arXiv subjects

Enrico Ramirez-Ruiz

Publications and source records attributed to Enrico Ramirez-Ruiz.

At least 253 records · Page 14Linked to original sources

arXiv2007

Deducing the Lifetime of Short Gamma-Ray Burst Progenitors from Host Galaxy Demography

The frequency of short gamma-ray bursts (GRBs) in galaxies with distinct star formation histories can be used to constrain the lifetime of the progenitor systems. As an illustration, we consider here the constraints that can be derived from separating the host galaxies into early and late types. On average, early-type galaxies have their stars formed earlier than late-type galaxies, and this difference, together with the time delay between progenitor formation and short GRB outburst, leads to different burst rates in the two types of hosts. Presently available data suggest, but not yet prove, that the local short GRB rate in early-type galaxies may be comparable to that in late-type galaxies. This suggests that, unlike Type Ia supernovae, at least half of the short GRB progenitors that can outburst within a Hubble time have lifetimes greater than about 7 Gyr. Models of the probability distribution of time delays, here parametrized as P(τ) \propto τ^n, with n > -1 are favored. This apparent long time delay and the fact that early-type galaxies in clusters make a substantial contribution to the local stellar mass inventory can explain the observed preponderance of short GRBs in galaxy clusters.

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arXiv2007

On the absence of Wind Signatures in GRB Afterglow Spectra: Constraints on the Wolf-Rayet Winds of GRB Progenitors

We investigate available constraints on the circumstellar medium (CSM) around long-duration gamma-ray burst (GRB) progenitors from afterglow spectra. We first establish a statistical sample of five GRB afterglow spectra that have been collected and analyzed with no prior knowledge of the line-of-sight properties. This sample is then adopted for a uniform search of Wolf-Rayet wind signatures, as represented by CIV 1548, 1550 absorption doublets at dv=-1000 to -5000 km/s from the GRBs (hereafter CIV_{15}). We report the detection of a single CIV_{15} absorber at dv ~ -1500 km/s from GRB 050730 and none in the rest. Our search yields an estimate of 20% for the incidence of CIV_{15} absorbers with rest-frame absorption equivalent width EW(CIV 1548)>0.2 Ang near GRB host galaxies, consistent with the incidence of intergalactic CIV_{15} near classical damped Lya absorbers toward quasar sightlines. Including the two CIV_{15} absorbers previously known toward GRB 021004, we further demonstrate that the presence of H^0, C^+, and Si^+ together with the absence of excited C^+ or Si^+ argue against a CSM origin. The null result is consistent with the expectation that the circumburst medium is fully ionized by the afterglow radiation field. We examine possible scenarios for the survival of the C^{3+} ions, including a clumpy wind model. We find that a clumpy wind is unable to effectively shield the ionizing radiation and allow C^{3+} to survive at r<10 pc from the afterglow. We conclude that the lack of CSM-originated CIV_{15} absorbers is consistent with Wolf-Rayet winds terminating at < 30 pc from their progenitor stars.

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arXiv2007

The Progenitors of Short Gamma-Ray Bursts

Recent months have witnessed dramatic progress in our understanding of SGRBs. There is now general agreement that SGRBs can produce directed outflows of relativistic matter with a kinetic luminosity exceeding by many millions that of AGN. The requirements of energy and compactness indicate that SGRB activity can be ascribed to a modest fraction of a solar mass of gas accreting onto a stellar mass BH or to a precursor stage leading inevitably to such an object. Scenarios involving the birth of a rapidly rotating NS, or an accreting BH in a merging binary driven by gravitational waves are reviewed, along with possible alternatives (collisions or collapse of compact objects). If a BH lies at the center of this activity, the fundamental pathways through which mass, angular momentum and energy can flow around and away from it play a key role in understanding how these prime movers can form collimated relativistic outflows. Hypercritical flows near BHs, where photons cannot supply the cooling, but neutrinos do so efficiently, are discussed in detail, and we believe that they offer the best hope of understanding the central engine. On the other hand, statistical investigations of SGRB niches provide valuable information on their nature and evolutionary behavior. In addition, compelling evidence now points to the continuous fueling of SGRB sources. We suggest that the observed late flaring activity could be due to a secondary accretion episode induced by the fall back of material stripped from a compact object during a merger or collision. Important open questions are identified, along with the types of observation that would discriminate among various models. SGRB jets may be one of the few observable consequences of how flows near nuclear density behave under the influence of strong gravitational fields. (abridged)

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arXiv2007

No supernovae associated with two long-duration gamma ray bursts

It is now accepted that long duration gamma-ray bursts (GRBs) are produced during the collapse of a massive star. The standard "collapsar" model predicts that a broad-lined and luminous Type Ic core-collapse supernova (SN) accompanies every long-duration GRB. This association has been confirmed in observations of several nearby GRBs. Here we present observations of two nearby long-duration GRBs that challenge this simple view. In the GRBs 060505 and 060614 we demonstrate that no SN emission accompanied these long-duration bursts, down to limits hundreds of times fainter than the archetypal SN 1998bw that accompanied GRB 980425, and fainter than any Type Ic SN ever observed. Multi-band observations of the early afterglows, as well as spectroscopy of the host galaxies, exclude the possibility of significant dust obscuration and show that the bursts originated in star-forming regions. The absence of a SN to such deep limits is qualitatively different from all previous nearby long GRBs and suggests a new phenomenological type of massive stellar death. From the supplementary material: Now we have observed SN-less GRBs in star-forming regions, suggesting that a non-detection of a SN does not preclude a massive progenitor. The position of the GRB, i.e. in a star-forming region or in an older component, may be the only way to discriminate between merging compact objects and massive stars as progenitors. In fact, several host galaxies for short GRBs have been found to be as actively star-forming as some host galaxies of long-duration GRBs. The GRB labels "long" and "short" have become synonymous with "massive stars" and "other progenitors". These distinctions may need to be relaxed.

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arXiv2006

A Simple Test of the External Shock Model for the Prompt Emission in Gamma-Ray Bursts

It is demonstrated here that if the prompt GRB emission is produced by the simplest version of the external shock model, a specific relation should prevail between the observed duration, isotropic equivalent energy, and photon peak energy. In essence, this relation arises because both the burst duration and the typical energy of the emitted synchrotron photons depend on the same combination of the, usually poorly constrained, external density at the deceleration radius, $n_{dec}$, and initial bulk Lorentz factor, $Γ_0$. This has the fortunate consequence of making the relation independent of both $Γ_0$ and $n_{dec}$. Unless the efficiency of electron acceleration is very low, synchrotron gamma-rays from the external shock would fail to meet the current observational constraints for the vast majority of GRBs, including those with a smooth, single peak temporal profile. This argues either against an external shock origin for the prompt emission in GRBs or for changes in our understanding of the microphysical and radiation processes occurring within the shocked region.

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arXiv2006

Accretion modes in collapsars - prospects for GRB production

We explore low angular momentum accretion flows onto black holes formed after the collapse of massive stellar cores. In particular, we consider the state of the gas falling quasi-spherically onto stellar-mass black holes in the hypercritical regime, where the accretion rates are in the range 0.001 - 0.5 solar masses per second and neutrinos dominate the cooling. Previous studies have assumed that in order to have a black hole switch to a luminous state, the condition l >> r_g c, where l is the specific orbital angular momentum of the infalling gas and r_g is the Schwarszchild radius, needs to be fulfilled. We argue that flows in hyperaccreting, stellar mass disks around black holes are likely to transition to a highly radiative state when their angular momentum is just above the threshold for disk formation, l ~ 2 r_g c. In a range where l lies between r_g c and 2 r_g c, a dwarf disk forms in which gas spirals rapidly into the black hole due to general relativistic effects, without any help from horizontal viscous stresses. For high rotation rates with l greater than 2 r_g c, the luminosity is supplied by large, hot equatorial bubbles around the black hole. The highest neutrino luminosities are obtained for l ~ 2 r_g c, and this value of angular momentum also produces the most energetic neutrinos, and thus also the highest energy deposition rates. Given the range of l explored in this work, we argue that, as long as l is greater than 2 r_g c, low angular momentum cores may in fact be better suited for producing neutrino--driven explosions following core collapse in supernovae and gamma ray bursts.

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arXiv2005

X-ray flares following short gamma-ray bursts from shock heating of binary stellar companions

The discovery of long-lasting (~100 s) X-ray flares following short gamma-ray bursts initially called into question whether they were truly classical short-hard bursts. Opinion over the last few years has coalesced around the view that the short-hard bursts arise from the merger of pairs of neutron stars, or a neutron star merging with a stellar-mass black hole. The natural timescales associated with these processes, however, essentially preclude an X-ray flare lasting ~100 s. Here we show that an interaction between the GRB outflow and a non-compact stellar companion at a distance of ~a light-minute provides a natural explanation for the flares. In the model, the burst is triggered by the collapse of a neutron star after accreting matter from the companion. This is reminiscent of type Ia supernovae, where there is a wide distribution of delay times between formation and explosion, leading to an association with both star-forming galaxies and old ellipticals.

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arXiv2005

The redshift distribution of gamma-ray bursts revisited

In this letter, we calculate the redshift distribution of gamma-ray bursts assuming that they trace (i) the globally averaged star formation rate or (ii) the average metallicity in the Universe. While at redshifts 5 and below, both the star formation rate and metallicity are observationally determined modulo some uncertainties, at higher redshifts there are few constraints. We extrapolate the star formation rate and metallicity to higher redshifts and explore models that are broadly consistent with bounds on the optical depth from WMAP results. In addition, we also include parametric descriptions of the luminosity function, and the typical spectrum for GRBs. With these essential ingredients included in the modeling, we find that a substantial fraction 75% of GRBs are expected to originate at redshifts below 4, in variance with some previous estimates. Conversely, if we assume as expected for the collapsar model that gamma-ray bursts favour a low metallicity environment and therefore, relate the GRB rate to a simple model of the average metallicity as a function of redshift, we find that a higher fraction of bursts, about 40% originate from redshifts greater than 4. We conclude with the implications of SWIFT GRB detections.

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arXiv2005

Photospheric signatures imprinted on the gamma-ray burst spectra

A solution is presented for the spectrum of high-energy gamma-ray burst photons confined to a quasi-thermal baryonic photosphere. The solution is valid in the steady-state limit assuming the region under consideration is optically thick to the continuously injected photons. It is shown that for a high luminosity photosphere, the non-thermal electrons resulting from gamma-ray Compton cooling lose their energy by upscattering the soft thermalised radiation. The resulting spectral modifications offer the possibility of diagnosing not only the burst comoving luminosity but also the baryon load of the ejecta. This model leads to a simple physical interpretation of X-ray rich bursts and anomalous low-energy slopes.

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arXiv2005

A compact binary merger model for the short, hard GRB 050509b

The first X-ray afterglow for a short (~30 ms), hard gamma-ray burst was detected by Swift on 9 May 2005 (GRB 050509b). No optical or radio counterpart was identified in follow-up observations. The tentative association of the GRB with a nearby giant elliptical galaxy at redshift z=0.2248 would imply a total energy release E(gamma, isotropic) ~3 x 10**48 erg, and that the progenitor had traveled several tens of kpc from its point of origin, in agreement with expectations linking these events to the final merger of compact binaries driven by gravitational wave emission. We model the dynamical merger of such a system and the time-dependent evolution of the accretion tori thus created. The resulting energetics, variability, and expected durations are consistent with GRB 050509b originating from the tidal disruption of a neutron star by a stellar mass black hole, or of the merger of two neutron stars followed by prompt gravitational collapse of the massive remnant. We discuss how the available gamma-ray and X-ray data provide a probe for the nature of the relativistic ejecta and the surrounding medium.

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arXiv2005

Dynamical evolution of neutrino--cooled accretion disks: detailed microphysics, lepton-driven convection, and global energetics

We present a detailed, two dimensional numerical study of the microphysical conditions and dynamical evolution of accretion disks around black holes when neutrino emission is the main source of cooling. Such structures are likely to form after the gravitational collapse of massive rotating stellar cores, or the coalescence of two compact objects in a binary (e.g., the Hulse--Taylor system). The physical composition is determined self consistently by considering two regimes: neutrino--opaque and neutrino--transparent, with a detailed equation of state which takes into account neutronization, nuclear statistical equilibrium of a gas of free nucleons and alpha particles, blackbody radiation and a relativistic Fermi gas of arbitrary degeneracy. Various neutrino emission processes are considered, with electron/positron capture onto free nucleons providing the dominant contribution to the cooling rate. We find that important temporal and spatial scales, related to the optically thin/optically thick transition are present in the disk, and manifest themselves clearly in the energy output in neutrinos. This transition produces an inversion of the lepton gradient in the innermost regions of the flow which drives convective motions, and affects the density and disk scale height radial profiles. The electron fraction remains low in the region close to the black hole, and if preserved in an outflow, could give rise to heavy element nucleosynthesis. Our specific initial conditions arise from the binary merger context, and so we explore the implications of our results for the production of gamma ray bursts.

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arXiv2005

The State of the Circumstellar Medium Surrounding Gamma-Ray Burst Sources and its Effect on the Afterglow Appearance

We present a numerical investigation of the contribution of the presupernova ejecta of Wolf-Rayet stars to the environment surrounding gamma-ray bursts (GRBs), and describe how this external matter can affect the observable afterglow characteristics. An implicit hydrodynamic calculation for massive stellar evolution is used here to provide the inner boundary conditions for an explicit hydrodynamical code to model the circumstellar gas dynamics. The resulting properties of the circumstellar medium are then used to calculate the deceleration of a relativistic, gas-dynamic jet and the corresponding afterglow light curve produced as the shock wave propagates through the shocked-wind medium. We find that variations in the stellar wind drive instabilities that may produce radial filaments in the shocked-wind region. These comet-like tails of clumps could give rise to strong temporal variations in the early afterglow lightcurve. Afterglows may be expected to differ widely among themselves, depending on the angular anisotropy of the jet and the properties of the stellar progenitor; a wide diversity of behaviors may be the rule, rather than the exception.

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arXiv2005

An Off-Axis Model for GRB 031203

The low luminosity radio emission of the unusually faint GRB 031203 has been argued to support the idea of a class of intrinsically sub-energetic gamma-ray bursts (GRBs), currently comprising two members. While low energy GRBs probably exist, we show that the collective prompt and multiwavelength observations of the afterglow of GRB 031203 do not necessarily require a sub-energetic nature for that event. In fact, the data are more consistent with a typical, powerful GRB seen at an angle of about twice the opening angle of the central jet. The (redshift corrected) peak energy, E_p, of GRB 031203 then becomes ~ 2 MeV, similar to many other GRBs.

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arXiv2005

Supernovae and Gamma-Ray Bursts Powered by Hot Neutrino-Cooled Coronae

Cosmological explosions such as core-collapse supernovae (SNe) and gamma-ray bursts (GRBs) are thought to be powered by the rapid conversion of roughly a solar mass' worth of gravitational binding energy into a comparatively small amount of outgoing observable kinetic energy. A fractional absorption of the emitted neutrinos, the particles which carry away the binding energy, by the expelled matter is a widely discussed mechanism for powering such explosions. Previous work addressing neutrino emission from core-collapse like environments assumes that the outgoing neutrino spectrum closely resembles a black body whose effective temperature is determined by both the rate of energy release and the surface area of the entire body. Unfortunately, this assumption minimizes the net efficiency for both neutrino-driven explosion mechanisms. Motivated by this fact, we qualitatively outline a scenario where a hot corona deforms the neutrino spectrum away from that of a cool thermal emitter. Our primary result is that in principle, a coronal-driven explosion mechanism can enhance the net efficiency of neutrino-driven SNe and GRBs by more than an order of magnitude.

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arXiv2005

Afterglow Observations Shed New Light on the Nature of X-ray Flashes

X-ray flashes (XRFs) and X-ray rich gamma-ray bursts (XRGRBs) share many observational characteristics with long duration GRBs, but the reason for which their prompt emission peaks at lower photon energies, $E_p$, is still under debate. Although many different models have been invoked in order to explain the lower $E_p$ values, their implications for the afterglow emission were not considered in most cases, mainly because observations of XRF afterglows have become available only recently. Here we examine the predictions of the various XRF models for the afterglow emission, and test them against the observations of XRF 030723 and XRGRB 041006, the events with the best monitored afterglow light curves in their respective class. We show that most existing XRF models are hard to reconcile with the observed afterglow light curves, which are very flat at early times. Such light curves are, however, naturally produced by a roughly uniform jet with relatively sharp edges that is viewed off-axis (i.e. from outside of the jet aperture). This type of model self consistently accommodates both the observed prompt emission and the afterglow light curves of XRGRB 041006 and XRF 030723, implying viewing angles $θ_{obs}$ from the jet axis of $(θ_{obs}-θ_0)\sim 0.15θ_0$ and $\sim θ_0$, respectively, where $θ_0\sim 3$ deg is the jet half-opening angle. This suggests that GRBs, XRGRBs and XRFs are intrinsically similar relativistic jets viewed from different angles, corresponding to $γ(θ_{obs}-θ_0)$ of less than 1, between 1 and a few, and more than a few, respectively, where $γ$ is the Lorentz factor. Future observations with Swift could help test this unification scheme in which GRBs, XRGRBs and XRFs share the same basic physics and differ only by their orientation relative to our line of sight.

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arXiv2004

A Deep Search with HST for Late Time Supernova Signatures in the Hosts of XRF 011030 and XRF 020427

X-ray Flashes (XRFs) are, like Gamma-Ray Bursts (GRBs), thought to signal the collapse of massive stars in distant galaxies. Many models posit that the isotropic equivalent energies of XRFs are lower than those for GRBs, such that they are visible from a reduced range of distances when compared with GRBs. Here we present the results of two epoch Hubble Space Telescope imaging of two XRFs. These images taken approximately 45 and 200 days post burst reveal no evidence for an associated supernova in either case. Supernovae such as SN 1998bw would have been visible out to z ~1.5 in each case, while fainter supernovae such as SN 2002ap would have been visible to z ~ 1. If the XRFs lie at such large distances, their energies would not fit the observed correlation between the GRB peak energy and isotropic energy release, in which soft bursts are less energetic. We conclude that, should these XRFs reside at low redshifts ($z<0.6$), either their line of sight is heavily extinguished, or they are associated with extremely faint supernovae, or, unlike GRBs, these XRFs do not have temporally coincident supernovae.

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arXiv2004

Supernova 1993J: a veiled pulsar in a binary system?

The recent report of a binary companion to the progenitor of supernova 1993J may provide important clues for identifying the nature of the nascent compact object. Given the estimates of the progenitor mass, the potential power source is probably a pulsar rather than an accreting black hole. If there is a pulsar, one would expect the rotational luminosity to be stored and reprocessed by the supernova remnant, but no pulsar nebula has yet been seen. The lack of detection of an X-ray synchrotron nebula should be taken as strong evidence against the presence of a bright pulsar. This is because absorption by the surrounding supernova gas should be negligible for the light of the companion star to have been detected. A model is developed here for the luminosity of the pulsar nebula in SN 1993J, which is then used to predict the spin-down power of the putative pulsar. If one exists, it can be providing no more than about 6 x 10^{39} erg/s. With an initial rotation period of 10-30 ms, as extrapolated from young galactic pulsars, the nascent neutron star can have either a weak magnetic field, B < 10^{11} G, or one so strong, B > 10^{15} G, that its spin was rapidly slowed down. The companion star, if bound to the neutron star, should provide ample targets for the pulsar wind to interact and produce high-energy gamma-rays. The expected non-pulsed, GeV signal is calculated; it could be detected by current and future experiments provided that the pulsar wind velocity is similar to that of the Crab Nebula.

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arXiv2004

Implications of the Measured Image Size for the Radio Afterglow of GRB 030329

We use data on the image size of the radio afterglow of GRB 030329 (Taylor et al. 2004) to constrain the physical parameters of this explosion. Together with the observed broad band spectrum, this data over-constrains the physical parameters, thus enabling to test different GRB jet models for consistency. We consider two extreme models for the lateral spreading of the jet: model 1 with relativistic expansion in the local rest frame, and model 2 with little lateral expansion as long as the jet is highly relativistic. We find that both models are consistent with the data for a uniform external medium, while for a stellar wind environment model 1 is consistent with the data but model 2 is disfavored by the data. Our derivations can be used to place tighter constraints on the dynamics and structure of GRB jets in future afterglows, following a denser monitoring campaign for the temporal evolution of their image size.

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