arXiv subjects

P. Meszaros

Publications and source records attributed to P. Meszaros.

At least 73 records · Page 4Linked to original sources

arXiv2004

The Swift Gamma-Ray Burst Mission

The Swift mission, scheduled for launch in early 2004, is a multiwavelength observatory for gamma-ray burst (GRB) astronomy. It is the first-of-its-kind autonomous rapid-slewing satellite for transient astronomy and pioneers the way for future rapid-reaction and multiwavelength missions. It will be far more powerful than any previous GRB mission, observing more than 100 bursts per year and performing detailed X-ray and UV/optical afterglow observations spanning timescales from 1 minute to several days after the burst. The objectives are to determine the origin of GRBs; classify GRBs and search for new types; study the interaction of the ultra-relativistic outflows of GRBs with their surrounding medium; and use GRBs to study the early universe out to z>10. The mission is being developed by a NASA-led international collaboration. It will carry three instruments: a new-generation wide-field gamma-ray (15-150 keV) detector; a narrow-field X-ray telescope; and a narrow-field UV/optical telescope. Redshift determinations will be made for most bursts. In addition to the primary GRB science, the mission will perform a hard X-ray survey to a sensitivity of ~1 mCrab (~2x10^{-11} erg cm^{-2} s^{-1} in the 15-150 keV band), more than an order of magnitude better than HEAO A-4. A flexible data and operations system will allow rapid follow-up observations of all types of high-energy transients, with rapid data downlink and uplink available through the NASA TDRSS system. The mission is currently funded for 2 years of operations and the spacecraft will have a lifetime to orbital decay of ~8 years. [ABRIDGED]

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arXiv2003

Detectability of Long GRB Afterglows from Very High Redshifts

Gamma-ray bursts are promising tools for tracing the formation of high redshift stars, including the first generation. At very high redshifts the reverse shock emission lasts longer in the observer frame, and its importance for detection and analysis purposes relative to the forward shock increases. We consider two different models for the GRB environment, based on current ideas about the redshift dependence of gas properties in galaxies and primordial star formation. We calculate the observed flux as a function of the redshift and observer time for typical GRB afterglows, taking into account intergalactic photoionization and Lyman-$α$ absorption opacity as well as extinction by the Milky Way Galaxy. The fluxes in the X-ray and near IR bands are compared with the sensitivity of different detectors such as Chandra, XMM, Swift XRT and JWST. Using standard assumptions, we find that Chandra, XMM and Swift XRT can potentially detect GRBs in the X-ray band out to very high redshifts $z\gtrsim$ 30. In the K and M bands, the JWST and ground-based telescopes are potentially able to detect GRBs even one day after the trigger out to $z\sim$ 16 and 33, if present. While the X-ray band is insensitive to the external density and to reverse shocks, the near IR bands provide a sensitive tool for diagnosing both the environment and the reverse shock component.

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arXiv2003

High energy neutrinos from magnetars

Magnetars can accelerate cosmic rays to high energies through the unipolar effect, and are also copious soft photon emitters. We show that young, fast-rotating magnetars whose spin and magnetic moment point in opposite directions emit high energy neutrinos from their polar caps through photomeson interactions. We identify a neutrino cut-off band in the magnetar period-magnetic field strength phase diagram, corresponding to the photomeson interaction threshold. Within uncertainties, we point out four possible neutrino emission candidates among the currently known magnetars, the brightest of which may be detectable for a chance on-beam alignment. Young magnetars in the universe would also contribute to a weak diffuse neutrino background, whose detectability is marginal, depending on the typical neutrino energy.

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arXiv2003

Gamma-ray bursts as X-ray depth-gauges of the Universe

We discuss the X-ray flux of gamma-ray burst afterglows at redshifts in the range 3-30, including the effects of the intergalactic He II absorption. We point out that strong X-ray lines may form locally in burst afterglows starting minutes after the trigger. This can provide distinctive X-ray distance indicators out to the redshifts where the first generation of massive stars form.

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arXiv2003

Iron K Lines from Gamma Ray Bursts

We present models for reprocessing of an intense flux of X-rays and gamma rays expected in the vicinity of gamma ray burst sources. We consider the transfer and reprocessing of the energetic photons into observable features in the X-ray band, notably the K lines of iron. Our models are based on the assumption that the gas is sufficiently dense to allow the microphysical processes to be in a steady state, thus allowing efficient line emission with modest reprocessing mass and elemental abundances ranging from solar to moderately enriched. We show that the reprocessing is enhanced by down-Comptonization of photons whose energy would otherwise be too high to absorb on iron, and that pair production can have an effect on enhancing the line production. Both "distant" reprocessors such as supernova or wind remnants and "nearby" reprocessors such as outer stellar envelopes can reproduce the observed line fluxes with Fe abundances 30-100 times above solar, depending on the incidence angle. The high incidence angles required arise naturally only in nearby models, which for plausible values can reach Fe line to continuum ratios close to the reported values.

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arXiv2003

High Energy Photons, Neutrinos and Gravitational Waves from Gamma-Ray Bursts

Most of the current knowldege about GRB is based on electromagnetic observations at MeV and lower energies. Here we focus on some recent theoretical work on GRB, in particular the higher energy (GeV-TeV) photon emission, and two potentially important non-electromagnetic channels, the TeV and higher energy neutrino signals, and the gravitational wave signals expected from GRB.

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arXiv2003

GRB 021004: a Massive Progenitor Star Surrounded by Shells

We present spectra of the optical transient of GRB021004 obtained with the Hobby-Eberly telescope starting 15.48, 20.31 hours, and 4.84 days after the burst and a spectrum obtained with the H. J. Smith 2.7 m Telescope starting 14.31 hours after the burst. GRB021004 is the first afterglow whose spectrum is dominated by absorption lines from high ionization species with multiple velocity components separated by up to 3000 km/s. We argue that these lines are likely to come from shells around a massive progenitor star. The high velocities and high ionizations arise from a combination of acceleration and flash-ionization by the burst photons and the wind velocity and steady ionization by the progenitor. We also analyze the broad-band spectrum and the light curve. We distinguish six components along the line of sight: (1) The z~2.293 absorption lines arise from the wind of a massive star. For a mass loss rate of ~6 x 10^{-5} solar masses per year, this component also provides the external medium to create the afterglow light. (2) A second shell produces absorption lines with a relative velocity of 560 km/s, and this is associated with the shell created by the fast massive star wind blowing a bubble in the preceding slow wind at a radial distance of order 10 pc. (3) More distant clouds within the host galaxy lie between 30-2500 pc, where they have been ionized by the burst. (4-6) The massive star wind has clumps with radii and over-densities of 0.022, 0.063, and 0.12 parsecs and 50%, 10%, and 10% respectively. The immediate progenitor of the burster could either be a WC-type Wolf-Rayet star or a highly evolved star whose original mass was just too small for it to become a WN-type Wolf-Rayet star.

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arXiv2003

'On the difference between the short and long gamma-ray bursts'

We argue that the distributions of both the intrinsic fluence and the intrinsic duration of the gamma-ray emission in gamma-ray bursts from the BATSE sample are well represented by log-normal distributions, in which the intrinsic dispersion is much larger than the cosmological time dilatation and redshift effects. We perform separate bivariate log-normal distribution fits to the BATSE short and long burst samples. The bivariate log-normal behaviour results in an ellipsoidal distribution, whose major axis determines an overall statistical relation between the fluence and the duration. We show that this fit provides evidence for a power-law dependence between the fluence and the duration, with a statistically significant different index for the long and short groups. We discuss possible biases, which might affect this result, and argue that the effect is probably real. This may provide a potentially useful constraint for models of long and short bursts.

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arXiv2002

Collapsar Uncorking and Jet Eruption in GRB

We show that the collapsar model of gamma-ray bursts results in a series of successive shocks and rarefaction waves propagating in the "cork" of stellar material being pushed ahead of the jet, as it emerges from the massive stellar progenitor. These shocks result in a series of characteristic, increasingly shorter and harder thermal X-ray pulses, as well as a non-thermal gamma-ray pulse, which precede the usual non-thermal MeV gamma-rays. We consider jets escaping from both compact (CO or He dwarf) and blue supergiant stellar progenitors. The period, fluence and hardness of the pulses serves as a diagnostic for the size and density of the outer envelope of the progenitor star.

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arXiv2002

GeV Emission from TeV Blazars and Intergalactic Magnetic Fields

Several high-frequency peaked BL Lac objects such as Mrk 501 are strong TeV emitters. However, a significant fraction of the TeV gamma rays emitted are likely to be absorbed in interactions with the diffuse IR background, yielding electron-positron pairs. Hence, the observed TeV spectrum must be steeper than the intrinsic one. Using the recently derived intrinsic $γ$-ray spectrum of Mrk 501 during its 1997 high state, we study the inverse-Compton scattering of cosmic microwave photons by the resulting electron-positron pairs, which implies the existence of a hitherto undiscovered GeV emission. The typical duration of the GeV emission is determined by the flaring activity time and the energy-dependent magnetic deflection time. We numerically calculate the scattered photon spectrum for different intergalactic magnetic field (IGMF) strengths, and find a spectral turnover and flare duration at GeV energies which are dependent on the field strength. We also estimate the scattered photon flux in the quiescent state of Mrk 501. The GeV flux levels predicted are consistent with existing EGRET upper limits, and should be detectable above the synchrotron -- self Compton (SSC) component with the {\em Gamma-Ray Large Area Space Telescope} ({\em GLAST}) for IGMFs $\lesssim 10^{-16}$ G, as expected in voids. Such detections would provide constraints on the strength of weak IGMFs.

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arXiv2002

X-ray Rich GRB, Photospheres and Variability

We investigate the relationship between the quasi-thermal baryon-related photosphere in relativistic outflows, and the internal shocks arising outside them, which out to a limiting radius may be able to create enough pairs to extend the optically thick region. Variable gamma-ray light curves are likely to arise outside this limiting pair-forming shock radius, while X-ray excess bursts may arise from shocks occurring below it; a possible relation to X-ray flashes is discussed. This model leads to a simple physical interpretation of the observational gamma-ray variability-luminosity relation.

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arXiv2001

Theories of Gamma-Ray Bursts

The gamma ray burst phenomenon is reviewed from a theoretical point of view, with emphasis on the fireball shock scenario of the prompt emission and the longer wavelenght afterglow. Recent progress and issues are discussed, including spectral-temporal evolution, localizations, jets, spectral lines, environmental and cosmological aspects, as well as some prospects for future experiments in both electromagnetic and non-electromagnetic channels.

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arXiv2001

TeV Neutrinos from Successful and Choked Gamma-Ray Bursts

Core collapse of massive stars resulting in a relativistic fireball jet which breaks through the stellar envelope is a widely discussed scenario for gamma-ray burst production. For very extended or slow rotating stars, the fireball may be unable to break through the envelope. Both penetrating and choked jets will produce, by photo-meson interactions of accelerated protons, a burst of neutrinos with energies in excess of 5 TeV while propagating in the envelope. The predicted flux, from both penetrating and chocked fireballs, should be easily detectable by planned cubic kilometer neutrino telescopes.

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arXiv2001

Collapsar Jets, Bubbles and Fe Lines

In the collapsar scenario, gamma ray bursts are caused by relativistic jets expelled along the rotation axis of a collapsing stellar core. We discuss how the structure and time-dependence of such jets depends on the stellar envelope and central engine properties, assuming a steady jet injection. It takes a few seconds for the jet to bore its way through the stellar core; most of the energy output during that period goes into a cocoon of relativistic plasma surrounding the jet. This material subsequently forms a bubble of magnetized plasma that takes several hours to expand, subrelativistically, through the envelope of a high-mass supergiant. Jet break-through and a conventional burst would be expected not only in He stars but possibly also in blue supergiants. Shock waves and magnetic dissipation in the escaping bubble can contribute a non thermal UV/X-ray afterglow, and also excite Fe line emission from thermal gas, in addition to the standard jet deceleration power-law afterglow.

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arXiv2001

The X-ray afterglow of GRB000926 observed by BeppoSAX and Chandra: a mildly collimated fireball in a dense medium ?

We present X-ray observations of the afterglow of GRB000926 performed around and after the break observed in the optical light curve two days after the burst. The steep X-ray light curve observed around the break confirms the presence of this feature in X-rays. However, the spectral and temporal properties are not consistent with a standard jet scenario based on synchrotron emission, requiring a more complicated model. We find that X-ray and optical data are compatible with a moderately collimated fireball (with opening angle $θ\approx25 °$) expanding in a dense medium ($n\approx 4\times 10^4$ cm$^{-3}$). This produces two breaks in the light curve. The first, at $t\approx$ 2 days, is due to jet behavior. The second, around 5 days, is attributed to the transition of the fireball to a non-relativistic expansion. This transition predicts a{\it flattening} of the light curve, that explains the late X-ray measurement in excess above the extrapolation of the simple jet scenario, and is also consistent with optical data.

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arXiv2001

A Possible Intrinsic Fluence-Duration Power-Law Relation in Gamma-ray Bursts

We argue that the distributions of both the intrinsic fluence and the intrinsic duration of the gamma-ray emission in gamma-ray bursts from the BATSE sample are well represented by log-normal distributions, in which the intrinsic dispersion is much larger than the cosmological time dilatation and redshift effects. We perform separate bivariate log-normal distribution fits to the BATSE short and long burst samples. The bivariate log-normal behavior results in an ellipsoidal distribution, whose major axis determines an overall statistical relation between the fluence and the duration. We show that this fit provides evidence for a power-law dependence between the fluence and the duration, with a statistically significant different index for the long and short groups. We discuss possible biases which might affect this result, and argue that the effect is probably real. This may provide a potentially useful constraint for models of long and short bursts.

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arXiv2001

Electron-Positron Pair Cascades and Precursors in Gamma-Ray Bursts

Gamma-ray burst sources with a high luminosity can produce electron-positron pair cascades in their environment as a result of back-scattering of a seed fraction of their original hard spectrum. The resulting spectral modifications offer the possibility of diagnosing not only the compactness parameter of the gamma-ray emitting region but also the baryonic density of the environment external to the burst.

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arXiv2001

GRB and Environment Interaction

We discuss three aspects of the interaction between GRB and their surroundings. The illumination of the progenitor remnant and/or the surroundings by the X-ray afterglow continuum can produce substantial Fe K-alpha line and edge emission, with implications for the progenitor model. The presence of large dust column densities, capable of obscuring the GRB optical afterglow, will lead to characteristic delayed X-ray and far-IR light curve signatures. Pair production induced by the initial gamma-rays in the nearby environment will modify the initial spectrum and the afterglow light curve, and the magnitude of these changes provides a diagnostic for the external density.

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