There are by now ten published detections of fast radio bursts (FRBs), single bright GHz-band millisecond pulses of unknown origin. Proposed explanations cover a broad range from exotic processes at cosmological distances to atmospheric and terrestri
al sources. Loeb et al. have previously suggested that FRB sources could be nearby flare stars, and pointed out the presence of a W-UMa-type contact binary within the beam of one out of three FRB fields that they examined. Using time-domain optical photometry and spectroscopy, we now find possible flare stars in additional FRB fields, with one to three such cases among eight FRB fields studied. We evaluate the chance probabilities of these possible associations to be in the range 0.1% to 9%, depending on the input assumptions. Further, we re-analyze the probability that two FRBs recently discovered 3 years apart within the same radio beam are unrelated. Contrary to other claims, we conclude with 99% confidence that the two events are from the same repeating source. The different dispersion measures between the two bursts then rule out a cosmological origin for the dispersion measure, but are consistent with the flare-star scenario with a varying plasma blanket between bursts. Finally, we review some theoretical objections that have been raised against a local flare-star FRB origin, and show that they are incorrect.
Shocks around clusters of galaxies accelerate electrons which upscatter the Cosmic Microwave Background photons to higher-energies. We use an analytical model to calculate this inverse Compton (IC) emission, taking into account the effects of additio
nal energy losses via synchrotron and Coulomb scattering. We find that the surface brightness of the optical IC emission increases with redshift and halo mass. The IC emission surface brightness, 32--34~mag~arcsec$^{-2}$, for massive clusters is potentially detectable by the newly developed Dragonfly Telephoto Array.
The observed properties of high redshift galaxies depend on the underlying foreground distribution of large scale structure, which distorts their intrinsic properties via gravitational lensing. We focus on the regime where the dominant contribution o
riginates from a single lens and examine the statistics of gravitational lensing by a population of virialized and non-virialized structures using sub-mm galaxies at z ~ 2.6 and Lyman-break galaxies at redshifts z ~ 6 - 15 as the background sources. We quantify the effect of lensing on the luminosity function of the high redshift sources, focusing on the intermediate and small magnifications, mu < 2, which affect the majority of the background galaxies, and comparing to the case of strong lensing. We show that, depending on the intrinsic properties of the background galaxies, gravitational lensing can significantly affect the observed luminosity function even when no obvious strong lenses are present. Finally, we find that in the case of the Lyman-break galaxies it is important to account for the surface brightness profiles of both the foreground and the background galaxies when computing the lensing statistics, which introduces a selection criterion for the background galaxies that can actually be observed. Not taking this criterion into account leads to an overestimation of the number densities of very bright galaxies by nearly two orders of magnitude.
We study the circularization of tidally disrupted stars on bound orbits around spinning supermassive black holes by performing three-dimensional smoothed particle hydrodynamic simulations with Post-Newtonian corrections. Our simulations reveal that d
ebris circularization depends sensitively on the efficiency of radiative cooling. There are two stages in debris circularization if radiative cooling is inefficient: first, the stellar debris streams self-intersect due to relativistic apsidal precession; shocks at the intersection points thermalize orbital energy and the debris forms a geometrically thick, ring-like structure around the black hole. The ring rapidly spreads via viscous diffusion, leading to the formation of a geometrically thick accretion disk. In contrast, if radiative cooling is efficient, the stellar debris circularizes due to self-intersection shocks and forms a geometrically thin ring-like structure. In this case, the dissipated energy can be emitted during debris circularization as a precursor to the subsequent tidal disruption flare. The possible radiated energy is up to ~2*10^{52} erg for a 1 Msun star orbiting a 10^6 Msun black hole. We also find that a retrograde (prograde) black hole spin causes the shock-induced circularization timescale to be shorter (longer) than that of a non-spinning black hole in both cooling cases. The circularization timescale is remarkably long in the radiatively efficient cooling case, and is also sensitive to black hole spin. Specifically, Lense-Thirring torques cause dynamically important nodal precession, which significantly delays debris circularization. On the other hand, nodal precession is too slow to produce observable signatures in the radiatively inefficient case. We also discuss the relationship between our simulations and the parabolic TDEs that are characteristic of most stellar tidal disruptions.
The dominant baryonic component of galaxy clusters is hot gas whose distribution is commonly probed through X-ray emission arising from thermal bremsstrahlung. The density profile thus obtained has been traditionally modeled with a beta-profile, a si
mple function with only three parameters. However, this model is known to be insufficient for characterizing the range of cluster gas distributions, and attempts to rectify this shortcoming typically introduce additional parameters to increase the fitting flexibility. We use cosmological and physical considerations to obtain a family of profiles for the gas with fewer parameters than the beta-model but which better accounts for observed gas profiles over wide radial intervals.
Gamma-ray bursts (GRBs) show a bimodal distribution of durations, separated at a duration of ~2 s. Observations have confirmed the association of long GRBs with the collapse of massive stars. The origin of short GRBs is still being explored. We exami
ne constraints on the emission region size in short and long GRBs detected by Fermi/GBM. We find that the emission region size during the prompt emission, R, and the burst duration, T$_{90}$, are consistent with the relation R ~ c x T$_{90}$, for both long and short GRBs. We find the characteristic size for the prompt emission region to be ~2 x 10$^{10}$ cm, and ~4 x 10$^{11}$ cm for short and long GRBs, respectively.
The unusual morphologies of the Andromeda spiral galaxy (M31) and its dwarf companion M32 have been characterized observationally in great detail. The two galaxies apparent proximity suggests that Andromedas prominent star-forming ring as well as M32
s compact elliptical structure may result from a recent collision. Here we present the first self-consistent model of the M31-M32 interaction that simultaneously reproduces observed positions, velocities, and morphologies for both galaxies. Andromedas spiral structure is resolved in unprecedented detail, showing that a rare head-on orbit is not necessary to match Andromedas ring-like morphology. The passage of M32 through Andromedas disk perturbs the disk velocity structure. We find tidal stripping of M32s stars to be inefficient during the interaction, suggesting that some cEs are intrinsically compact. Additionally, the orbital solution implies that M32 is currently closer to the Milky Way than models have typically assumed, a prediction that may be testable with upcoming observations.
The discovery of the gas cloud G2 on a near-radial orbit about Sgr A* has prompted much speculation on its origin. In this Letter, we propose that G2 formed out of the debris stream produced by the removal of mass from the outer envelope of a nearby
giant star. We perform hydrodynamical simulations of the returning tidal debris stream with cooling, and find that the stream condenses into clumps that fall periodically onto Sgr A*. We propose that one of these clumps is the observed G2 cloud, with the rest of the stream being detectable at lower Br-$gamma$ emissivity along a trajectory that would trace from G2 to the star that was partially disrupted. By simultaneously fitting the orbits of S2, G2, and $sim$ 2,000 candidate stars, and by fixing the orbital plane of each candidate star to G2 (as is expected for a tidal disruption), we find that several stars have orbits that are compatible with the notion that one of them was tidally disrupted to produce G2. If one of these stars were indeed disrupted, it last encountered Sgr A* hundreds of years ago, and has likely encountered Sgr A* repeatedly. However, while these stars are compatible with the giant disruption scenario given their measured positions and proper motions, their radial velocities are currently unknown. If one of these stars radial velocity is measured to be compatible with a disruptive orbit, it would strongly suggest its disruption produced G2.
We simulate the effects of gravitational lensing on the source count of high redshift galaxies as projected to be observed by the Hubble Frontier Fields program and the James Webb Space Telescope (JWST) in the near future. Taking the mass density pro
file of the lensing object to be the singular isothermal sphere (SIS) or the Navarro-Frenk-White (NFW) profile, we model a lens residing at a redshift of z_L = 0.5 and explore the radial dependence of the resulting magnification bias and its variability with the velocity dispersion of the lens, the photometric sensitivity of the instrument, the redshift of the background source population, and the intrinsic maximum absolute magnitude (M_{max}) of the sources. We find that gravitational lensing enhances the number of galaxies with redshifts z >= 13 detected in the angular region theta_E/2 <= theta <= 2theta_E (where theta_E is the Einstein angle) by a factor of ~ 3 and 1.5 in the HUDF (df/d u_0 ~ 9 nJy) and medium-deep JWST surveys (df/d u_0 ~ 6 nJy). Furthermore, we find that even in cases where a negative magnification bias reduces the observed number count of background sources, the lensing effect improves the sensitivity of the count to the intrinsic faint-magnitude cut-off of the Schechter luminosity function. In a field centered on a strong lensing cluster, observations of z >= 6 and z >= 13 galaxies with JWST can be used to infer this cut-off magnitude for values as faint as M_{max} ~ -14.4 and -16.1 mag (L_{min} ~ 2.5*10^{26} and 1.2*10^{27} erg s^{-1} Hz^{-1}) respectively, within the range bracketed by existing theoretical models. Gravitational lensing may therefore offer an effective way of constraining the low-luminosity cut-off of high-redshift galaxies.
We investigate unbound dark matter particles in halos by tracing particle trajectories in a simulation run to the far future (a = 100). We find that the traditional sum of kinetic and potential energies is a very poor predictor of which dark matter p
articles will eventually become unbound from halos. We also study the mass fraction of unbound particles, which increases strongly towards the edges of halos, and decreases significantly at higher redshifts. We discuss implications for dark matter detection experiments, precision calibrations of the halo mass function, the use of baryon fractions to constrain dark energy, and searches for intergalactic supernovae.
