Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

Accretion flows having low angular momentum and low viscosity can have standing shock waves. These shocks arise because of the presence of multiple s

The well-known black hole candidate (BHC) H 1743-322 exhibited temporal and spectral variabilities during several outbursts. The variation of the accretion rates and flow geometry that change on a daily basis during each of the outbursts can be very well understood using the recent implementation of the two-component advective flow solution of the viscous transonic flow equations as an additive table model in XSPEC. This has dramatically improved our understanding of accretion flow dynamics. Most interestingly, the solution allows us to treat the mass of the BHC as a free parameter and its mass could be estimated from spectral fits. In this paper, we fitted the data of two successive outbursts of H 1743-322 in 2010 and 2011 and studied the evolution of accretion flow parameters, such as two-component (Keplerian and sub-Keplerian) accretion rates, shock location (i.e., size of the Compton cloud), etc. We assume that the model normalization remains the same across the states in both these outbursts. We used this to estimate the mass of the black hole and found that it comes out in the range of . For the sake of comparison, we also estimated mass using the Photon index versus Quasi Periodic Oscillation frequency correlation method, which turns out to be using GRO J1655-40 as a reference source. Combining these two estimates, the most probable mass of the compact object becomes .

One of the major obstacles to accurately modeling the interstellar chemistry is inadequate knowledge of the binding energy (BE) of interstellar species with dust grains. In denser regions of molecular clouds, where very complex chemistry is active, interstellar dust is predominantly covered by H₂O molecules, thus it is essential to know the interaction of gas-phase species with water ice to trace realistic physical and chemical processes. To this end, we consider water (cluster) ice to calculate the BE of several atoms, molecules, and radicals of astrochemical interest. Systematic studies have been carried out to come up with a relatively more accurate BE of astrophysically relevant species on water ice. We increase the size of the water cluster methodically to capture the realistic situation. Sequentially, one, three, four, five, and six water molecules are considered to represent water ice analogs in increasing order of complexity. We note that for most of the species considered here, as we increase the cluster size, our calculated BE value starts to converge toward the experimentally obtained value. More specifically, our computed results with the water c-pentamer (average deviation from experiment ∼±15.8%) and c-hexamer (chair) (average deviation from experiment ∼±16.7%) configurations are found to be nearer to an experimentally obtained value other than the value found for the water clusters we consider.

We present spectral and timing properties of the newly discovered X-ray transient source, MAXI J1535-571, which is believed to be a Galactic X-ray binary containing a black hole candidate (BHC) as the primary object. Since its discovery on 2017 September 2, it has been monitored regularly in multi-wavelength bands by several satellites. We use archival data of the Swift (XRT and BAT) and MAXI (GSC) satellite instruments to study accretion flow dynamics of the source during the outburst. During its outburst, the source became very bright in the sky with a maximum observed flux of 5 Crab in the 2–10 keV GSC band. Similar to other transient BHCs, it also shows signatures of low-frequency quasi-periodic oscillations (QPOs) during the outburst. Spectral data of different instruments are fitted with the transonic flow, solution-based two-component advective flow model fits file to find the direct accretion flow parameters. The evolution of spectral states and their transitions are understood from the model-fitted physical flow parameters and nature of QPOs. We also estimate the probable mass of the black hole from our spectral analysis as 7.9–9.9 M _⊙ or 8.9 ± 1.0 M _⊙.

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics