the orbital period by Dolan & Tapia (1989). Optical FLP measurements from the 1975 outburst (Dolan 1976) are also included, as are NIR (LIRIS) and optical (Dubus et al 2008) FLP measurements of field stars close to A0620-00, for comparison.…”

“…The optical FLP is likely to be due to Thomson scattering (e.g. Brown, McLean, & Emslie 1978;Bochkarev et al 1979) within the binary system, since it has been seen to vary on the orbital period (Dolan & Tapia 1989). For such scattering the FLP is expected to be either constant with wavelength, or slightly decreasing from optical towards NIR wavelengths (e.g.…”

“…4d) describes well the change in PA. Scattering is therefore a plausible origin to the optical and some of the NIR polarization, and can account for the FLP and PA in all bands except the H and KS-bands. Since the optical FLP can vary with phase of the orbit (Dolan & Tapia 1989), one possible reason that the NIR FLP is lower than the optical FLP is that the NIR data could have been acquired at a specific phase of the orbit. The optical quiescent data of A0620-00 in Fig.…”