No Arabic abstract
We consider radio emission from a newborn black hole (BH), which is accompanied by a mini-disk with a mass of $lesssim M_odot$. Such a disk can be formed from an outer edge of the progenitors envelope, especially for metal-poor massive stars and/or massive stars in close binaries. The disk accretion rate is typically super-Eddington and an ultrafast outflow with a velocity of $sim 0.1mbox{-}0.3,c$ will be launched into the circumstellar medium. The outflow forms a collisionless shock, and electrons are accelerated and emit synchrotron emission in radio bands with a flux of $sim 10^{26-30} rm erg s^{-1} Hz^{-1}$ days to decades after the BH formation. The model predicts not only a fast UV/optical transient but also quasi-simultaneous inverse-Compton X-ray emission $sim$ a few days after the BH formation, and the discovery of the radio counterpart with coordinated searches will enable us to identify this type of transients. The occurrence rate can be $0.1-10 %$ of the core-collapse supernova rate, which makes them a promising target of dedicated radio observations such as the Jansky VLA Sky Survey.
Newborn black holes in collapsing massive stars can be accompanied by a fallback disk. The accretion rate is typically super-Eddington and strong disk outflows are expected. Such outflows could be directly observed in some failed explosions of compact (blue supergiants or Wolf-Rayet stars) progenitors, and may be more common than long-duration gamma-ray bursts. Using an analytical model, we show that the fallback disk outflows produce blue UV-optical transients with a peak bolometric luminosity of ~10^(42-43) erg s^-1 (peak R-band absolute AB magnitudes of -16 to -18) and an emission duration of ~ a few to ~ 10 days. The spectra are likely dominated intermediate mass elements, but will lack much radioactive nuclei and iron-group elements. The above properties are broadly consistent with some of the rapid blue transients detected by Pan-STARRS and PTF. This scenario can be distinguished from alternative models using radio observations within a few years after the optical peak.
We explore the possibility to detect the continuum radio signal from direct collapse black holes (DCBHs) by upcoming radio telescopes such as the SKA and ngVLA, assuming that after formation they can launch and sustain powerful jets at the accretion stage. We assume that the high-$z$ DCBHs have similar jet properties as the observed radio-loud AGNs, then use a jet model to predict their radio flux detectability. If the jet power $P_{rm jet}gtrsim10^{42-43}$ erg s$^{-1}$, it can be detectable by SKA/ngVLA, depending on the jet inclination angle. Considering the relation between jet power and black hole mass and spin, generally, jetted DCBHs with mass $gtrsim10^5~M_odot$ can be detected. For a total jetted DCBH number density of $sim2.5times10^{-3}$ Mpc$^{-3}$ at $z=10$, about 100 deg$^{-2}z^{-1}$ DCBHs are expected to be above the detection threshold of SKA1-mid (100 hours integration). If the jet blob emitting most of the radio signal is dense and highly relativistic, then the DCBH would only feebly emit in the SKA-low band, because of self-synchrotron absorption (SSA) and blueshift. Moreover, the free-free absorption in the DCBH envelope may further reduce the signal in the SKA-low band. Thus, combining SKA-low and SKA-mid observations might provide a potential tool to distinguish a DCBH from a normal star-forming galaxy.
We consider black hole formation in failed supernovae when a dense circumstellar medium (CSM) is present around the massive star progenitor. By utilizing radiation hydrodynamical simulations, we calculate the mass ejection of blue supergiants and Wolf-Rayet stars in the collapsing phase and the radiative shock occurring between the ejecta and the ambient CSM. We find that the resultant emission is redder and dimmer than normal supernovae (bolometric luminosity of $sim 10^{40}-10^{41} {rm erg s^{-1}}$, effective temperature of $sim 5times 10^3$ K, and timescale of 10-100 days) and shows a characteristic power-law decay, which may comprise a fraction of intermediate luminosity red transients (ILRTs) including AT 2017be. In addition to searching for the progenitor star in the archival data, we encourage X-ray follow-up observations of such ILRTs $sim$ 1-10 yr after the collapse, targeting the fallback accretion disk.
Apart from the few tens of stellar-mass black holes discovered in binary systems, an order of $10^8$ isolated black holes (IBHs) are believed to be lurking in our Galaxy. Although some IBHs are able to accrete matter from the interstellar medium, the accretion flow is usually weak and thus radiatively inefficient, which results in significant material outflow. We study electron acceleration generated by the shock formed between this outflow and the surrounding material, and the subsequent radio synchrotron emission from accelerated electrons. By numerically calculating orbits of IBHs to obtain their spatial and velocity distributions, we estimate the number of IBHs detectable by surveys using SKA1-mid (SKA2) as $sim 30$ ($sim 700$) for the most optimistic case. The SKAs parallax measurements may accurately give their distances, possibly shedding light on the properties of the black holes in our Galaxy.
Sixteen years of observations of black hole transients with the Rossi X-ray Timing Explorer, complemented by other X-ray observatories and ground-based optical/infrared/radio telescopes have given us a clear view of the complex phenomenology associated with their bright outbursts. This has led to the definition of a small number of spectral/timing states which are separated by marked transitions in observables. The association of these states and their transitions to changes in the radio emission from relativistic radio jets completes the picture and have led to the study of the connection between accretion and ejection. A good number of fundamental questions are still unanswered, but the existing picture provides a good framework on which to base theoretical studies. We discuss the current observational standpoint, with emphasis onto the spectral and timing evolution during outbursts, as well as the prospects for future missions such as ASTROSAT (2012) and LOFT (>2020 if selected).