Multiwavelength Observations of A0620-00 in Quiescence

Abstract

We present contemporaneous X-ray, ultraviolet, optical, near-infrared, and radio observations of the black hole binary system, A0620-00, acquired in 2010 March. Using the Cosmic Origins Spectrograph on the Hubble Space Telescope, we have obtained the first FUV spectrum of A0620-00, as well as NUV observations with STIS. The observed spectrum is flat in the FUV and very faint (with continuum fluxes ≃ 1e − 17 ergs cm −2 s −1Å−1 ). The UV spectra also show strong, broad (FWHM∼2000 km s −1 ) emission lines of Si IV, C IV, He II, Fe II, and Mg II. The C IV doublet is anomalously weak compared to the other lines, which is consistent with the low carbon abundance seen in NIR spectra of the source. Comparison of these observations with previous NUV spectra of A0620-00 show that the UV flux has varied by factors of 2-8 over several years. We compiled the dereddened, broadband spectral energy distribution of A0620-00 and compared it to previous SEDs as well as theoretical models. The SEDs show that the source varies at all wavelengths for which we have multiple samples. Contrary to previous observations, the optical-UV spectrum does not continue to drop to shorter wavelengths, but instead shows a recovery and an increasingly blue spectrum in the FUV. We created an optical-UV spectrum of A0620-00 with the donor star contribution removed. The non-stellar spectrum peaks at ≃3000Å. The peak can be fit with a T=10,000 K blackbody with a small emitting area, probably originating in the hot spot where the accretion

A clean sightline to quiescence: multiwavelength observations of the high Galactic latitude black hole X-ray binary Swift J1357.2−0933

“…Our observations show that the quiescent UV flux is almost two orders of magnitude fainter than the extrapolation of the UV/X-ray correlation to low X-ray fluxes, and the shape of the quiescent UV spectrum is also steeper. An outer UV accretion disk is likely present in quiescence (Hynes & Robinson 2012, also see, e.g., McClintock et al 2003Froning et al 2011, who detect broad UV emission lines in the quiescent UV spectra of both XTE J1118+480 and A0620−00). If the outer accretion disk were to account for all of the UV emission in quiescence, then the observed "inversion" in UV color between the hard state and quiescence would require a rapid decrease in the disk temperature, so that the UV probes the Rayleigh-Jeans tail in the hard state and the Wien tail in quiescence (the UV is very unlikely dominated by thermal emission from the companion star, given the companion's low stellar mass, the lack of observed orbital NIR/optical flux modulations, and the lack of stellar absorption features in the quiescent optical spectrum).…”

Constraints on relativistic jets in quiescent black hole X-ray binaries from broad-band spectral modelling

“…For example, excess UV emission over the expected contribution from the companion star is often detected from quiescent BHXBs (see Hynes & Robinson 2012 and references therein; although also see Hynes et al 2009 who did not find a significant UV excess in the relatively luminous quiescent SED of V404 Cyg). Some of this UV excess is likely thermal radiation from the outer disk, as UV spectroscopy often reveals broad emission lines (including for both J1118 and A0620-00; McClintock et al 2003;Froning et al 2011). However, a caveat is that explaining typical UV excess fluxes purely via thermal blackbody radiation requires a hotter and/or more compact emission region than expected for the outer regions of quiescent accretion flows (e.g., McClintock, Horne & Remillard 1995;McClintock et al 2003;Froning et al 2011;Hynes & Robinson 2012), and complex geometries may be required (e.g., McClintock et al 2003).…”

“…Some of this UV excess is likely thermal radiation from the outer disk, as UV spectroscopy often reveals broad emission lines (including for both J1118 and A0620-00; McClintock et al 2003;Froning et al 2011). However, a caveat is that explaining typical UV excess fluxes purely via thermal blackbody radiation requires a hotter and/or more compact emission region than expected for the outer regions of quiescent accretion flows (e.g., McClintock, Horne & Remillard 1995;McClintock et al 2003;Froning et al 2011;Hynes & Robinson 2012), and complex geometries may be required (e.g., McClintock et al 2003). Our model fits on J1118 suggest that jet-related synchrotron radiation (from a relativistic population of quasithermal electrons in the jet base) could also substantially contribute to the UV waveband and should be taken into account.…”

“…Our model fits on J1118 suggest that jet-related synchrotron radiation (from a relativistic population of quasithermal electrons in the jet base) could also substantially contribute to the UV waveband and should be taken into account. Note that synchrotron radiation from the jet base can also explain the bulk of the UV excess from A0620-00 in quiescence (Gallo et al 2007;Froning et al 2011; although see §4 of Froning et al 2011 for other potential scenarios).…”

“…In particular, additional high-resolution UV spectra (e.g., with COS on HST) would be helpful for constraining the disk contribution to the UV (e.g., Froning et al 2011). Observational constraints on the high-energy cutoff would also be extremely useful, for which coordinated UV and X-ray observations (of unabsorbed systems) could be a promising avenue.…”