Burst Progenitors Research Articles

The BeppoSAX catalog of GRB X-ray afterglow observations

We present the catalog of X-ray afterglow observed by BeppoSAX from the launch of the satellite to the end of the mission. Thirty-three X-ray afterglows out of 39 observations were securely identified based on their fading behavior. We have extracted the continuum parameters (decay index, spectral index, flux, absorption) for all available afterglows. We point out a possible correlation between the X-ray afterglow luminosity and the energy emitted during the prompt γ-ray event. We do not detect a significant jet signature within the afterglows, implying a lower limit on the beaming angle, nor a standard energy release when X-ray fluxes are corrected for beaming. Our data support the hypothesis that the burst should be surrounded by an interstellar medium rather than a wind environment, and that this environment should be dense. This may be explained by a termination shock located near the burst progenitor. We finally point out that some dark bursts may be explained by an intrinsic faintness of the event, while others may be strongly absorbed.

Evidence for intrinsic absorption in the Swift X-ray afterglows

Gamma-ray burst (GRB) progenitors are observationally linked to the death of massive stars. X-ray studies of the GRB afterglows can deepen our knowledge of the ionization status and metal abundances of the matter in the GRB environment. Moreover, the presence of local matter can be inferred through its fingerprints in the X-ray spectrum, i.e. the presence of absorption higher than the Galactic value. A few studies based on BeppoSAX and XMM-Newton found evidence of higher than Galactic values for the column density in a number of GRB afterglows. Here we report on a systematic analysis of 17 GRBs observed by Swift up to April 15, 2005. We observed a large number of GRBs with an excess of column density. Our sample, together with previous determinations of the intrinsic column densities for GRBs with known redshift, provides evidence for a distribution of absorption consistent with that predicted for randomly occurring GRB within molecular clouds.

Closing in on a Short‐Hard Burst Progenitor: Constraints from Early‐Time Optical Imaging and Spectroscopy of a Possible Host Galaxy of GRB 050509b

The localization of the short-duration, hard-spectrum gamma-ray burst GRB 050509b by the Swift satellite was a watershed event. We report the discovery of the probable host galaxy, a bright elliptical galaxy at z = 0.2248. This is the first known redshift and host of a short-hard GRB and shows that at least some short-hard GRBs are cosmological in origin. We began imaging the GRB field 8 minutes after the burst and continued for 8 days. We present a reanalysis of the XRT afterglow and report the absolute position of the GRB. Based on positional coincidences, the GRB and the elliptical are likely to be physically related, unlike any known connection between a long-duration GRB and an early-type galaxy. Similarly unique, GRB 050509b likely also originated from within a rich cluster of galaxies with detectable diffuse X-ray emission. We demonstrate that while the burst was underluminous, the ratio of the blast wave energy to the γ-ray energy is consistent with that of long-duration GRBs. Based on this analysis, on the location of the GRB (40 ± 13 kpc from the putative host), on the galaxy type (elliptical), and the lack of a coincident supernova, we suggest that there is now observational support for the hypothesis that short-hard bursts arise during the merger of a compact binary. We limit the properties of any Li-Paczyński "minisupernova" that is predicted to arise on ~1 day timescales. Other progenitor models are still viable, and new Swift bursts will undoubtedly help to further clarify the progenitor picture.

Collisions and close encounters involving massive main-sequence stars

We study close encounters involving massive main sequence stars and the evolution of the exotic products of these encounters as common--envelope systems or possible hypernova progenitors. We show that parabolic encounters between low-- and high--mass stars and between two high--mass stars with small periastrons result in mergers on timescales of a few tens of stellar freefall times (a few tens of hours). We show that such mergers of unevolved low--mass stars with evolved high--mass stars result in little mass loss ($\sim0.01$ M$_{\odot}$) and can deliver sufficient fresh hydrogen to the core of the collision product to allow the collision product to burn for several million years. We find that grazing encounters enter a common--envelope phase which may expel the envelope of the merger product. The deposition of energy in the envelopes of our merger products causes them to swell by factors of $\sim100$. If these remnants exist in very densely-populated environments ($n\gtrsim10^{7}$ pc$^{-3}$), they will suffer further collisions which may drive off their envelopes, leaving behind hard binaries. We show that the products of collisions have cores rotating sufficiently rapidly to make them candidate hypernova/gamma--ray burst progenitors and that $\sim0.1%$ of massive stars may suffer collisions, sufficient for such events to contribute significantly to the observed rates of hypernovae and gamma--ray bursts.

Could electron-positron annihilation lines in the Galactic center result from pulsar winds?

Observations of a strong and extended positron-electron annihilation line emission in the Galactic center (GC) region by the Spectrometer on the International Gamma-Ray Astrophysical Laboratory (SPI/INTEGRAL) are challenging to the existing models of positron sources in the Galaxy. In this paper, we study the possibility that pulsar winds in the GC produce the 511 keV line. We propose that three possible scenarios of pulsar winds may exist as the positron sources: normal pulsars, rapidly spinning strongly magnetized neutron stars (magnetars) in gamma-ray burst (GRB) progenitors, a population of millisecond pulsars in the Galactic center. These e ± pairs could be trapped in the region by the magnetic field in the Galactic center, and cool through synchrotron radiation and Coulomb interactions with the medium, thereby becoming non-relativistic particles. The cooling timescales are shorter than the diffuse timescale of positrons, so low-energy positrons could annihilate directly with electrons into 511 keV photons or could form positronium before annihilation. We find that normal pulsars cannot be a significant contributor to the positron sources. Although magnetars in the GC could be potential sources of positrons, their birth rate and birth locations may pose some problems for this scenario. We believe that the most likely candidates for positron sources in the GC may be a population of millisecond pulsars in the GC. Our preliminary estimations predict that the e ± annihilation rate in the GC is >5 x 10 42 s -1 , which is consistent with the present observational constraints. Therefore, the e ± pairs from pulsars winds can contribute significantly to the positron sources in the Galactic center region. Furthermore, since the diffusion length of positrons is short, we predict that the intensity distribution of the annihilation line should follow the distribution of millisecond pulsars, which should then correlate to the mass distribution in the GC.

Which massive stars are gamma-ray burst progenitors?

The collapsar model for gamma-ray bursts requires three essential ingredients: a massive core, removal of the hydrogen envelope, and enough angular momentum in the core. We study current massive star evolution models of solar metallicity to determine which massive star physics is capable of producing these ingredients. In particular, we investigate the role of hydrodynamic and magnetic internal angular momentum transport and binary mass and angular momentum transfer. We follow the evolution of rotating single stars and of binary systems that include rotational processes for both stars. Neglecting magnetic fields, we show that the cores of massive single stars can maintain a high specific angular momentum when evolved with the assumption that mean molecular weight gradients suppress rotational mixing processes. In binary systems that undergo mass transfer during core hydrogen burning the mass receiving star accretes large amounts of high angular momentum material, leading to a spin-up of the core. We find, however, that this merely compensates for the tidal angular momentum loss due to spin-orbit coupling, which leads to synchronous rotation before the mass transfer event. Therefore the resulting cores do not rotate faster than in single stars. We also present models that include magnetic fields generated by differential rotation and we consider the internal angular momentum transport by magnetic torques. We investigate the capability of magnetic torques to efficiently pump angular momentum into the cores of accreting stars. Despite our finding that this mechanism works, the magnetic coupling of core and envelope after the accreting star ends core hydrogen burning leads to slower rotation than in the non-magnetic case.

Binary Merger Progenitors for Gamma‐Ray Bursts and Hypernovae

The collapsar model, the now leading model for the engine behind gamma-ray bursts and hypernovae, requires that a star collapses to form a black hole surrounded by an accretion disk of high-angular momentum material. The current best theoretical stellar models, however, do not retain enough angular momentum in the core of the star to make a centrifugally supported disk. In this paper, we present the first calculations of the helium-star/helium-star merger progenitors for the collapsar model. These progenitors invoke the merger of two helium cores during the common envelope inspiral phase of a binary system. We find that, in some cases, the merger can produce cores that are rotating 3-10 times faster than single stars. He-star/He-star gamma-ray burst progenitors have a very different redshift distribution than their single-star gamma-ray burst progenitors and we discuss how gamma-ray burst observations can constrain these progenitors.

The Surroundings of Gamma-Ray Bursts: Constraints on Progenitors

SummaryThe association of a supernova with a gamma-ray burst (GRB 030329) implies a massive star progenitor, which is expected to have an environment formed by pre-burst stellar winds. Although some sources are consistent with the expected wind environment, many are not, being better fit by a uniform density environment. One possibility is that this is a shocked wind, close to the burst because of a high interstellar pressure and a low mass loss density. Alternatively, there is more than one kind of burst progenitor, some of which interact directly with the interstellar medium. Another proposed environment is a pulsar wind bubble that has expanded inside a supernova, which requires that the supernova precede the burst.

The ultimate outcome of black hole-neutron star mergers

We present a simple, semi--analytical description for the final stages of mergers of black hole (BH) -- neutron star (NS) systems. Such systems are of much interest as gravitational wave sources and gamma--ray burst progenitors. Numerical studies show that in general the neutron star is not disrupted at the first phase of mass transfer. Instead, what remains of the neutron star is left on a wider, eccentric, orbit. We consider the evolution of such systems as they lose angular momentum via gravitational radiation and come into contact for further phases of mass transfer. During each mass transfer event the neutron star mass is reduced until a critical value where mass loss leads to a rapid increase in the stellar radius. At this point Roche lobe overflow shreds what remains of the neutron star, most of the mass forming a disc around the black hole. Such a disc may be massive enough to power a gamma--ray burst. The mass of the neutron star at the time of disruption (and therefore the disc mass) is largely independent of the initial masses of the black hole and neutron star, indicating that BH--NS star mergers may be standard candles.

Probing a Gamma-Ray Burst Progenitor at a Redshift of z = 2: A Comprehensive Observing Campaign of the Afterglow of GRB 030226

Probing a Gamma-Ray Burst Progenitor at a Redshift of z = 2: A Comprehensive Observing Campaign of the Afterglow of GRB 030226

Multiwavelength Afterglows of Gamma-Ray Bursts

Our knowledge of Gamma-Ray Burst (GRB) progenitors is based on three cases at relatively low redshift (between 0.01 and 0.2) in which the association with a supernova (SN) has been firmly established. In a number of higher redshift GRBs the presence of a SN has been suggested, although the properties of the SN could not be precisely determined. However, the study of several tens of multiwavelength afterglows has now provided evidence that GRBs are associated with star formation. The observational results which point to this connection are reviewed, and the high energy properties of afterglows and SNe are compared.

The Nature of GRB 980425 and the Search for Off‐Axis Gamma‐Ray Burst Signatures in Nearby Type Ib/c Supernova Emission

The identification of Type Ib/c supernovae (SNe Ib/c) as gamma-ray burst (GRB) progenitors is motivated by the association of GRB 980425 with SN 1998bw and of GRB 030329 with SN 2003dh. While the gamma-ray luminosity of GRB 030329 was typical to cosmological GRBs, the luminosity of the nearby (40 Mpc) GRB 980425 was ~5 orders of magnitude lower. The large luminosity difference is commonly explained by hypothesizing either that SNe Ib/c produce two different classes of GRBs or that GRB 980425 was a typical cosmological GRB jet viewed off-axis. In the latter scenario, strong radio emission, Lν ~ 1030ν ergs s-1 Hz-1, is expected at ~1 yr delay as a result of jet deceleration to subrelativistic speed, as observed from GRB 970508. The radio luminosity of SN 1998bw was 3 orders of magnitude lower than this value. We show that the low radio flux may be consistent with the off-axis jet interpretation, if the density of the wind surrounding the progenitor is lower than typically expected, ≡ (/10-5 M☉ yr-1)/(vw/103 km s-1) 0.1 instead of 1. The lower value of is consistent with the observed radio emission from the SN shock driven into the wind. This interpretation predicts transition to subrelativistic expansion at ~10 yr delay, with current ≈1 mJy 10 GHz flux and mV ≈ 23 optical flux, and with ≈10 mas angular source size. It also implies that in order to search for the signature of off-axis GRBs associated with nearby SNe Ib/c, follow-up observations should be carried out on a multiyear timescale.

The Role of Helium Stars in the Formation of Double Neutron Stars

We have calculated the evolution of 60 model binary systems consisting of helium stars in the mass range of MHe = 2.5-6 M☉ with a 1.4 M☉ neutron star companion to investigate the formation of double neutron star systems. Orbital periods ranging from 0.09 to 2 days are considered, corresponding to Roche lobe overflow starting from the helium main sequence to after the ignition of carbon burning in the core. We have also examined the evolution into a common envelope phase via secular instability, delayed dynamical instability, and the consequence of matter filling the neutron star's Roche lobe. The survival of some close He-star neutron star binaries through the last mass transfer episode (either dynamically stable or unstable mass transfer phase) leads to the formation of extremely short-period double neutron star systems (with P ≲ 0.1 days). In addition, we find that systems throughout the entire calculated mass range can evolve into a common envelope phase, depending on the orbital period at the onset of mass transfer. The critical orbital period below which common envelope evolution occurs generally increases with MHe. In addition, a common envelope phase may occur during a short time for systems characterized by orbital periods of 0.1-0.5 days at low He-star masses (~2.6-3.3 M☉). The existence of a short-period population of double neutron stars increases the predicted detection rate of in-spiral events by ground-based gravitational-wave detectors and impacts their merger location in host galaxies and their possible role as γ-ray burst progenitors. We use a set of population synthesis calculations and investigate the implications of the mass transfer results for the orbital properties of DNS populations.

The late stages of evolution of helium star-neutron star binaries and the formation of double neutron star systems

With a view to understanding the formation of double neutron star binaries, we investigate the late stages of evolution of helium stars with masses of 2.8–6.4 M⊙ in binary systems with a 1.4-M⊙ neutron star companion. We found that mass transfer from 2.8- to 3.3-M⊙ helium stars (originating from main-sequence stars with masses of 10–12 M⊙ that underwent case B evolution, or 9–10 M⊙ that experienced case C mass transfer) as well as from 3.3 to 3.8 M⊙ in very close orbits (Porb≲ 0.25 d) will end up in a common-envelope and spiral-in phase due to the development of a convective helium envelope at the end of the calculation. If the neutron star has sufficient time to complete the spiralling-in process in the envelope of the helium star before the core collapses, the system will produce very tight double neutron star binaries (Porb ∼ 0.01 d) with a very short merger time-scale, i.e. of the order of 1 Myr or less. These systems would have important consequences for the detection rate of gravitational-wave radiation and for the understanding of γ-ray burst progenitors. On the other hand, if the time left until the explosion is shorter than the orbital-decay time-scale, the system will undergo a supernova (SN) explosion during the common-envelope phase. Helium stars with masses 3.3–3.8 M⊙ in wider orbits (Porb≳ 0.25 d) and those more massive than 3.8 M⊙ do not develop a convective envelope and therefore are not expected to go through common-envelope evolution. The remnants of these massive helium stars are double neutron star pulsars with periods in the range of 0.1–1 d. This suggests that this range of mass (originating from main-sequence stars more massive than 12 M⊙ that underwent case B evolution, or more massive than 10 M⊙ that experienced case C mass transfer) includes the progenitors of the galactic double neutron star pulsars with close orbits (B1913+16 and B1534+12). A minimum kick velocity of 70 and 0 km s−1 (for B1913+16 and B1534+12, respectively) must have been imparted at the birth of the companion to the pulsar. The double neutron stars with wider orbits (J1518+4904 and probably J1811−1736) are produced from helium star-neutron star binaries that avoid Roche lobe overflow, with the helium star being more massive than 2.5 M⊙, i.e. the remnants of main-sequence stars more massive than 10 M⊙ in relatively wide orbits. For these systems, the minimum kick velocities are 50 and 10 km s−1 (for J1518+4904 and J1811−1736, respectively).

Gravitational Radiation from Gamma‐Ray Burst Progenitors

We study gravitational radiation from various proposed gamma-ray burst (GRB) progenitor models, in particular compact mergers and massive stellar collapses. These models have in common a high angular rotation rate, and the final stage involves a rotating black hole and accretion disk system. We consider the in-spiral, merger and ringing phases, and for massive collapses we consider the possible effects of asymmetric collapse and break-up, as well bar-mode instabilities in the disks. We calculate the strain and frequency of the gravitational waves expected from various progenitors, at distances based on occurrence rate estimates. Based on simplifying assumptions, we give estimates of the probability of detection of gravitational waves by the advanced LIGO system from the different GRB scenarios.

Galactic distribution of merging neutron stars and black holes - prospects for short gamma-ray burst progenitors and LIGO/VIRGO

We have performed detailed population synthesis on a large number (20 million) of binary systems in order to investigate the properties of massive double degenerate binaries. We have included new important results in our input physics in order to obtain more reliable estimates of the merging timescales and relative formation rates. These improvements include refined treatment of the binding energy in a common envelope, helium star evolution and reduced kicks imparted to newborn black holes. The discovery and observations of GRB afterglows and the identification of host galaxies have allowed comparisons of theoretical distributions of merger sites with the observed distribution of afterglow positions relative to host galaxies. To help investigate the physical nature of short- and long-duration GRBs, we compute the distances of merging neutron stars (NS) and/or black holes (BH) from the centers of their host galaxies, as predicted by their formation scenario combined with motion in galactic potentials. Furthermore, we estimate the formation rate and merging rate of these massive double degenerate binaries. The latter is very important for the prospects of detecting gravitational waves with LIGO/VIRGO. We find that the expected detection rate for LIGO II is ~850 per year for galactic field sources and that this rate is completely dominated by merging BHBH binaries. Even LIGO I may detect such an event (~0.25 per year). Our preferred model estimate the Galactic field NSNS merger rate to be ~1.5*10^(-6) per year. For BHBH systems this model predicts a merger rate of ~9.7*10^{-6} per year. Our studies also reveal an accumulating numerous population of very wide orbit BHBH systems which never merge (t >> t_Hubble).

Formation of the binary pulsars J1141–6545 and B2303+46

The binaries PSR J1141-6545 and PSR B2303+46 each appear to contain a white dwarf which formed before the neutron star. We describe an evolutionary pathway to produce these two systems. In this scenario, the primary transfers its envelope onto the secondary which is then the more massive of the two stars, and indeed sufficiently massive later to produce a neutron star via a supernova. The core of the primary produces a massive white dwarf which enters into a common envelope with the core of the secondary when the latter evolves off the main sequence. During the common envelope phase, the white dwarf and the core of the secondary spiral together as the envelope is ejected. The evolutionary history of PSR J1141-6545 and PSR B2303+46 differ after this phase. In the case of PSR J1141--6545, the secondary (now a helium star) evolves into contact transferring its envelope onto the white dwarf. We propose that the vast majority of this material is in fact ejected from the system. The remains of the secondary then explode as a supernova producing a neutron star. Generally the white dwarf and neutron star will remain bound in tight, often eccentric, systems resembling PSR J1141-6545. These systems will spiral in and merge on a relatively short timescale and may make a significant contribution to the population of gamma ray burst progenitors. In PSR B2303+46, the helium-star secondary and white dwarf never come into contact. Rather the helium star loses its envelope via a wind, which increases the binary separation slightly. Only a small fraction of such systems will remain bound when the neutron star is formed (as the systems are wider). Those systems which are broken up will produce a population of high-velocity white dwarfs and neutron stars.

Merger Sites of Double Neutron Stars and Their Host Galaxies

Using the StarTrack population synthesis code, we analyze the formation channels possibly available to double neutron star binaries and find that they can be richer than previously thought. We identify a group of short-lived, tight binaries, which do not live long enough to escape their host galaxies, despite their large center-of-mass velocities. We present our most recent results on all possible evolutionary paths leading to the formation of double neutron stars, calculate their coalescence rates, and also revisit the question of the distribution of merger sites around host galaxies. For a wide variety of binary evolution models and galaxy potentials, we find that most of the neutron star mergers take place within galaxies. Our results stem from allowing for radial and common envelope evolution of helium-rich stars (testable in the future with detailed stellar-structure and hydrodynamic calculations) and indicate that double neutron star binaries may not be excluded as gamma-ray burst progenitors solely on the basis of their spatial distribution around host galaxies. We also find, in contrast to Bethe & Brown, that in a significant fraction of common envelope (CE) phases, neutron stars do not accrete enough material to become black holes, and thus the channels involving CEs are still open for the formation of double neutron stars.

Transient Absorption Features in Gamma‐Ray Bursts and Their Implications for Gamma‐Ray Burst Progenitors

The recent detection of a transient absorption feature in the prompt emission of GRB 990705 has sparked multiple attempts to fit this feature in terms of photoelectric absorption or resonance scattering out of the line of sight to the observer. However, the physical conditions required to reproduce the observed absorption feature turn out to be rather extreme compared to the predictions of current gamma-ray burst (GRB) progenitor models. In particular, strong clumping of ejecta from the GRB progenitor seems to be required. Using detailed three-dimensional hydrodynamic simulations of supernova explosions as a guideline, we have investigated the dynamics and structure of pre-GRB ejecta predicted in various GRB progenitor models. Based on our results, combined with population synthesis studies relevant to the He-merger model, we estimate the probability of observing X-ray absorption features as seen in GRB 990705 to 1%. Alternatively, if the supranova model is capable of producing highly collimated, long-duration GRBs, it may be a more promising candidate to produce observable, transient X-ray absorption features.

Time-dependent photoionization opacities in dense gamma-ray burst environments

The recent detection of a transient absorption feature in the X–ray prompt emission of GRB 990705 showed the importance of such observations in the understanding of gamma–ray bursts and their progenitors. We investigate the time dependence of photoionization edges during the prompt emission of bursts in different environments. We show that their variability can be used to infer the density and geometry of the surrounding medium, giving important clues to unveil the nature of the burst progenitor.