We describe new optically thin solutions for rotating accretion flows around black holes and neutron stars. These solutions are advection dominated, so that most of the viscously dissipated energy is advected radially with the flow. We model the accreting gas as a two temperature plasma and include cooling by bremsstrahlung, synchrotron, and Comptonization. We obtain electron temperatures $T_e\sim 10^{8.5}-10^{10}$K. The new solutions are present only for mass accretion rates $\dot M$ less than a critical rate $\dot M_{crit}$ which we calculate as a function of radius $R$ and viscosity parameter $\alpha$. For $\dot M<\dot M_{crit}$ we show that there are three equilibrium branches of solutions. One of the branches corresponds to a cool optically thick flow which is the well-known thin disk solution of Shakura \& Sunyaev (1973). Another branch corresponds to a hot optically thin flow, discovered originally by Shapiro, Lightman \& Eardley (1976, SLE). This solution is thermally unstable. The third branch corresponds to our new advection-dominated solution. This solution is hotter and more optically thin than the SLE solution, but is viscously and thermally stable. It is related to the ion torus model of Rees et al. (1982) and may potentially explain the hard X-ray and $\gamma$-ray emission from X-ray binaries and active galactic nuclei.

/pdf/advection-dominated-accretion-underfed-black-holes-and-cc41v8tdhl.pdf

Advection-Dominated Accretion: Underfed Black Holes and Neutron Stars

The ESA observatory INTEGRAL (International Gamma-Ray Astrophysics Laboratory) is dedicated to the fine spectroscopy (2.5 keV FWHM @ 1 MeV) and fine imaging (angular resolution: 12 arcmin FWHM) of celestial gamma-ray sources in the energy range 15 keV to 10 MeV with concurrent source monitoring in the X-ray ($3{-}35$ keV) and optical ( V -band, 550 nm) energy ranges. INTEGRAL carries two main gamma-ray instruments, the spectrometer SPI (Vedrenne et al. [CITE]) – optimized for the high-resolution gamma-ray line spectroscopy (20 keV–8 MeV), and the imager IBIS (Ubertini et al. [CITE]) – optimized for high-angular resolution imaging (15 keV–10 MeV). Two monitors, JEM-X (Lund et al. [CITE]) in the ($3{-}35$) keV X-ray band, and OMC (Mas-Hesse et al. [CITE]) in optical Johnson V -band complement the payload. The ground segment includes the Mission Operations Centre at ESOC, ESA and NASA ground stations, the Science Operations Centre at ESTEC and the Science Data Centre near Geneva. INTEGRAL was launched on 17 October 2002. The observing programme is well underway and sky exposure (until June 2003) reaches ~1800 ks in the Galactic plane. The prospects are excellent for the scientific community to observe the high energy sky using state-of-the-art gamma-ray imaging and spectroscopy. This paper presents a high-level overview of INTEGRAL.

The INTEGRAL Mission

The mathematical simplicity of black holes, combined with their links to some of the most energetic events in the universe, means that black holes are key objects for fundamental physics and astrophysics. Until recently, it was generally believed that black holes in nature appear in two broad mass ranges: stellar-mass (M ∼ 3 − 20 M⊙), which are produced by the core collapse of massive stars, and supermassive (M ∼ 10 6 − 10 10 M⊙), which are found in the centers of galaxies and are produced by a still uncertain combination of processes. In the last few years, however, evidence has accumulated for an intermediate-mass class of black holes, with M ∼ 10 2 − 10 4 M⊙. If such objects exist they have important implications for the dynamics of stellar clusters, the formation of supermassive black holes, and the production and detection of gravitational waves. We review the evidence for intermediate-mass black holes and discuss future observational and theoretical work that will help clarify numerous outstanding questions about these objects.

Intermediate - mass black holes

With the advent of more sensitive all-sky instruments, the transient Universe is being probed in greater depth than ever before. Taking advantage of available resources, we have established a comprehensive database of black hole (and black hole candidate) X-ray binary (BHXB) activity between 1996 and 2015 as revealed by all-sky instruments, scanning surveys, and select narrow-field X-ray instruments aboard the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL), Monitor of All-Sky X-ray Image (MAXI), Rossi X-ray Timing Explorer (RXTE), and Swift telescopes; the Whole-sky Alberta Time-resolved Comprehensive black-Hole Database Of the Galaxy or WATCHDOG. Over the past two decades, we have detected 132 transient outbursts, tracked and classified behavior occurring in 47 transient and 10 persistently accreting BHs, and performed a statistical study on a number of outburst properties across the Galactic population. We find that outbursts undergone by BHXBs that do not reach the thermally dominant accretion state make up a substantial fraction ($\sim$ 40%) of the Galactic transient BHXB outburst sample over the past $\sim20$ years. Our findings suggest that this "hard-only" behavior, observed in transient and persistently accreting BHXBs, is neither a rare nor recent phenomenon and may be indicative of an underlying physical process, relatively common among binary BHs, involving the mass-transfer rate onto the BH remaining at a low level rather than increasing as the outburst evolves. We discuss how the larger number of these "hard-only" outbursts and detected outbursts in general have significant implications for both the luminosity function and mass-transfer history of the Galactic BHXB population.

https://iopscience.iop.org/article/10.3847/0067-0049/222/2/15/pdf

Watchdog: a comprehensive all-sky database of galactic black hole x-ray binaries

OSSE has observed seven transient black hole candidates: GRO J0422+32, GX339-4, GRS 1716-249, GRS 1009-45, 4U 1543-47, GRO J1655-40, and GRS 1915+105. Two gamma-ray spectral states are evident and, based on a limited number of contemporaneous X-ray and gamma-ray observations, these states appear to be correlated with X-ray states. The former three objects show hard spectra below 100 keV (photon number indices Gamma < 2) that are exponentially cut off with folding energy ~100 keV, a spectral form that is consistent with thermal Comptonization. This "breaking gamma-ray state" is the high-energy extension of the X-ray low, hard state. In this state, the majority of the luminosity is above the X-ray band, carried by photons of energy ~100 keV. The latter four objects exhibit a "power-law gamma-ray state" with a relatively soft spectral index (Gamma ~ 2.5-3) and no evidence for a spectral break. For GRO J1655-40, the lower limit on the break energy is 690 keV. GRS 1716-249 exhibits both spectral states, with the power-law state having significantly lower gamma-ray luminosity. The power-law gamma-ray state is associated with the presence of a strong ultrasoft X-ray excess (kT ~ 1 keV), the signature of the X-ray high, soft (or perhaps very high) state. The physical process responsible for the unbroken power law is not well understood, although the spectra are consistent with bulk-motion Comptonization in the convergent accretion flow.

Gamma-Ray Spectral States of Galactic Black Hole Candidates

We investigate the accretion disk geometry in Galactic black hole sources by measuring the time delay between soft and hard X-ray emissions. Similar to the recent discoveries of anticorrelated hard X-ray time lags in Cygnus X-3 and GRS 1915+105, we find that the hard X-rays are anticorrelated with soft X-rays with a significant lag in another source, XTE J1550-564. We also find the existence of pivoting in the model-independent X-ray spectrum during these observations. We investigate time-resolved X-ray spectral parameters and find that the variation in these parameters is consistent with the idea of a truncated accretion disk. The QPO frequency, which is a measure of the size of truncated accretion disks, also changes, indicating that the geometric size of the hard X-ray emitting region changes along with the spectral pivoting and soft X-ray flux. A similar kind of delay is also noticed in 4U 1630-47.

Anticorrelated Hard X-Ray Time Lags in Galactic Black Hole Sources

http://iopscience.iop.org/article/10.3847/0004-6256/151/3/69/pdf

ASAS J083241+2332.4: A New Extreme Low Mass Ratio Overcontact Binary System

We investigate the accretion disk geometry in Galactic black hole sources by measuring the time delay between soft and hard X-ray emissions. Similar to the recent discoveries of anti-correlated hard X-ray time lags in Cyg X-3 and GRS 1915+105, we find that the hard X-rays are anti-correlated with soft X-rays with a significant lag in another source: XTE J1550-564. We also find the existence of pivoting in the model independent X-ray spectrum during these observations. We investigate time-resolved X-ray spectral parameters and find that the variation in these parameters is consistent with the idea of a truncated accretion disk. The QPO frequency, which is a measure of the size of truncated accretion disk, too changes indicating that the geometric size of the hard X-ray emitting region changes along with the spectral pivoting and soft X-ray flux. Similar kind of delay is also noticed in 4U 1630-47.

https://arxiv.org/pdf/astro-ph/0703074.pdf

Anti-correlated hard X-ray time lags in Galactic black hole sources

Multiwavelength observations of Galactic black hole candidate sources indicate a close connection between the accretion disk emission and the jet emission. The recent discovery of an anticorrelated time lag between the soft and hard X-rays in Cygnus X-3 (Choudhury & Rao) constrains the geometric picture of the disk-jet connection into a truncated accretion disk, the truncation radius being quite close to the black hole. Here we report the detection of similar anticorrelated time lag in the superluminal jet source GRS 1915+105. We show the existence of pivoting in the X-ray spectrum during the delayed anticorrelation, and we also find that the quasi-periodic oscillation frequency changes along with the spectral pivoting. We explore theoretical models to understand this phenomenon.

https://iopscience.iop.org/article/10.1086/432675/pdf

Anticorrelated hard x-ray time lag in grs 1915+105: evidence for a truncated accretion disk

Context. The evolution of different types of quasi-periodic oscillations (QPOs) and the coupled radiative/physical changes in the accretion disk are still poorly understood. In a few black hole binaries it was found that fast evolution of QPOs is associated with spectral variations. These studies in other black hole binaries are important to understand the QPO phenomenon. Aims. We study fast QPO transitions and accompanying spectral variations of the black hole transient XTE J1817-330 to investigate the causes of the spectral variation during the QPO transition. Methods. Recently, QPOs in ten RXTE observations of XTE J1817-330 were found. We found that among these, only one observation shows erratic dips in its X-ray light curve. The power density spectra and the corresponding energy spectra were extracted and analyzed for the dip and non-dip sections of the light curve. Results. We found that type-B ∼6 Hz QPO changes into type-A QPO in a few tens of seconds along with a flux decrease. This transient evolution is accompanied by a significant spectral variation. Conclusions. We report a transient QPO feature and accompanying spectral variation in XTE J1817-330. Based on our findings, we discuss the origin of fast evolution of QPOs and spectral variations.

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Kandulapati Sriram

Papers

Anticorrelated Hard X-Ray Time Lags in Galactic Black Hole Sources

ASAS J083241+2332.4: A New Extreme Low Mass Ratio Overcontact Binary System

Anti-correlated hard X-ray time lags in Galactic black hole sources

Anticorrelated hard x-ray time lag in grs 1915+105: evidence for a truncated accretion disk

Fast transition of type-B quasi-periodic oscillation in the black hole transient XTE J1817-330