The Underluminous Nature of Sgr A* Available to Purchase

In the last several years, a number of observing campaigns of the massive black hole Sgr A* has been carried out in order to address two important issues: one concerns the underluminous nature of Sgr A* with its bolometric luminosity being several orders of magnitude less than those of its more massive counterparts. It turns out that the angular momentum of the ionized stellar winds from orbiting stars in one or two disks orbiting Sgr A* could be a critical factor in estimating accurately the accretion rate unto Sgr A*. A net angular momentum of ionized gas feeding Sgr A* could lower the Bondi rate. Furthermore, the recent time delay picture of the peak flare emission can be understood in the context of adiabatic expansion of hot plasma. The expansion speed of the plasma is estimated to be sub‐relativistic. However, relativistic bulk motion of the plasma could lead to outflow from Sgr A*. Significant outflow from Sgr A* could then act as a feedback which could then reduce Bondi accretion rate. These uncertain factors can in part explain the underluminous nature of Sgr A*. The other issue is related to the emission mechanism and the cause of flare activity in different wavelength bands. The emission mechanism of flaring in the IR is now recognized to be due to synchrotron emission dominating the variable and quiescent emissions from Sgr A* in all wavelength bands with the possible exception of the X‐ray emission. Modeling of X‐ray and near‐IR flares suggests that inverse Compton scattering (ICS) of IR flare photons by the energetic electrons responsible for the submm emission can account for for the X‐ray flares. A time delay of minutes to tens of minutes is predicted between the peak flaring in the near‐IR and X‐rays, not due to adiabatic expansion of optically thick hot plasma, but to the time taken for IR flare photons to cross the accretion flow before being upscattered. Indeed, the observed X‐ray fluxes and lack of detected time delays place significant restrictions on the electron density and temperature profile in the inner accretion flow. These constraints are tightened if the X‐ray flares are instead produced by other mechanisms, such as synchrotron emission or the synchrotron self‐Compton process as then the inverse Compton process can only provide a small fraction of the observed X‐ray flare emission.