arXiv subjects

Robert Mochkovitch

Publications and source records attributed to Robert Mochkovitch.

At least 19 recordsLinked to original sources

arXiv2024

Ending the prompt phase in photospheric models of gamma-ray bursts

The early steep decay, a rapid decrease in X-ray flux as a function of time following the prompt emission, is a robust feature seen in almost all gamma-ray bursts with early enough X-ray observations. This peculiar phenomenon has often been explained as emission from high latitudes of the last flashing shell. However, in photospheric models of gamma-ray bursts, the timescale of high-latitude emission is generally short compared to the duration of the steep decay phase, and hence an alternative explanation is needed. In this paper, we show that the early steep decay can directly result from the final activity of the dying central engine. We find that the corresponding photospheric emission can reproduce both the temporal and spectral evolution observed. This requires a late-time behaviour that should be common to all GRB central engines, and we estimate the necessary evolution of the kinetic power and the Lorentz factor. If this interpretation is correct, observation of the early steep decay can grant us insights into the last stages of central activity, and provide new constraints on the late evolution of the Lorentz factor and photospheric radius.

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arXiv2023

The Signature of Refreshed Shocks in the of Afterglow of GRB030329

GRB030329 displays one clear and, possibly, multiple less intense fast-rising ($Δt / t \sim 0.3$) jumps in its optical afterglow light curve. The decay rate of the optical light curve remains the same before and after the photon flux jumps. This may be the signature of energy injection into the forward and reverse shocked material at the front of the jet. In this study, we model the Gamma-Ray Burst (GRB) ejecta as a series of shells of material. We follow the dynamical evolution of the ejecta as it interacts with itself (i.e., internal shocks) and with the circumburst medium (i.e., external forward and reverse shocks), and we calculate the emission from each shock event assuming synchrotron emission. We confirm the viability of the model proposed by \citet{2003Natur.426..138G} in which the jumps in the optical afterglow light curve of GRB030329 are produced via refreshed shocks. The refreshed shocks may be the signatures of the collisions between earlier ejected material with an average Lorentz factor $\barΓ\gtrsim 100$ and later ejected material with $\barΓ \sim 10$ once the early material has decelerated due to interaction with the circumburst medium. We show that even if the late material is ejected with a spread of Lorentz factors, internal shocks naturally produce a narrow distribution of Lorentz factors ($ΔΓ/Γ\lesssim0.1$), which is a necessary condition to produce the observed quick rise times of the jumps. These results imply a phase of internal shocks at some point in the dynamical evolution of the ejecta, which requires a low magnetization in the outflow.

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arXiv2022

Flares in gamma-ray burst X-ray afterglows as prompt emission from slightly misaligned structured jets

We develop a model to explain the flaring activity in gamma-ray burst X-ray afterglows within the framework of slightly misaligned observers to structured jets. We suggest that flares could be the manifestation of prompt dissipation within the core of the jet, appearing to a misaligned observer in the X-ray band because of less favorable Doppler boosting. These flares appear during the afterglow phase because of core--observer light travel delays. In this picture, the prompt emission recorded by this observer comes from material along their line of sight, in the lateral structure of the jet, outside the jet's core. We start by laying down the basic analytical framework to determine the flares characteristics as a function of those of the gamma-ray pulse an aligned observer would see. We show that there is viable parameter space to explain flares with typical observing times and luminosities. We then analytically explore this model, showing that it naturally produces flares with small aspect ratios, as observed. We perform fits of our model to two $Swift$/XRT flares representing two different types of morphology, to show that our model can capture both. The ejection time of the core jet material responsible of the flare is a critical parameter. While it always remains small compared to the observed time of the flare, confirming that our model does not require very late central engine activity, late ejection times are strongly favored, sometimes larger than the observed duration of the parent gamma-ray burst's prompt emission as measured by $T_{90}$.

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arXiv2021

Prospects for kilonova signals in the gravitational-wave era

The binary neutron star merger gravitational-wave signal GW170817 was followed by three electromagnetic counterparts, including a kilonova arising from the radioactivity of freshly synthesized $r$-process elements in ejecta from the merger. Finding kilonovae after gravitational-wave triggers is crucial for (i) the search for further counterparts, such as the afterglow, (ii) probing the diversity of kilonovae and their dependence on the system's inclination angle, and (iii) building a sample for multi-messenger cosmology. During the third observing run of the gravitational-wave interferometer network, no kilonova counterpart was found. We aim to predict the expected population of detectable kilonova signals for the upcoming O4 and O5 observing runs of the LIGO-Virgo-KAGRA instruments. Using a simplified criterion for gravitational-wave detection and a simple GW170817-calibrated model for the kilonova peak magnitude, we determine the rate of kilonovae in reach of follow-up campaigns and their distributions in magnitude for various bands. We briefly consider the case of GW190425, the only binary neutron star merger confirmed since GW170817, and obtain constraints on its inclination angle from the non-detection of its kilonova, assuming the source was below the follow-up thresholds. We also show that non-gravitational-wave-triggered kilonovae can be a numerous class of sources in future surveys and briefly discuss associations with short bright gamma-ray bursts. We finally discuss the detection of the jetted outflow afterglow in addition to the kilonova.

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arXiv2021

Probing binary neutron star mergers in dense environments using afterglow counterparts

The only binary neutron star merger gravitational wave event with detected electromagnetic counterparts recorded to date is GRB170817A. This merger occurred in a rarefied medium with a density smaller than $10^{-3}-10^{-2}~{\rm cm}^{-3}$. Since kicks are imparted to neutron star binaries upon formation, and due to their long delay times before merger, such low-density circum-merger media are generally expected. However, there is some indirect evidence for fast-merging or low-kick binaries, which would coalesce in denser environments. Nonetheless, present astronomical data are largely inconclusive on the possibility of these high-density mergers. We describe a method to directly probe this hypothetical population of high-density mergers through multi-messenger observations of binary neutron star merger afterglows, exploiting the high sensitivity of these signals to the density of the merger environment. This method is based on a sample of merger afterglows that has yet to be collected. Its constraining power is large, even with a small sample of events. We discuss the method's limitations and applicability. In the upcoming era of third-generation gravitational wave detectors, this method's potential will be fully realized as it will allow us to probe mergers that occurred soon after the peak of cosmic star formation, provided the follow-up campaigns are able to locate the sources.

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arXiv2020

X-ray Plateaus in Gamma Ray Bursts' light-curves from jets viewed slightly off-axis

Using multiple observational arguments, recent work has shown that cosmological GRBs are typically viewed at angles within, or close to the cores of their relativistic jets. One of those arguments relied on the lack of tens-of-days-long periods of very shallow evolution that would be seen in the afterglow light-curves of GRBs viewed at large angles. Motivated by these results, we consider that GRBs efficiently produce $γ$-rays only within a narrow region around the core. We show that, on these near-core lines-of-sight, structured jets naturally produce shallow phases in the X-ray afterglow of GRBs. These plateaus would be seen by a large fraction of observers and would last between $10^2-10^5$ s. They naturally reproduce the observed distributions of time-scales and luminosities as well as the inter-correlations between plateau duration, plateau luminosity and prompt $γ$-ray energy. An advantage of this interpretation is that it involves no late time energy injection which would be both challenging from the point of view of the central engine and, as we show here, less natural given the observed correlations between plateau and prompt properties.

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arXiv2019

Radio afterglows of binary neutron star mergers: a population study for current and future gravitational wave observing runs

Following the historical observations of GW170817 and its multi-wavelength afterglow, more radio afterglows from neutron star mergers are expected in the future as counterparts to gravitational wave inspiral signals. We wish to describe these events using our current knowledge of the population of neutron star mergers coming from gamma-ray burst science, and taking into account the sensitivities of current and future gravitational wave and radio detectors. We combine analytical models for the merger gravitational wave and radio afterglow signals to a population model prescribing the energetics, circum-merger density and other relevant parameters of the mergers. We report the expected distributions of observables (distance, orientation, afterglow peak time/flux, etc.) from future events and study how these can be used to further probe the population of binary neutron stars, their mergers and related outflows during future observing campaigns. In the case of the O3 run of the LIGO-Virgo Collaboration, the radio afterglow of one third of gravitational-wave-detected mergers should be detectable (and detected if the source is localized thanks to the kilonova counterpart) by the Very Large Array, and these events should have viewing angles similar to that of GW170817. These findings confirm the radio afterglow as a powerful insight on these events, though some key afterglow-related techniques, such as very long baseline interferometry imaging of the merger remnant, may no longer be feasible as the gravitational wave horizon increases.

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arXiv2019

Neutron Star Merger Afterglows: Population Prospects for the Gravitational Wave Era

Following the historical observation of GW170817 and its electromagnetic follow-up, new neutron star merger afterglows are expected to be observed as counterparts to gravitational wave signals during the next science runs of the gravitational interferometer network. The diversity of the observed population of afterglows of these future events is subject to various factors, which are (i) intrinsic, such as the energy of the ejecta, (ii) environmental, such as the ambient medium density or (iii) observational, such as the viewing angle and distance of the source. Through prescribing a population of mergers and modelling their afterglows, we study the diversity of those events to be observed jointly in gravitational waves and electromagnetic bands. In the future, observables of detected events such as viewing angle, distance, afterglow peak flux or proper motion will form distributions which together with predictions from our study will provide insight on neutron star mergers and their environments.

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arXiv2019

Detecting the imprint of a kilonova or supernova \\ in short GRB afterglows

Short gamma-ray bursts result from mergers of two neutron stars or from collapsars, but probably at a smaller rate. In the first case, a kilonova occurs while in the second case a Type Ic supernova is expected. } {Even if future observations of kilonovae in association with gravitational wave events provide better data, detecting a kilonova during an afterglow follow-up would remain useful for exploring the diversity of the kilonova phenomenon. As supernovae produce a weaker gravitational signal, afterglow follow-up will be the only possible method to find one. In this work, we identify the conditions of the burst energy, external density, kilonova mass, supernova luminosity, that are necessary for the detection of a kilonova or supernova in the follow-up of short GRB afterglows.} {We have used a simple kilonova model to obtain the peak luminosities and times as a function of mass, expansion velocity and ejected matter opacity. Afterglow light curves are computed for a uniform medium and a stellar wind, in the kilonova and supernova cases, respectively.} {We represent, using diagrams of the burst kinetic energy vs. density of the external medium, the domains where the kilonova or supernova at maximum is brighter than the afterglow. %\LEt{Please check I have retained your intended meaning -> It's OK

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arXiv2017

What can we learn from "internal plateaus"? The peculiar afterglow of GRB 070110

Context: The origin of GRBs' prompt emission is highly debated. Proposed scenarios involve dissipation processes above or below the photosphere of an ultra-relativistic outflow. Aims: We search for observational features that would favour one scenario over the others by constraining the dissipation radius, the outflow magnetization or by indicating the presence of shocks. Bursts showing peculiarities can emphasize the role of a specific physical ingredient, which becomes more apparent under certain circumstances. Methods: We study GRB 070110, which exhibited several remarkable features during its early afterglow: a very flat plateau terminated by an extremely steep drop and immediately followed by a bump. We model the plateau as photospheric emission from a long lasting outflow of moderate Lorentz factor ($Γ\sim 20$) which lags behind an ultra-relativistic ($Γ> 100$) ejecta responsible for the prompt emission. We compute the dissipation of energy in the forward and reverse shocks resulting from this ejecta's deceleration by the external medium. Results: Photospheric emission from the long-lasting outflow can account for the plateau properties (luminosity and spectrum) assuming some dissipation takes place in the flow. The geometrical timescale at the photospheric radius is so short that the observed decline at the end of the plateau likely corresponds to the shut-down of the central engine. The following bump results from dissipated power in the reverse shock, which develops when the slower material catches up with the initially fast component, after it had been decelerated. Conclusions: Our interpretation suggests that the prompt phase resulted from dissipation above the photosphere while the plateau had a photospheric origin. If the bump is produced by the reverse shock, it implies an upper limit ($σ\lesssim 0.1$) on the magnetization of the slower material.

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arXiv2016

Statistical Study of Observed and Intrinsic Durations among BATSE and Swift/BAT GRBs

Studies of \textit{BATSE} bursts \citep{kouveliotou:93} have resulted in the widespread adoption of a two-group categorization: long bursts (those with durations $\geq 2$ seconds) and short bursts (those with durations $\leq 2$ seconds). This categorization, one must recall, used the observed $T_{90}$ time durations for bursts (during which 90\% of a burst's fluence is measured). In this work, we have explored two ideas: 1) a statistical search for a possible third, intermediate category of bursts (between the "short" and the "long" ones) among 2041 \textit{BATSE} GRBs and 757 \textit{Swift/BAT} ones, 2) a study of bursts' intrinsic durations, where durations in the bursts' reference frames (instead of the observed durations) are considered, for this, 248 \textit{Swift/BAT} bursts that have redshift measurements were statistically analyzed for the same categorization goal. We first use a Monte Carlo method to determine the proper binning of each GRB, considering that bursts come with different uncertainties on their durations. Then, using the method of minimization of chi-square $χ^2$, we search for the best fit of the normalized frequency distributions $\frac{1}{N_0}\frac{dN}{d\ln{T}}$ of durations, this allows us to compare fits with two groups ("short" and "long") with fits with three groups ("short", "long", and "intermediate"). Our results indicate that the distributions of observed durations are better fitted by three groups than two groups for \textit{Swift/BAT} data, interestingly, the "intermediate" group appears rather clearly for both observed and intrinsic durations. For BATSE data, the statistical test does not prefer three groups over two. We discuss the results, their possible underlying causes, and reasonable interpretations.

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arXiv2014

UHECR acceleration at GRB internal shocks

We study the acceleration of CR protons and nuclei at GRB internal shocks. Physical quantities and their time evolution are estimated using the internal shock modeling implemented by Daigne & Mochkovitch 1998. We consider different hypotheses about the way the energy dissipated at internal shocks is shared between accelerated CR, e- and B field. We model CR acceleration at mildly relativistic shocks, including all the significant energy loss processes. We calculate CR and neutrino release from single GRBs, assuming that nuclei heavier than protons are present in the relativistic wind. Protons can only reach maximum energies of ~ 10^19.5 eV, while intermediate and heavy nuclei are able to reach values of ~ 10^20 eV and above. The spectra of nuclei escaping from the acceleration site are found to be very hard while the combined spectrum of protons and neutrons is much softer. We calculate the diffuse UHECR flux expected on Earth using the GRB luminosity function from Wanderman & Piran 2010. Only the models assuming that the prompt emission represent a very small fraction of the energy dissipated at internal shocks, and that most of this dissipated energy is communicated to accelerated CR, are able to reproduce the magnitude of the UHECR flux observed. For these models, the observed shape of the UHECR spectrum can be well reproduced and the evolution of the composition is compatible with the trend suggested by Auger. We discuss implications of the softer proton component for the GCR to EGCR transition in the light of the recent composition analyses (KASCADE-Grande experiment). The associated secondary particle diffuse fluxes do not upset any current observational limit. Diffuse neutrino flux from GRB sources should however be detected with the lifetime of neutrino observatories.

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arXiv2012

Dynamics and Afterglow Light Curves of GRB Blast Waves with a Long-lived Reverse Shock

We perform a detailed study on the dynamics of a relativistic blast wave with the presence of a long-lived reverse shock (RS). Although a short-lived RS has been widely considered, the RS is believed to be long-lived as a consequence of a stratification expected on the ejecta Lorentz factors. The existence of a long-lived RS makes the forward shock (FS) dynamics to deviate from a self-similar Blandford-McKee solution. Employing the "mechanical model" that correctly incorporates the energy conservation, we present an accurate solution for both the FS and RS dynamics. We conduct a sophisticated calculation of the afterglow emission. Adopting a Lagrangian description of the blast wave, we keep track of an adiabatic evolution of numerous shells between the FS and RS. An evolution of the electron spectrum is also followed individually for every shell. We then find the FS and RS light curves by integrating over the entire FS and RS shocked regions, respectively. In particular, we make use of an analytic expression for observed spectral flux, which we derive here in terms of an observed frequency and observer time. Exploring a total of 20 different ejecta stratifications, we explain in detail how a stratified ejecta affects its blast wave dynamics and afterglow light curves. We show that, while the FS light curves are not sensitive to the ejecta stratifications, the RS light curves exhibit much richer features, including steep declines, plateaus, bumps, re-brightenings, and a variety of temporal decay indices. These distinctive RS features may be observable if the RS has higher values of the micophysics parameters than the FS. We discuss possible applications of our results in understanding the GRB afterglow data.

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arXiv2007

Is the time lag-luminosity relation of GRBs a consequence of the Amati relation?

The lag-luminosity relation (LLR) provides a way of estimating GRB luminosity by measuring the spectral lags between different energy bands. We want to understand the origin of the LLR and test its validity. This appears especially important if the LLR is to be used as a distance indicator. We perform a linear analysis of the lag between two spectral bands. The lag is obtained as the time interval between the maxima of a given pulse in the two bands. We get a simple expression for the lag, which shows in a very simple way how it is related to the spectral evolution of the burst via the variation of the peak energy and spectral indices. When this expression is coupled to the Amati relation, it leads to a LLR that agrees with the observational results only if the burst's spectral evolution is limited to a decrease in peak energy during pulse decay. However, when the variation of the spectral indices is also taken into account, the predicted LLR differs from the observed one. We briefly discuss some ways to solve this problem, such as a possible correlation between pulse spikiness and burst luminosity.

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arXiv2007

Can the early X-ray afterglow of GRBs be explained by a contribution from the reverse shock?

We propose to explain the recent observations of GRB early X-ray afterglows with SWIFT by the dissipation of energy in the reverse shock which crosses the ejecta as it is decelerated by the burst environment. We compute the evolution of the dissipated power and discuss the possibility that a fraction of it can be radiated in the X-ray range. We show that this reverse shock contribution behaves in a way very similar to the observed X-ray afterglows if the following two conditions are satisfied: (i) the Lorentz factor of the material which is ejected during the late stages of source activity decreases to small values Gamma < 10 and (ii) a large part of the shock dissipated energy is transferred to a small fraction (zeta < 0.1) of the electron population. We also discuss how our results may help to solve some puzzling problems raised by multiwavelength early afterglow observations such as the presence of chromatic breaks.

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arXiv2003

The expected photospheric emission of GRBs in the internal shock model

The prompt emission of gamma-ray bursts (hereafter GRBs) probably comes from a highly relativistic wind which converts its kinetic energy into radiation via the formation of shocks within the wind itself. Such "internal shocks" can occur if the wind is generated with a highly non uniform distribution of the Lorentz factor Gamma. Taking into account such a variable distribution of Gamma, we estimate the expected thermal emission of the relativistic wind when it becomes transparent. We compare this emission (temporal profile + spectrum) to the emission produced by the internal shocks. In most cases we predict a rather bright thermal emission that could easily be detected. This favors acceleration mechanisms for the wind where the main energy reservoir is under magnetic rather than thermal form. Such scenarios can produce thermal X-ray precursors comparable to those observed by GINGA and WATCH/GRANAT.

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arXiv2003

On the nature of X-ray Flashes

We have developed a toy model for internal shocks which has been used to generate a large number of synthetic GRBs in order to find in the parameter space the conditions which can lead to the formation of X-ray flashes. The key condition appears to be a small contrast of the Lorentz factor in the relativistic wind emitted by the central engine.

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arXiv2003

The physics of pulses in gamma-ray bursts: emission processes, temporal profiles and time lags

We present a simple, semi-analytical model to explain GRB temporal and spectral properties in the context of the internal shock model. Each individual pulse in the temporal profiles is produced by the deceleration of fast moving material by a comparatively slower layer within a relativistic wind. The spectral evolution of synthetic pulses is first obtained with standard equipartition assumptions to estimate the post-shock magnetic field and electron Lorentz factor. We get Ep propto t^-delta with delta=7/2 which is much steeper than the observed slopes delta(obs) <= 1.5. We therefore consider the possibility that the equipartition parameters depend on the shock strength and post-shock density. We then get a much better agreement with the observations and our synthetic pulses satisfy both the hardness-intensity and hardness-fluence correlations. We also compute time lags between profiles in different energy channels and we find that they decrease with increasing hardness. We finally compare our predicted time lag - luminosity relation to the Norris et al. (2000) result obtained from 6 bursts with known redshift.

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