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Register a new userLong-term Multi-wavelength Observations of GRS 1758-258 and the ADAF Model
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John W. Keck
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We present a long-term multi-wavelength light curve of Galactic black hole candidate GRS 1758-258 by combining previously published and archival data from GRANAT, ROSAT, CGRO, RXTE, SAX, ASCA, EXOSAT, and the VLA. In addition we include first spectral results from the balloon-borne Gamma-ray Arcminute Telescope Imaging System (GRATIS). In light of divergent analyses of the 1991-1993 ROSAT observations, we have re-analyzed these data; we find the soft X-rays track the hard X-rays, and that the fits require no black-body component-- indicating that GRS 1758-258 did not go to the high state in 1993. We offer an interpretation based on the ADAF model for a system with mdot ltsim mdot_crit. We find the 1990-1993 coeval hard and soft X-ray observations support the ADAF predictions. We discuss a new way to constrain black-hole mass with spectral data and the ADAF theory, and apply this technique to GRS 1758-258 to find M_1 gtsim 8--9 M_sol at an assumed distance of 8.5 kpc. Further investigations of the ADAF model allow us to evaluate the model critically against the data and flux-flux diagram of Barret, McClintock & Grindlay (1996) and to understand the limits of the latters ``X-ray burster box.
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The results of GRANAT/SIGMA hard X-ray observations of GRS 1758-258 in 1990-1998 are presented. The source lies at ~5arcdeg from the Galactic Center and was within the SIGMA field of view during the GRANAT surveys of this region. The total exposure time of the Galactic Center was 11x10^6 s. The regular SIGMA observations revealed strong variability of the source: the 40-150 keV flux varied at least by a factor of 8 on a time scale of a year, between less than 13 mCrab and ~100-110 mCrab. The average flux was ~60 mCrab in 1990-1998. The sources spectrum is well fitted by a power law with a photon index alpha ~1.86 in the energy range 40 to 150 keV and becomes steeper at energies above ~100 keV. The radio and hard X-ray properties of GRS 1758-258 are similar to those of another Galactic Center source, 1E1740.7-2942. GRS 1758-258 and 1E1740.7-2942 are the two brightest hard X-ray sources in the Galactic Center region. Both sources have radio jets, similar X-ray luminosities (~10^37 erg/s), and spectra, and exhibit variations in the hard X-ray flux on long times scales by a factor of ~10 or more . In contrast to most of the known black hole candidates, which are X-ray transients, GRS 1758-258 and 1E1740.7-2942 were detected by SIGMA during most of the observations in 1990-1998. Assuming that this behavior of the sources implies the suppression of accretion-disk instability in the region of partial hydrogen ionization through X-ray heating, we impose constraints on the mass of the optical companion and on the orbital period of the binary system.
The family links between radio galaxies and microquasars have been strongly strengthened thanks to a new common phenomenon: the presence of extended winged features. The first detection of such structures in a Galactic microquasar, recently reported in Nature Communications (http://rdcu.be/zgX8), widens the already known analogy between both kinds of outflow sources (Marti et al. 2017). This observational result also has potential implications affecting the black hole merger scenarios that contribute to the gravitational wave background.
We report the results of a deep search for the optical and near infrared counterpart of the microquasar source GRS1758-258. At least two possible candidate counterparts of the binary star companion have been recognized on the basis of astrometric coincidence to within 1. Our photometric study shows that the brightest of them would be consistent with a K-type giant star, while the weakest one would be a main sequence F companion. Follow up spectroscopic observations in the near infrared H and K-bands have failed so far to provide evidence for emission lines that may support an unambiguous identification. However, the proximity of these two sources to the sub-arcsec VLA radio position of GRS 1758-258 makes them deserving further attention in the future.
Context. Understood to be a microquasar in the Galactic center region, GRS 1758-258 has not yet been unambiguously identified to have an optical/near-infrared counterpart, mainly because of the high absorption and the historic lack of suitable astrometric stars, which led to the use of secondary astrometric solutions. Although it is considered with 1E 1740.7-2942 as the prototypical microquasar in the Galactic center region, the Galactic origin of both sources has not yet been confirmed. Aims. We attempt to improve previous astrometry to identify a candidate counterpart to GRS 1758-258. We present observations with the Gran Telescopio de Canarias (GTC), in which we try to detect any powerful emission lines that would infer an extragalactic origin of this source. Methods. We use modern star catalogues to reanalyze archival images of the GRS 1758-258 field in the optical and near-infrared wavelengths, and compute a new astrometric solution. We also reanalyzed archival radio data of GRS 1758-258 to determine a new and more accurate radio position. Results. Our improved astrometric solution for the GRS 1758-258 field represents a significant advancement on previous works and allows us to identify a single optical/near-infrared source, which we propose as the counterpart of GRS 1758-258. The GTC spectrum of this source is however of low signal-to-noise ratio and does not rule out a Galactic origin. Hence, new spectral observations are required to confirm or discard a Galactic nature.
We present deep infrared ($2.2 mu$m) imaging of the Galactic microquasars 1E1740-2942 and GRS 1758-258 using the Keck-I 10-meter telescope in June 1998. The observations were taken under excellent seeing conditions ($sim 0.45 arcsec$ full-width half-maximum), making them exceptionally deep for these crowded fields. We used the USNO-A2.0 catalog to astrometrically calibrate the infrared images (along with an optical CCD image in the case of GRS 1758-258), providing independent frame ties to the known radio positions of the objects. For 1E1740-2942, we confirm potential candidates for the microquasar previously identified by Marti et al., and show that none of the objects near the microquasar have varied significantly from 1998 to 1999. For GRS 1758-258, our astrometry indicates a position shifted from previous reports of candidates for the microquasar. We find no candidates inside our 90% confidence radius to a $2 sigma$ limiting magnitude of $K_s = 20.3$ mag. We discuss the implications of these results for the nature of the microquasar binary systems.
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