arXiv subjects
Publications and source records attributed to Tsvi Piran.
We explore numerically the evolution of a collapsing spherical shell of charged, massless scalar field. We obtain an external \RN space-time, and an inner space-time that is bounded by a singularity on the Cauchy Horizon. We compare these results with previous analysis and discuss some of the numerical problems encountered.
GRB 030329 is unique in many aspects. It has a very low redshift for a GRB, $z=0.1685$, and is therefore very bright and easy to monitor, making it the most well studied afterglow to date. It shows a supernova bump in the light curve, with a spectrum very similar to SN 1998bw, thus establishing with much better confidence the connection between GRBs and core collapse SNe. There are also two important physical characteristics that make this burst especially interesting, aside from its remarkably low redshift. First, unlike most GRB afterglows, the light curve of GRB 030329 shows a very large variability a few days after the burst. These fluctuations show a roughly constact amplitude, and a constant duration $Δt$, while $Δt/t$ decreases with time $t$. Second, its $γ$-ray energy output and X-ray luminosity at $10 $hr are a factor of $\sim 20$ and $\sim 30$, respectively, below the average value around which most GRBs are narrowly clustered. We consider several interpretations for the variability in the light curve, in the context of different physical mechanisms, and find that the most likely cause is refreshed shocks, i.e. slow shells that are ejected from the source and catch up with the afterglow shock at late times. In GRB 030329 this happens after the jet break, which implies an approximately constant duration $Δt$ of the bumps, in agreement with the observations. This interpretation also explains the anomalously low initial energy of this burst, as the total energy of the afterglow shock is increased by a factor of $\sim 10$ due to the refreshed shocks, thus bringing the total energy output close to the average value for all GRBs.
We present general analytic expressions for GRB afterglow light curves arising from a variable external density profile and/or a variable energy in the blast wave. The former could arise from a clumpy ISM or a variable stellar wind; The latter could arise from refreshed shocks or from an angular dependent jet structure (patchy shell). Both scenarios would lead to a variable light curve. Our formalism enables us to invert the observed light curve and obtain possible density or energy profiles. The optical afterglow of GRB 021004 was detected 537 seconds AB (after the burst) (Fox et al. 2002). Extensive follow up observations revealed a significant temporal variability. We apply our formalism to the R-band light curve of GRB 021004 and we find that several models provide a good fit to the data. We consider the patchy shell model with $p=2.2$ as the most likely explanation. According to this model our line of sight was towards a "cold spot" that has lead to a relativity low $γ$-ray flux and an initially weak afterglow (while the X-ray afterglow flux after a day was above average). Observations above the cooling frequency, $ν_c$, could provide the best way to distinguish between our different models.
The fluctuations observed in the light curves of some GRB-afterglows (such as GRB 021004) provide a useful tool to probe the circum-burst density profile and to probe the variations in the energy of blast-wave with time. We present a general formalism that reduces the calculation of the observed light curve from a Blandford-Mackee blast-wave to the evaluation of a one dimensional integral. Using this formalism we obtain a simple approximation to the general light curve that arises in more complex situations where the afterglow's energy or the external density vary. The solution is valid for spherically symmetric profiles and it takes a full consideration of the angular time delay effects. We present the light curves of several external density profiles and demonstrate the effects of density variations on the light curve. We also re-visit the afterglow of GRB021004 and we find that the steep decay after the first bump ($\sim 4000sec$) cannot result from a spherically symmetric density variation or from the passage of the synchrotron frequency through the optical band. This suggests that an angular structure is responsible to some of the observed features in the light curve. This may be the first evidence that an angular structure is important in the early stages of the afterglow.
We explore time-symmetric hypersurfaces containing apparent horizons of black objects in a 5d spacetime with one coordinate compactified on a circle. We find a phase transition within the family of such hypersurfaces: the horizon has different topology for different parameters. The topology varies from $S^3$ to $S^2 \times S^1$. This phase transition is discontinuous -- the topology of the horizon changes abruptly. We explore the behavior around the critical point and present a possible phase diagram.
We investigate how the angular structure of GRB jets effects the afterglow light curves at different viewing angles, $θ_{v}$, from the jet symmetry axis. A numerical hydrodynamical modeling for the evolution of a relativistic collimated outflow, as it interacts with the surrounding medium, is carried out, and compared to two simple models that make opposite and extreme assumptions for the degree of lateral energy transfer. The Lorentz factor, $Γ$, and kinetic energy per unit solid angle, $ε$, are initially taken to be power laws of the angle $θ$ from the jet axis. We find that the lateral velocity in the comoving frame, $v'_θ$, is typically much smaller than the sound speed, $c_s$, as long as $Γ\gg 1$, and the dynamics of relativistic structured jets may be reasonably described by a simple analytic model where $ε$ is independent of time, as long as $Γ(θ=0)\ga$ a few. We perform a qualitative comparison between the resulting light curves and afterglow observations. This constrains the jet structure, and poses problems for a `universal' jet model, where all GRB jets are assumed to be intrinsically identical, and differ only by our viewing angle, $θ_{v}$.
In many GRB inner engine models the highly relativistic GRB jets are engulfed by slower moving matter. This could result in different beaming for the prompt $γ$-ray emission and for the lower energy afterglow. In this case we should expect that some observer will see {\it on-axis orphan afterglows}: X-ray, optical and radio afterglows within the initial relativistic ejecta with no preceding GRB; The prompt $γ$-ray emission is pointing elsewhere. We show that the observations of the WFC on BeppoSAX constrain with high certainty the prompt X-ray beaming factor to be less than twice the prompt $γ$-ray beaming. The results of Ariel 5 are consistent with this interpretation. The RASS from ROTSE and HEAO-1 constrain the X-ray beaming factor at 400 and 20 minutes after the burst respectively to be comparable and certainly not much larger than the $γ$-ray beaming factor. There is no direct limit on the optical beaming. However, we show that observations of several months with existing hardware could result in a useful limit on the optical beaming factor of GRB afterglows.
The realization that GRBs release a constant amount of energy implies that the post jet-break afterglow evolution would be largely universal. For a given redshift all afterglows should be detected up to a fixed observer angle. We estimate the observed magnitude and the implied detectability of orphan afterglows. We show that for reasonable limiting magnitudes ($m_{lim}=25$) orphan afterglows would typically be detected from small ($\sim 10^\circ$) angles away from the GRB jet axis. A detected orphan afterglow would generally correspond to a "near-miss" of the GRB whose jet was pointing just slightly away from us. With our most optimistic parameters we expect that 15 orphan afterglows will be recorded in the SDSS and 35 transients will be recorded in a dedicated 2m class telescope operating full time for a year in an orphan afterglow search. The rate is smaller by a factor of 15 for our "canonical" parameters. We show that for a given facility an optimal survey should be shallower, covering larger area rather than deeper. The limiting magnitude should not be, however, lower than $\sim$ 23rd as in this case more transients from on-axis GRBs will be discovered than orphan afterglows. About 15% of the transients could be discovered with a second exposure of the same area provided that it follows after 3, 4 and 8 days for $m_{lim}=23$, 25 and 27.
Gamma-Ray Bursts (GRBs) - short bursts of 100-1MeV photons arriving from random directions in the sky are probably the most relativistic objects discovered so far. Still, somehow they did not attract the attention of the relativistic community. In this short review I discuss briefly GRB observations and show that they lead us to the fireball model - GRBs involve macroscopic relativistic motion with Lorentz factors of a few hundred or more. I show that GRB sources involve, most likely, new born black holes, and their progenitors are Supernovae or neutron star mergers. I show that both GRB progenitors and the process of GRB itself produce gravitational radiation and I consider the possibility of detecting this emission. Finally I show that GRBs could serve as cosmological indicators that could teach us about the high redshift ($z \approx 5-15$) dark ages of the universe.
The realization that GRBs are narrowly beamed implied that the actual rate of GRBs is much larger than the observed one. There are 500 unobserved GRBs for each observed one. The lack of a clear trigger makes it hard to detect these unobserved GRBs as orphan afterglows. At late time, hundreds or thousands of years after a GRB, we expect to observe a GRB remnant (GRBR). These remnants could be distinguished from the more frequent SNRs using their different morphology. While SNRs are spherical, GRBRs that arise from a highly collimated flow, are expected to be initially nonspehrical. Using SPH simulations we follow the evolution of a GRBR and calculate the image of the remnant produced by bremsstrahlung and by synchrotron emission. We find that the GRBR becomes spherical after $ \sim 3000 {\rm yr}(E_{51}/n)^{1/3}$ at $R\sim 12{\rm pc} (E_{51}/n)^{1/3}$, where $E_{51}$ is the initial energy in units of $10^{51}{\rm erg}$ and $n$ is the surrounding ISM number density in ${\rm cm}^{-3}$. We expect $0.5 (E_{51}/n)^{1/3}$ non-spherical GRBs per galaxy. Namely, we expect $\sim ~ 20$ non spherical GRBRs with angular sizes $\sim 1 μ$arcsec within a distance of 10Mpc. These results are modified if there is an underlying spherical supernova. In this case the GRBR will remain spherical only for $\sim150 {\rm yr} (E_{51}/n)^{1/3}$ and the number of non-spherical GRBRs is smaller by a factor of 10 and their size is smaller by a factor of 3.
According to the fireball model GRBs are produced when a relativistic flow is dissipated by shocks. The questions how does a GRB's "inner engine" accelerates and collimated the relativistic flow is today the most interesting (and most difficult) open question concerning GRBs. We summarize, here, the known constrains on the "inner engines". These constrains arise mostly from the temporal structure seen in the light curves of GRBs. We discuss the well-known NO GO theorem: An explosive source cannot produce a variable GRB. We show that various attempts to find a way around this theorem are rules out by new observational data: similarity between pulse width and pulse intervals and a correlation between a pulse and the preceding interval. We show that a simple toy model explains these features in terms of the internal shocks model. We discuss the implications of these findings to GRBs' inner engines.
We analyze a sample of bright short bursts from the BATSE 4B-catalog and find that many short bursts are highly variable ($δt_{min}/T\ll 1$, where $δt_{min}$ is the shortest pulse duration and T is the burst duration). This indicates that it is unlikely that short bursts are produced by external shocks. We also analyze the available (first 1-2 seconds) high resolution (TTE) data of some of the long bursts. We find that variability on a 10ms time-scale is common in long bursts. This result shows that some long bursts are even more variable than it was thought before ($δt_{min}/T\approx 10^{-4}-10^{-3}$ )
We analyze a sample of bright long bursts and find that the pulses duration have a lognormal distribution while the intervals between pulses have an excess of long intervals (relative to lognormal distribution). This excess can be explained by the existence of quiescent times, long periods with no signal above the background. The lognormal distribution of the intervals (excluding the quiescent times) is similar to the distribution of the pulses width. This result suggests that the quiescent times are made by a different mechanism than the rest of the intervals. It also suggests that the intervals (excluding the quiescent times) and the pulse width are connected to the same parameters of the source. We find that there is a correlation between a pulse width and the duration of the interval preceding it. There is a weaker, but still a significant, correlation between a pulse width and the interval following it. The significance of the correlation drops substantially when the intervals considered are not adjacent to the pulse.
The energy of a Gamma-Ray Burst is one of the most interesting factors that can help determining the origin of these mysterious explosions. After the discovery that GRBs are cosmological it was thought, for a while that they are standard candles releasing $\sim 10^{51}$ergs. Redshift measurements that followed the discovery of GRB afterglow revealed that GRBs are (i) further than what was initially believed and (ii) have a very wide luminosity function. Some bursts revealed an enormous energy release with a record estimate of $ 1.4 \times 10^{54}$ergs for GRB990123. The energy budget was stretched even further when it was suggested that the conversion efficiency of producing gamma-rays is low. The realization that GRBs are beamed changed this perspective and brought the energy budget of GRBs down back to a "modest" $\sim 5 \times 10^{50}$ergs. I discuss here various estimates showing that GRB energy is narrowly distributed and discuss the implications of this conclusion to GRB models.
The energy release in gamma-ray bursts is one of the most interesting clues on the nature of their "inner engines". We show here that the total energy release in GRBs varies by less than one order of magnitude from one burst to another while the energy emitted in $γ$-ray photons varies by more than an order of magnitude. This result indicates that the central engine of long duration GRB has a remarkably constant energy output which provides very important constraint on the nature of these enigmatic explosions. The broader distribution of the observed $γ$-ray flux, about three orders of magnitude in width, can be attributed, in part, to a variation in the opening angle of the collimated explosion, and in part to the variation of Lorentz factor across the jet as well as a variable efficiency for converting the kinetic energy of explosion to $γ$-rays.
The most interesting current open question in the theory of GRB afterglow is the propagation of jetted afterglows during the sideway expansion phase. Recent numerical simulations show hydrodynamic behavior that differs from the one suggested by simple analytic models. Still, somewhat surprisingly, the calculated light curves show a `jet break' at about the expected time. These results suggest that the expected rate of orphan optical afterglows should be smaller than previously estimated.
Many models of gamma-ray bursts (GRBs) involve accretion onto a compact object, usually a black hole, at a mass accretion rate of order a fraction of a solar mass per second. If the accretion disk is larger than a few tens or hundreds of Schwarzschild radii, the accretion will proceed via a convection-dominated accretion flow (CDAF) in which most of the matter escapes to infinity rather than falling onto the black hole. Models involving the mergers of black hole white dwarf binaries and black hole helium star binaries fall in this category. These models are unlikely to produce GRBs since very little mass reaches the black hole. If the accretion disk is smaller, then accretion will proceed via neutrino cooling in a neutrino-dominated accretion disk (NDAF) and most of the mass will reach the center. Models involving the mergers of double neutron star binaries and black hole neutron star binaries fall in this category and are capable of producing bright GRBs. If the viscosity parameter $α$ in the NDAF has a standard value $\sim0.1$, these mergers can explain short GRBs with durations under a second, but they are unlikely to produce long GRBs with durations of tens or hundred of seconds. If the accretion disk is fed by fallback of material after a supernova explosion, as in the collapsar model, then the time scale of the burst is determined by fallback, not accretion. Such a model can produce long GRBs. Fallback models again require that the accretion should proceed via an NDAF rather than a CDAF in order for a significant amount of mass to reach the black hole. This condition imposes an upper limit on the radius of injection of the gas.
If a light gluino exists, the lightest gluino-containing baryon, the \OSO, is a possible candidate for self-interacting dark matter. In this scenario, the simplest explanation for the observed ratio $Ω_{dm}/Ω_b \approx 6-10$ is that $m_{S^0} \sim 900$\MeVcs; this is not at present excluded by particle physics. Such an \OSO could be present in neutron stars, with hyperon formation serving as an intermediate stage. We calculate equilibrium compositions and equation of state for high density matter with the \OSO, and find that for a wide range of parameters the properties of neutron stars with the \OSO are consistent with observations. In particular, the maximum mass of a nonrotating star is $1.7-1.8 M_\odot$, and the presence of the \OSO is helpful in reconciling observed cooling rates with hyperon formation.