arXiv subjects

Rodolfo Barniol Duran

Publications and source records attributed to Rodolfo Barniol Duran.

34 records · Page 2Linked to original sources

arXiv2016

Modeling the early afterglow in the short and hard GRB 090510

The bright, short and hard GRB 090510 was detected by all instruments aboard Fermi and Swift satellites. The multiwavelength observations of this burst presented similar features with the Fermi-LAT-detected gamma-ray bursts. In the framework of the external shock model of early afterglow, a leptonic scenario that evolves in a homogeneous medium is proposed to revisit GRB 090510 and explain the multiwavelength light curve observations presented in this burst. These observations are consistent with the evolution of a jet before and after the jet break. The long-lasting LAT, X-ray and optical fluxes are explained in the synchrotron emission from the adiabatic forward shock. Synchrotron self-Compton emission from the reverse shock is consistent with the bright LAT peak provided that progenitor environment is entrained with strong magnetic fields. It could provide compelling evidence of magnetic field amplification in the neutron star merger.

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arXiv2016

Radio SNRs in the Magellanic Clouds as probes of shock microphysics

A large number of supernova remnants (SNRs) in our Galaxy and galaxies nearby have been resolved in various radio bands. This radio emission is thought to be produced via synchrotron emission from electrons accelerated by the shock that the supernova ejecta drives into the external medium. Here we consider the sample of radio SNRs in the Magellanic Clouds. Given the size and radio flux of a SNR, we seek to constrain the fraction of shocked fluid energy in non-thermal electrons ($ε_e$) and magnetic field ($ε_B$), and find $ε_e ε_B \sim 10^{-3}$. These estimates do not depend on the largely uncertain values of the external density and the age of the SNR. We develop a Monte Carlo scheme that reproduces the observed distribution of radio fluxes and sizes of the population of radio SNRs in the Magellanic Clouds. This simple model provides a framework that could potentially be applied to other galaxies with complete radio SNRs samples.

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arXiv2015

An anisotropic minijets model for the GRB prompt emission

In order to explain rapid light curve variability without invoking a variable source, several authors have proposed "minijets" that move relativistically relative to the main flow of the jet. Here we consider the possibility that these minijets, instead of being isotropically distributed in the comoving frame of the jet, form primarily perpendicular to the direction of the flow, as the jet dissipates its energy at a large emission radius. This yields two robust features. First, the emission is significantly delayed compared with the isotropic case. This delay allows for the peak of the afterglow emission to appear while the source is still active, in contrast to the simplest isotropic model. Secondly, the flux decline after the source turns off is steeper than the isotropic case. We find that these two features are realized in gamma-ray bursts (GRBs): 1. The peak of most GeV light curves (ascribed to the external shock) appears during the prompt emission phase. 2. Many X-ray light curves exhibit a period of steep decay, which is faster than that predicted by the standard isotropic case. The gamma-ray generation mechanism in GRBs, and possibly in other relativistic flows, may therefore be anisotropic.

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arXiv2015

Radio rebrightening of the GRB afterglow by the accompanying supernova

The gamma-ray burst (GRB) jet powers the afterglow emission by shocking the surrounding medium, and radio afterglow can now be routinely observed to almost a year after the explosion. Long-duration GRBs are accompanied by supernovae (SNe) that typically contain much more energy than the GRB jet. Here we consider the fact that the SN blast wave will also produce its own afterglow (supernova remnant emission), which will peak at much later time (since it is non-relativistic), when the SN blast wave transitions from a coasting phase to a decelerating Sedov-Taylor phase. We predict that this component will peak generally a few tens of years after the explosion and it will outshine the GRB powered afterglow well-before its peak emission. In the case of GRB 030329, where the external density is constrained by the $\sim 10$-year coverage of the radio GRB afterglow, the radio emission is predicted to start rising over the next decade and to continue to increase for the following decades up to a level of $\sim$ mJy. Detection of the SN-powered radio emission will greatly advance our knowledge of particle acceleration in $ \sim 0.1$c shocks.

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arXiv2015

Energies of GRB blast waves and prompt efficiencies as implied by modeling of X-ray and GeV afterglows

We consider a sample of ten GRBs with long lasting ($\gtrsim10^2\rm\,sec$) emission detected by Fermi/LAT and for which X-ray data around $1\,$day are also available. We assume that both the X-rays and the GeV emission are produced by electrons accelerated at the external forward shock, and show that the X-ray and the GeV fluxes lead to very different estimates of the initial kinetic energy of the blast wave. The energy estimated from GeV is on average $\sim50$ times larger than the one estimated from X-rays. We model the data (accounting also for optical detections around $1\,$day, if available) to unveil the reason for this discrepancy and find that good modelling within the forward shock model is always possible and leads to two possibilities: either the X-ray emitting electrons (unlike the GeV emitting electrons) are in the slow cooling regime or ii) the X-ray synchrotron flux is strongly suppressed by Compton cooling, whereas, due to the Klein-Nishina suppression, this effect is much smaller at GeV energies. In both cases the X-ray flux is no longer a robust proxy for the blast wave kinetic energy. On average, both cases require weak magnetic fields ($10^{-6}\lesssim ε_B \lesssim 10^{-3}$) and relatively large isotropic kinetic blast wave energies $10^{53}\rm\,erg<E_{0,kin}<10^{55}\rm\,erg$ corresponding to large lower limits on the collimated energies, in the range $10^{52}\rm\,erg<E_{θ,kin}<5\times10^{52}\rm\,erg$ for an ISM environment with $n\sim 1\mbox{cm}^{-3}$ and $10^{52}\rm\,erg<E_{θ,kin}<10^{53}\rm\,erg$ for a wind environment with $A_* \sim 1$. These energies are larger than those estimated from the X-ray flux alone, and imply smaller inferred values of the prompt efficiency mechanism, reducing the efficiency requirements on the still uncertain mechanism responsible for prompt emission.

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arXiv2015

Shedding light on the prompt high efficiency paradox - self consistent modeling of GRB afterglows

We examine GRBs with both Fermi-LAT and X-ray afterglow data. Assuming that the 100MeV (LAT) emission is radiation from cooled electrons accelerated by external shocks, we show that the kinetic energy of the blast wave estimated from the 100MeV flux is 50 times larger than the one estimated from the X-ray flux. This can be explained if either: i) electrons radiating at X-rays are significantly cooled by SSC (suppressing the synchrotron flux above the cooling frequency) or ii) if the X-ray emitting electrons, unlike those emitting at 100MeV energies, are in the slow cooling regime. In both cases the X-ray flux is no longer an immediate proxy of the blast wave kinetic energy. We model the LAT, X-ray and optical data and show that in general these possibilities are consistent with the data, and explain the apparent disagreement between X-ray and LAT observations. All possible solutions require weak magnetic fields: $10^{-6}< ε_B < 10^{-3}$ (where $ε_B$ is the fraction of shocked plasma energy in magnetic fields). Using the LAT emission as a proxy for the blast wave kinetic energy we find that the derived prompt efficiencies are of order 15%. This is considerably lower compared with previous estimates (87% and higher for the same bursts). This provides at least a partial solution to the "prompt high efficiency paradox".

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arXiv2015

The afterglow of a relativistic shock breakout and low-luminosity GRBs

The prompt emission of low-luminosity gamma-ray bursts (llGRBs) indicates that these events originate from a relativistic shock breakout. In this case we can estimate, based on the properties of the prompt emission, the energy distribution of the ejecta. We develop a general formalism to estimate the afterglow produced by synchrotron emission from the forward shock resulting from the interaction of this ejecta with the circum-burst matter. We assess whether this emission can produce the observed radio and X-ray afterglows of the available sample of 4 llGRBs. All 4 radio afterglows can be explained within this model, providing further support for shock breakouts being the origin of llGRBs. We find that in one of the llGRBs (GRB 031203) the predicted X-ray emission, using the same parameters that fit the radio, can explain the observed one. In another one (GRB 980425) the observed X-rays can be explained if we allow for a slight modification of the simplest model. For the last two cases (GRBs 060218 and 100316D), we find that, as is the case for previous attempts to model these afterglows, the simplest model that fits the radio emission underpredicts the observed X-ray afterglows. Using general arguments, we show that the most natural location of the X-ray source is, like the radio source, within the ejecta-external medium interaction layer but that emission is due to a different population of electrons or to a different emission process.

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arXiv2014

The Nature of ULX Source M101 X-1: Optically Thick Outflow from A Stellar Mass Black Hole

The nature of ultra-luminous X-ray sources (ULXs) has long been plagued by an ambiguity about whether the central compact objects are intermediate-mass (IMBH, >~ 10^3 M_sun) or stellar-mass (a few tens M_sun) black holes (BHs). The high luminosity (~ 10^39 erg/s) and super-soft spectrum (T ~ 0.1 keV) during the high state of the ULX source X-1 in the galaxy M101 suggest a large emission radius (>~ 10^9 cm), consistent with being an IMBH accreting at a sub-Eddington rate. However, recent kinematic measurement of the binary orbit of this source and identification of the secondary as a Wolf-Rayet star suggest a stellar-mass BH primary with a super-Eddington accretion. If that is the case, a hot, optically thick outflow from the BH can account for the large emission radius and the soft spectrum. By considering the interplay of photons' absorption and scattering opacities, we determine the radius and mass density of the emission region of the outflow and constrain the outflow mass loss rate. The analysis presented here can be potentially applied to other ULXs with thermally dominated spectra, and to other super-Eddington accreting sources.

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arXiv2014

Constraining the magnetic field in GRB relativistic collisionless shocks using radio data

Using GRB radio afterglow observations, we calculate the fraction of shocked plasma energy in the magnetic field in relativistic collisionless shocks ($ε_B$). We obtained $ε_B$ for 38 bursts by assuming that the radio afterglow light curve originates in the external forward shock and that its peak at a few to tens of days is due to the passage of the minimum (injection) frequency through the radio band. This allows for the determination of the peak synchrotron flux of the external forward shock, $f_p$, which is $f_p \propto ε_B^{1/2}$. The obtained value of $ε_B$ is conservatively a minimum if the time of the "jet break" is unknown, since after the "jet break" $f_p$ is expected to decay with time faster than before it. Claims of "jet breaks" have been made for a subsample of 23 bursts, for which we can estimate a measurement of $ε_B$. Our results depend on the blast wave total energy, $E$, and the density of the circum-stellar medium (CSM), $n$, as $ε_B \propto E^{-2}n^{-1}$. However, by assuming a CSM magnetic field ($\sim 10$ $μ$G), we can express the lower limits/measurements on $ε_B$ as a density-independent ratio, $B/B_{sc}$, of the magnetic field behind the shock to the CSM shock-compressed magnetic field. We find that the distribution on both the lower limit on and the measurement of $B/B_{sc}$ spans $\sim 3.5$ orders of magnitude and both have a median of $B/B_{sc} \sim 30$. This suggests that some amplification, beyond simple shock-compression, is necessary to explain these radio afterglow observations.

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arXiv2014

Magnetic Fields In Relativistic Collisionless Shocks

We present a systematic study on magnetic fields in Gamma-Ray Burst (GRB) external forward shocks (FSs). There are 60 (35) GRBs in our X-ray (optical) sample, mostly from Swift. We use two methods to study epsilon_B (fraction of energy in magnetic field in the FS). 1. For the X-ray sample, we use the constraint that the observed flux at the end of the steep decline is $\ge$ the X-ray FS flux. 2. For the optical sample, we use the condition that the observed flux arises from the FS (optical sample light curves decline as ~t^-1, as expected for the FS). Making a reasonable assumption on E (jet isotropic equivalent kinetic energy), we converted these conditions into an upper limit (measurement) on epsilon_B n^{2/(p+1)} for our X-ray (optical) sample, where n is the circumburst density and p is the electron index. Taking n=1 cm^-3, the distribution of epsilon_B measurements (upper limits) for our optical (X-ray) sample has a range of ~10^-8 -10^-3 (~10^-6 -10^-3) and median of ~few x 10^-5 (~few x 10^-5). To characterize how much amplification is needed, beyond shock compression of a seed magnetic field ~10 muG, we expressed our results in terms of an amplification factor, AF, which is very weakly dependent on n (AF propto n^0.21 ). The range of AF measurements (upper limits) for our optical (X-ray) sample is ~ 1-1000 (~10-300) with a median of ~50 (~50). These results suggest that some amplification, in addition to shock compression, is needed to explain the afterglow observations.

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arXiv2013

Radius constraints and minimal equipartition energy of relativistically moving synchrotron sources

A measurement of the synchrotron self-absorption flux and frequency provides tight constraints on the physical size of the source and a robust lower limit on its energy. This lower limit is also a good estimate of the magnetic field and electrons' energy, if the two components are at equipartition. This well-known method was used for decades to study numerous astrophysical sources moving at non-relativistic (Newtonian) speeds. Here we generalize the Newtonian equipartition theory to sources moving at relativistic speeds including the effect of deviation from spherical symmetry expected in such sources. Like in the Newtonian case, minimization of the energy provides an excellent estimate of the emission radius and yields a useful lower limit on the energy. We find that the application of the Newtonian formalism to a relativistic source would yield a smaller emission radius, and would generally yield a larger lower limit on the energy (within the observed region). For sources where the Synchrotron-self-Compton component can be identified, the minimization of the total energy is not necessary and we present an unambiguous solution for the parameters of the system.

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arXiv2013

On the origin of the radio emission of Sw 1644+57

We apply relativistic equipartition synchrotron arguments to the radio data of the tidal disruption event candidate Sw 1644+57. We find that, regardless of the details of the equipartition scenario considered, the energy required to produce the observed radio (i.e., energy in magnetic field and radio emitting electrons) must increase by a factor of ~20 during the first 200 days. It then saturates. This energy increase cannot be alleviated by a varying geometry of the system. The radio data can be explained by: (i) An afterglow like emission of the X-ray emitting narrow relativistic jet. The additional energy can arise here from a slower moving material ejected in the first few days that gradually catches up with the slowing down blast wave (Berger et al. 2012). However, this requires at least ~4x10^{53} erg in the slower moving outflow. This is much more than the energy of the fast moving outflow that produced the early X-rays and it severely constrains the overall energy budget. (ii) Alternatively, the radio may arise from a mildly relativistic quasi-spherical outflow. Here, the energy for the radio emission increases with time to at least ~10^{51} erg after 200 days. This scenario requires, however, a second X-ray emitting collimated relativistic component. Given these results, it is worthwhile to consider models in which the energy of the magnetic field and/or of the radio emitting electrons increases with time without a continuous energy supply to the blast wave. This can happen, for example, if the energy is injected initially mostly in one form (Poynting flux or baryonic) and it is gradually converted to the other form, leading to a deviation from equipartition. Another intriguing possibility is that a gradually decreasing Inverse Compton cooling modifies the synchrotron emission and leads to an increase of the available energy in the radio emitting electrons (Kumar et al. 2013).

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arXiv2011

Evidence for mild deviation from power-law distribution of electrons in relativistic shocks: GRB 090902B

Many previous studies have determined that the long lasting emission at X-ray, optical and radio wavelengths from gamma-ray bursts (GRBs), called the afterglow, is likely produced by the external forward shock model. In this model, the GRB jet interacts with the circum-stellar medium and drives a shock that heats the medium, which radiates via synchrotron emission. In this work, we carried out a detailed analysis of the late time afterglow data of GRB 090902B using a very careful accounting of the Inverse Compton losses. We find that in the context of the external forward shock model, the only viable option to explain the X-ray and optical data of GRB 090920B is to have the electron energy distribution deviate from a power-law shape and exhibit some slight curvature immediately downstream of the shock front (we explored other models that rely on a single power-law assumption, but they all fail to explain the observations). We find the fraction of the energy of shocked plasma in magnetic field to be ~10^{-6} using late time afterglow data, which is consistent with the value obtained using early gamma-ray data. Studies like the present one might be able to provide a link between GRB afterglow modeling and numerical simulations of particle acceleration in collisionless shocks. We also provide detailed calculations for the early (< 10^3 s) high energy (> 100 MeV) emission and confirm that it is consistent with origin in the external forward shock. We investigated the possibility that the ~10 keV excess observed in the spectrum during the prompt phase also has its origin in the external shock and found the answer to be negative.

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arXiv2010

Implications of electron acceleration for high-energy radiation from gamma-ray bursts

In recent work we suggested that photons of energy >100 MeV detected from GRBs by the Fermi Satellite are produced via synchrotron emission in the external forward shock with a weak magnetic field - consistent with shock compressed upstream magnetic field of a few tens of micro-Gauss. Here we investigate whether electrons can be accelerated to energies such that they radiate synchrotron photons with energy up to about 10 GeV in this particular scenario. We do this using two methods: (i) we check if these electrons can be confined to the shock front; and (ii) we calculate radiative losses while they are being accelerated. We find that these electrons remain confined to the shock front, as long as the upstream magnetic field is >~ 10 micro-Gauss, and don't suffer substantial radiative losses, the only condition required is that the external reverse shock emission be not too bright: peak flux less than 1 Jy in order to produce photons of 100 MeV, and less than ~100 mJy for producing 1-GeV photons. We also find that the acceleration time for electrons radiating at 100 MeV is a few seconds (in observer frame), and the acceleration time is somewhat longer for electrons radiating at a few GeV. This could explain the lack of >100 MeV photons for the first few seconds after the trigger time for long GRBs reported by the Fermi Satellite, and also the slight lag between photons of GeV and 100 MeV energies. We model the onset of the external forward shock light curve in this scenario and find it consistent with the sharp rise observed in the 100-MeV light curve of GRB080916C and similar bursts.

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arXiv2010

External forward shock origin of high energy emission for three GRBs detected by Fermi

We analyze the >100MeV data for 3 GRBs detected by Fermi (GRBs 080916C, 090510, 090902B) and find that these photons were generated via synchrotron emission in the external forward shock. We arrive at this conclusion by four different methods as follows. (1) We check the light curve and spectral behavior of the >100MeV data, and late time X-ray and optical data, and find them consistent with the closure relations for the external forward shock radiation. (2) We calculate the expected external forward shock synchrotron flux at 100MeV, and it matches the observed flux value. (3) We determine the external forward shock model parameters using the >100MeV data, and with these we calculate the expected X-ray and optical fluxes at late times (hours to days after the burst) and find these to be in good agreement with the observed data. (4) We calculate the external forward shock model parameters using only the late time X-ray, optical and radio data and from these estimate the expected flux at >100 MeV at the end of the sub-MeV burst (and at subsequent times) and find that to be entirely consistent with the high energy data obtained by Fermi/LAT. The ability of a simple external forward shock, to fit the entire data from the end of the burst (1-50s) to about a week, covering more than eight-decades in photon frequency provides compelling confirmation of the external forward shock synchrotron origin of the >100MeV radiation from these Fermi GRBs. Moreover, the parameters determined in points (3) and (4) show that the magnetic field required in these GRBs is consistent with shock-compressed magnetic field in the circum-stellar medium with pre-shocked values of a few tens of micro-Gauss.

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arXiv2009

On the generation of high energy photons detected by the Fermi Satellite from gamma-ray bursts

Observations of gamma-ray bursts by the Fermi satellite, capable of detecting photons in a very broad energy band: 8keV to >300GeV, have opened a new window for the study of these enigmatic explosions. It is widely assumed that photons of energy larger than 100 MeV are produced by the same source that generated lower energy photons -- at least whenever the shape of the spectrum is a Band function. We report here a surprising discovery -- the Fermi data for a bright burst, GRB 080916C, unambiguously shows that the high energy photons (>= 100MeV) were generated in the external shock via the synchrotron process, and the lower energy photons had a distinctly different source. The magnetic field in the region where high energy photons were produced (and also the late time afterglow emission region) is found to be consistent with shock compressed magnetic field of the circum-stellar medium. This result sheds light on the important question of the origin of magnetic fields required for gamma-ray burst afterglows. The external shock model for high energy radiation makes a firm prediction that can be tested with existing and future observations.

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