62800795
Sw 1644+57/GRB 110328A: The Physical Origin and the Composition of the Relativistic Outflow
Sw 1644+57/GRB 110328A is a remarkable cosmological X-ray outburst detected by the Swift satellite. Its early-time (t less than or similar to 0.1 days since the trigger) X-ray emission resembles some gamma-ray bursts (GRBs), e. g., GRB 090417B. But the late-time flaring X-ray plateau lasting >40 days renders it unique. We examine the possibilities that the outburst is a super-long GRB powered either by the fallback accretion onto a nascent black hole or by a millisecond pulsar, and find out that these two scenarios can address some but not all of the main observational features. We then focus on the model of tidal disruption of a (giant) star by a massive black hole. The mass of the tidal-disrupted star is estimated to be greater than or similar to a few solar masses. A simple/straightforward argument for a magnetic origin of the relativistic outflow is presented.
7516929
Early optical afterglow light curves of neutron-fed gamma-ray bursts
The neutron component is likely an inevitable ingredient of a gamma-ray burst (GRB) baryonic fireball, in essentially all progenitor scenarios. The suggestion that the neutron composition may alter the early afterglow behavior has been proposed, but there is no detailed calculation so far. In this paper, within the popular internal shock scenario of GRBs, we calculate the early optical afterglow light curves of a neutron-fed GRB fireball for different assumed neutron fractions in the fireball and for both ISM- and wind-interaction models. The cases for both long and short GRBs are considered. We show that as long as the neutron fraction is significant (e. g., the number of neutrons is comparable to that of protons), rich afterglow signatures would show up. For a constant-density (ISM) model, a neutron-rich early afterglow is characterized by a slowly rising light curve followed by a sharp rebrightening bump caused by a collision between the leading neutron decay trail ejecta and the trailing ion ejecta. For a massive star stellar wind model, the neutron-rich early afterglow shows an extended plateau lasting for about 100 s before the light curve starts to decay. The plateau is mainly attributed to the emission from the unshocked neutron decay trail. When the overlapping of the initial prompt gamma-rays with the shocks and the trail is important, as is common for the wind model and is also possible in the ISM model under some conditions, the IC cooling effect suppresses the very early optical afterglow significantly, making the neutron-fed signature dimmer. For short GRBs powered by compact star mergers, a neutron decay-induced steplike rebrightening is predicted, although the amplitude is not large. All these neutron-fed signatures are likely detectable by the Ultraviolet Optical Telescope (UVOT) on board the Swift observatory if GRB fireballs are indeed baryonic and neutron-rich. Close monitoring of early afterglows from tens to thousands of seconds, when combined with detailed theoretical modeling, could be used to potentially diagnose the existence of the neutron component in GRB fireballs.
119286609
Fast radio bursts as a cosmic probe
We discuss the possibility of using fast radio bursts (FRBs)-if cosmological-as a viable cosmic probe. We find that the contribution of the host galaxies to the detected dispersion measures can be inapparent for the FRBs that are not from galaxy centers or star-forming regions. The inhomogeneity of the intergalactic medium (IGM), however, causes significant deviation of the dispersion measure from that predicted in the simplified homogeneous IGM model for an individual event. Fortunately, with sufficient FRBs along different sightlines but within a very narrow redshift interval (e. g., Delta z similar to 0.05), the mean obtained from averaging observed dispersion measures does not suffer such a problem and hence may be used as a cosmic probe. We show that in the optimistic case (e. g., about 20 FRBs in each Delta z have been measured; the most distant FRBs were at redshift >= 3; the host galaxies and the FRB sources contribute little to the detected dispersion measures) and with all the uncertainties (i.e., the inhomogeneity of the IGM, the contribution and uncertainty of host galaxies, and the evolution and error of f(IGM)) considered, FRBs could help constrain the equation of state of dark energy.
14054854
Diverse Temporal Properties of GRB Afterglow
The detection of delayed X-ray, optical and radio emission, " afterglow " , associated with γ-ray bursts (GRBs) is consistent with fireball models, where the emission are produced by relativistic expanding blast wave, driven by expanding fireball at cosmogical distances. The emission mechanisms of GRB afterglow have been discussed by many authors and synchrotron radiation is believed to be the main mechanism. The observations show that the optical light curves of two observed gamma-ray bursts, GRB970228 and GRB GRB970508, can be described by a simple power law, which seems to support the synchrotron radiation explanation. However, here we shall show that under some circumstances, the inverse Compton scattering (ICS) may play an important role in emission spectrum and this may influence the temporal properties of GRB afterglow. We expect that the light curves of GRB afterglow may consist of multi-components, which depends on the fireball parameters.
119112478
Model-dependent estimate on the connection between fast radio bursts and ultra high energy cosmic rays
The existence of fast radio bursts (FRBs), a new type of extragalatic transient, has recently been established, and quite a few models have been proposed. In this work, we discuss the possible connection between the FRB sources and ultra high energy (>10(18) eV) cosmic rays. We show that in the blitzar model and the model of merging binary neutron stars, which includes the huge energy release of each FRB central engine together with the rather high rate of FRBs, the accelerated EeV cosmic rays may contribute significantly to the observed ones. In other FRB models, including, for example, the merger of double white dwarfs and the energetic magnetar radio flares, no significant EeV cosmic ray is expected. We also suggest that the mergers of double neutron stars, even if they are irrelevant to FRBs, may play a nonignorable role in producing EeV cosmic ray protons if supramassive neutron stars are formed in a sufficient fraction of mergers and the merger rate is greater than or similar to 10(3) yr(-1) Gpc(-3). Such a possibility will be unambiguously tested in the era of gravitational wave astronomy.
14647645
The evolution of beamed GRB afterglow : non-relativistic case
There has been increasing evidence that at least some GRBs are emission beamed. The beamed GRB afterglow evolution has been discussed by several authors in the ultra-relativistic case. It has been shown that the dynamics of the blast wave will be significantly modified by the sideways expansion, and there may be a sharp break in the afterglow light curves under certain circumstances. However, this is true only when the fireball is still relativistic. Here we present an analytical approach to the evolution of the beamed GRB blast wave expanding in the surrounding medium (density n ∝ r−s) in the non-relativistic case, our purpose is to explore whether the sideways expansion will strongly affect the blast wave evolution as in the relativistic case. We find that the blast wave evolution is strongly dependent on the speed of the sideways expansion. If it expands with the sound speed, then the jet angle θ increases with time as θ ∝ lnt, which means that the sideways expansion has little effect on the afterglow light curves, the flux F ∝ t− 3(5α−1) 5 for s = 0 and F ∝ t− 7α+1 3 for s = 2. It is clear that the light curve of s = 2 is not always steeper than that of s = 0, as in the relativistic case. We also show that if the expansion speed is a constant, then the jet angle θ ∝ t, and the radius r ∝ t, in this case the sideways expansion has the most significant effect on the blast wave evolution, the flux F ∝ t−(5α−1) independent of s, and we expect that there should be a smooth and gradual break in the light curve. Subject headings: gamma-rays: bursts
14129116
Silicon and Nickel Enrichment in Planet Host Stars: Observations and Implications for the Core Accretion Theory of Planet Formation
We present evidence that stars with planets exhibit statistically significant silicon and nickel enrichment over the general metal-rich population. We also present simulations that predict silicon enhancement of planet hosts within the context of the core accretion hypothesis for giant planet formation. Because silicon and oxygen are both α-elements, [Si/Fe] traces [O/Fe], so the silicon enhancement in planet hosts predicts that these stars are oxygen-rich as well. We present new numerical simulations of planet formation by core accretion that establish the timescale on which a Jovian planet reaches rapid gas accretion, trga, as a function of solid surface density σsolid: (trga/1 Myr) = (σsolid/25.0 g cm-2)-1.44. This relation enables us to construct Monte Carlo simulations that predict the fraction of star-disk systems that form planets as a function of [Fe/H], [Si/Fe], disk mass, outer disk radius, and disk lifetime. Our simulations reproduce both the known planet-metallicity correlation and the planet-silicon correlation reported in this paper. The simulations predict that 15% of solar-type stars form Jupiter-mass planets, in agreement with 12% predicted from extrapolation of the observed planet frequency-semimajor axis distribution. Although a simple interpretation of core accretion predicts that the planet-silicon correlation should be much stronger than the planet-nickel correlation, we observe the same degree of silicon and nickel enhancement in planet hosts. If this result persists once more planets have been discovered, it might indicate a complexity in the chemistry of planet formation beyond the simple accumulation of solids in the core accretion theory.