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Register a new userBroadband X-ray Spectra of GX 339-4 and the Geometry of Accreting Black Holes in the Hard State
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John A. Tomsick
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A major question in the study of black hole binaries involves our understanding of the accretion geometry when the sources are in the hard state. In this state, the X-ray energy spectrum is dominated by a hard power-law component and radio observations indicate the presence of a steady and powerful compact jet. Although the common hard state picture is that the accretion disk is truncated, perhaps at hundreds of gravitational radii from the black hole, recent results for the recurrent transient GX 339-4 by Miller and co-workers show evidence for optically thick material very close to the black holes innermost stable circular orbit. That work focused on an observation of GX 339-4 at a luminosity of about 5% of the Eddington limit and used parameters from a relativistic reflection model and the presence of a soft, thermal component as diagnostics. In this work, we use similar diagnostics, but extend the study to lower luminosities (2.3% and 0.8% Ledd) using Swift and RXTE observations of GX 339-4. We detect a thermal component with an inner disk temperature of ~0.2 keV at 2.3% Ledd. We detect broad features due to iron Kalpha that are likely related to reflection of hard X-rays off the optically thick material. If these features are broadened by relativistic effects, they indicate that optically thick material resides within 10 Rg down to 0.8% Ledd, and the measurements are consistent with the inner radius of the disk remaining at ~4 Rg down to this level. However, we also discuss an alternative model for the broadening, and we note that the evolution of the thermal component is not entirely consistent with the constant inner radius interpretation. Finally, we discuss the results in terms of recent theoretical work on the possibility that material may condense to maintain an inner optically thick disk.
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One of the popular models for the low/hard state of Black Hole Binaries is that the standard accretion disk is truncated and the hot inner region produces via Comptonization, the hard X-ray flux. This is supported by the value of the high energy photon index, which is often found to be small $sim$ 1.7 ($<$ 2) implying that the hot medium is seed photons starved. On the other hand, the suggestive presence of a broad relativistic Fe line during the hard state would suggest that the accretion disk is not truncated but extends all the way to the inner most stable circle orbit. In such a case, it is a puzzle why the hot medium would remain photon starved. The broad Fe line should be accompanied by a broad smeared reflection hump at $sim$ 30 keV and it may be that this additional component makes the spectrum hard and the intrinsic photon index is larger, i.e. $>$ 2. This would mean that the medium is not photon deficient, reconciling the presence of a broad Fe line in the observed hard state. To test this hypothesis, we have analyzed the RXTE observations of GX 339-4 from the four outbursts during 2002-2011 and identify the observations when the system was in the hard state and showed a broad Fe line. We have then attempted to fit these observations with models, which include smeared reflection to understand whether the intrinsic photon index can indeed be large. We find that, while for some observations the inclusion of reflection does increase the photon index, there are hard state observations with broad Fe line that have photon indices less than 2.
We investigate variability of optical and near-infrared light curves of the X-ray binary GX 339-4 on a timescale of days. We use the data in four filters from six intervals corresponding to the soft state and from four intervals corresponding to the quiescent state. In the soft state, we find prominent oscillations with the average period P = 1.772 $pm$ 0.003 d, which is offset from the measured orbital period of the system by 0.7 per cent. We suggest that the measured periodicity originates from the superhumps. In line with this interpretation we find no periodicity in the quiescent state. The obtained period excess $epsilon$ is below typical values found for cataclysmic variables for the same mass ratio of the binary. We discuss implications of this finding in the context of the superhump theory.
X-ray and near-infrared ($J$-$H$-$K_{rm s}$) observations of the Galactic black hole binary GX 339--4 in the low/hard state were performed with Suzaku and IRSF in 2009 March. The spectrum in the 0.5--300 keV band is dominated by thermal Comptonization of multicolor disk photons, with a small contribution from a direct disk component, indicating that the inner disk is almost fully covered by hot corona with an electron temperature of $approx$175 keV. The Comptonizing corona has at least two optical depths, $tau approx 1,0.4$. Analysis of the iron-K line profile yields an inner disk radius of $(13.3^{+6.4}_{-6.0}) R_{rm g}$ ($R_{rm g} $ represents the gravitational radius $GM/c^2$), with the best-fit inclination angle of $approx50^circ$. This radius is consistent with that estimated from the continuum fit by assuming the conservation of photon numbers in Comptonization. Our results suggest that the standard disk of GX 339--4 is likely truncated before reaching the innermost stable circular orbit (for a non rotating black hole) in the low/hard state at $sim$1% of the Eddington luminosity. The one-day averaged near-infrared light curves are found to be correlated with hard X-ray flux with $F_{rm Ks} propto F_{rm X}^{0.45}$. The flatter near infrared $ u F_{ u}$ spectrum than the radio one suggests that the optically thin synchrotron radiation from the compact jets dominates the near-infrared flux. Based on a simple analysis, we estimate the magnetic field and size of the jet base to be $5times10^4$ G and $6times 10^8$ cm, respectively. The synchrotron self Compton component is estimated to be approximately 0.4% of the total X-ray flux.
We report BeppoSAX and optical observations of the black hole candidate GX 339-4 during its X-ray `off state in 1999. The broad-band (0.8-50 keV) X-ray emission can be fitted by a single power law with spectral index, alpha ~1.6. The observed luminosity is 6.6e33 erg s^{-1} in the 0.5-10 keV band, which is at the higher end of the flux distribution of black hole soft X-ray transients in quiescence, comparable to that seen in GS 2023+338 and 4U 1630-47. An optical observation just before the BeppoSAX observation shows the source to be very faint at these wavelengths as well (B=20.1, V=19.2). By comparing with previously reported `off and low states (LS), we conclude that the `off state is actually an extension of the LS, i.e. a LS at lower intensities. We propose that accretion models such as the advection-dominated accretion flows are able to explain the observed properties in such a state.
Black hole X-ray binaries show signs of non-thermal emission in the optical/near-infrared range. We analyze the optical/near-infrared SMARTS data on GX339$-$4 over the 2002--2011 period. Using the soft state data, we estimate the interstellar extinction towards the source and characteristic color temperatures of the accretion disk. We show that various spectral states of regular outbursts occupy similar regions on the color-magnitude diagrams, and that transitions between the states proceed along the same tracks despite substantial differences in the observed light curves morphology. We determine the typical duration of the hard-to-soft and soft-to-hard state transitions and the hard state at the decaying stage of the outburst to be one, two and four weeks, respectively. We find that the failed outbursts cannot be easily distinguished from the regular ones at their early stages, but if the source reaches 16 mag in $V$-band, it will transit to the soft state. By subtracting the contribution of the accretion disk, we obtain the spectra of the non-thermal component, which have constant, nearly flat shape during the transitions between the hard and soft states. In contrast to the slowly evolving non-thermal component seen at optical and near-infrared wavelengths, the mid-infrared spectrum is strongly variable on short timescales and sometimes shows a prominent excess with a cutoff below $10^{14}$ Hz. We show that the radio to optical spectrum can be modeled using three components corresponding to the jet, hot flow and irradiated accretion disk.
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