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A. Panaitescu

Publications and source records attributed to A. Panaitescu.

At least 19 recordsLinked to original sources
arXiv2026

Determination of the Angular Distributions of Dynamical and Emission Parameters of GRB Relativistic Outflows

A previously-developed analytical treatment of the "Larger-Angle Emission" model for GRBs and afterglows is further developed to include the effect of a fixed magnetic field orientation. This model, has the uncanny ability to unify the prompt and delayed emissions and to account easily for the entire diversity of Swift X-ray light-curves. The LAE model is applied to seven GRBs (060526, 060614, 060714, 060729, 061007, 061121, 100902, 130427A) with well-sampled 0.3 keV/10 keV Swift/XRT and optical light-curves, displaying well-monitored four phases (GRB, Tail, Plateau, post-Plateau), to illustrate its power and limitations and to determine its five fundamental parameters. The one-and-only Core parameter Gamma_c*theta_c is determined by the GRB and afterglow temporal milestones (i.e. by the dynamical range of the Tail and Plateau), and may have an universal value around 3.0, with narrower Cores (smaller theta_c) being more relativistic (higher Gamma_c). Flat Plateaus require an Envelope angular structure close to Gamma(theta) ~ 1/theta^2. A magnetic field parallel to the direction of the relativistic flow yields more curved prompt/GRB and delayed/afterglow light-curves than a perpendicular or isotropic field, with curvature $Δα$ defined as the difference between the flux asymptotic power-law decay indices at the beginning and end of each phase. The curvature Delta alpha_grb of the GRB light-curves arising from the uniform Core is set only by the B-field orientation, and a perpendicular or isotropic B-field is equally often found, but a parallel field is never identified for the Core. The curvature Delta alpha_ag of the afterglow light-curves from the power-law Envelope depends on B-orientation and three other parameters. The Envelope B-field orientation can be identified for only three afterglows, where a parallel B is found.

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arXiv2025

Analysis of Two Models for the Angular Structure of the Outflows Producing the Swift/XRT "Larger-Angle Emission" of Gamma-Ray Bursts

The instantaneous emission from a relativistic surface endowed with a Lorentz factor $Γ$ that decreases away from the outflow symmetry axis can naturally explain the three phases observed by Swift/XRT in GRBs and their afterglows (GRB tail, afterglow plateau and post-plateau). We expand the analytical formalism of the "Larger-Angle Emission" model previously developed for "Power-Law" outflows to "n-Exponential" outflows (e.g. exponential with $n=1$ and Gaussian with $n=2$) and compare their abilities to account for the X-ray emission of XRT afterglows. We assume power-law $Γ$-dependences of two spectral characteristics (peak-energy and peak intensity) and find that, unlike Power-Law outflows, n-Exponential outflows cannot account for plateaus with a temporal dynamical range larger than 100. To include all information existing in the Swift/XRT measurements of X-ray aferglows (0.3-10 keV unabsorbed flux and effective spectral slope), we calculate 0.3 keV and 10 keV light-curves using a broken power-law emission spectrum of peak-energy and low-and high-energy slopes that are derived from the effective slope measured by XRT. This economical peak-energy determination is found to be consistent with more expensive spectral fits. The angular distributions of the Lorentz factor, comoving frame peak-energy, and peak-intensity ($Γ(θ), E'_p (θ), i'_p(θ)$) constrain the (yet-to-be determined) convolution of various features of the production of relativistic jets by solar-mass black-holes and of their propagation through the progenitor/circumburst medium, while the $E'_p (Γ)$ and $i'_p (Γ)$ dependences may constrain the GRB dissipation mechanism and the GRB emission process.

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arXiv2020

X-ray Afterglows from the Gamma-Ray Burst "Large(r)-Angle" Emission

We derive basic analytical results for the timing and decay of the GRB-counterpart and delayed-afterglow light-curves for a brief emission episode from a relativistic surface endowed with angular structure, consisting of a uniform Core of size theta_c (Lorentz factor Gamma_c and surface emissivity i_nu are angle-independent) and an axially-symmetric power-law Envelope (Gamma ~ theta^{-g}). In this Large-Angle Emission (LAE) model, radiation produced during the prompt emission phase (GRB) at angles theta > theta_c arrives at observer well after the burst (delayed emission). The dynamical time-range of the very fast-decaying GRB "tail" and of the flat afterglow "plateau", and the morphology of GRB counterpart/afterglow, are all determined by two parameters: the Core's parameter Gamma_c*theta_c and the Envelope's Lorentz factor index g, leading to three types of light-curves that display three post-GRB phases (type 1: tail, plateau/slow-decay, post-plateau/normal-decay), two post-GRB phases (type 2: tail and fast-decay), or just one (type 3: normal decay). We show how X-ray light-curve features can be used to determine Core and Envelope dynamical and spectral parameters. Testing of the LAE model is done using the Swift/XRT X-ray emission of two afterglows of type 1 (060607A, 061121), one of type 2 (061110A), and one of type 3 (061007). We find that the X-ray afterglows with plateaus require an Envelope Lorentz factor Gamma ~ theta^{-2} and a comoving-frame emissivity i_nu ~ theta^2, thus, for a typical afterglow spectrum F_nu ~ nu^{-1}, the lab-frame energy release is uniform over the emitting surface.

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arXiv2019

Adiabatic and Radiative Cooling of Relativistic Electrons Applied to Synchrotron Spectra and Light-Curves of Gamma-Ray Burst Pulses

We investigate the adiabatic and radiative (synchrotron and inverse-Compton) cooling of relativistic electrons whose injected/initial distribution with energy is a power-law above a typical energy $γ_i$. Analytical and numerical results are presented for the cooling-tail and the cooled-injected distribution that develop below and above the typical energy of injected electrons, for the evolution of the peak-energy $E_p$ of the synchrotron emission spectrum, and for the pulse shape resulting from an episode of electron injection. The synchrotron emission calculated numerically is compared with the spectrum and shape of Gamma-Ray Burst (GRB) pulses. Both adiabatic and radiative cooling processes lead to a softening of the pulse spectrum, and both types of cooling processes lead to pulses peaking earlier and lasting shorter at higher energy, quantitatively consistent with observations. For adiabatic-dominated electron cooling, a power-law injection rate $R_i$ suffices to explain the observed power-law GRB low-energy spectra. Synchrotron-dominated cooling leads to power-law cooling-tails that yield the synchrotron standard slope alpha = -3/2 provided that $R_i \sim B^2$, which is exactly the expectation if the magnetic field is a constant fraction of the post-shock energy density. Increasing (decreasing) $R_i$ and decreasing (increasing) B(t) lead to slopes alpha harder (softer, respectively) than the standard value and to non--power-law (curved) cooling-tails. Inverse-Compton cooling yields four values for the slope alpha but, as for synchrotron, other $R_i$ or B histories yield a wider range of slopes and curved low-energy spectra. Feedback between the power-law segments that develop below and above the typical injected electron leads to a synchrotron spectrum with many breaks above and below the usual 10 keV-1 MeV observing range.

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arXiv2017

The 7.2/3 Years Collection of Well-Monitored Fermi-LAT GRB Afterglows

We present the light-curves and spectra of 24 afterglows that have been monitored by Fermi-LAT at 0.1-100 GeV over more than a decade in time. All light-curves (except 130427) are consistent with a single power-law starting from their peaks, which occurred, in most cases, before the burst end. The light-curves display a brightness-decay rate correlation, with all but one (130427) of the bright afterglows decaying faster than the dimmer afterglows. We attribute this dichotomy to a quick deposition of the relativistic ejecta energy in the external-shock for the brighter/faster-decaying afterglows and to an extended energy-injection in the afterglow shock for the dimmer/slower-decaying light-curves. The spectra of six afterglows (090328, 100414, 110721, 110731, 130427, 140619B) indicate the existence of a harder component above a spectral dip/ankle at energy 0.3-3 GeV, offering evidence for an inverse-Compton emission at higher energies, and suggesting that the harder power-law spectra of five other LAT afterglows (130327B, 131231, 150523, 150627, 160509) could also be inverse-Compton, while the remaining softer LAT afterglows should be synchrotron. Marginal evidence for a spectral break and softening at higher energies is found for two afterglows (090902B and 090926).

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arXiv2015

Optical Flashes from Internal Pairs Formed in Gamma-Ray Burst Afterglows

We develop a numerical formalism for calculating the distribution with energy of the (internal) pairs formed in a relativistic source from unscattered MeV--TeV photons. For GRB afterglows, this formalism is more suitable if the relativistic reverse-shock that energizes the ejecta is the source of the GeV photons. The number of pairs formed is set by the source GeV output (calculated from the Fermi-LAT fluence), the unknown source Lorentz factor, and the unmeasured peak energy of the LAT spectral component. We show synchrotron and inverse-Compton light-curves expected from pairs formed in the shocked medium and identify some criteria for testing a pair origin of GRB optical counterparts. Pairs formed in bright LAT afterglows with a Lorentz factor in the few hundreds may produce bright optical counterparts (R < 10) lasting for up to one hundred seconds. The number of internal pairs formed from unscattered seed photons decreases very strongly with the source Lorentz factor, thus bright GRB optical counterparts cannot arise from internal pairs if the afterglow Lorentz factor is above several hundreds.

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arXiv2014

Synchrotron and inverse-Compton emissions from pairs formed in GRB afterglows (analytical treatment)

We calculate the synchrotron and inverse-Compton emissions from pairs formed in GRB afterglows from high-energy photons (above 100 MeV), assuming a power-law photon spectrum C_nu ~ nu^{-2} and considering only the pairs generated from primary high-energy photons. The essential properties of these pairs (number, minimal energy, cooling energy, distribution with energy) and of their emission (peak flux, spectral breaks, spectral slope) are set by the observables GeV fluence Phi (t) = Ft and spectrum, and by the Lorentz factor Gamma and magnetic field B of the source of high-energy photons, at observer-time t. Optical and X-ray pseudo--light-curves F_nu (Gamma) are calculated for given B; proper synchrotron self-Compton light-curves are calculated by setting the dynamics Gamma(t) of the high-energy photons source to be that of a decelerating, relativistic shock. It is found that the emission from pairs can accommodate the flux and decays of the optical flashes measured during the prompt (GRB) phase and of the faster-decaying X-ray plateaus observed during the delayed (afterglow) phase. The brightest pair optical emission is obtained for 100 < Gamma < 500, and depends mostly on the GeV fluence, being independent of the source redshift. Emission from pairs formed during the GRB phase offers an alternate explanation to reverse-shock optical flashes. These two models may be distinguished based on their corresponding flux decay index--spectral slope relations, different correlations with the LAT fluence, or through modeling of the afterglow multiwavelength data.

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arXiv2014

"Self-absorbed" GeV light-curves of Gamma-Ray Burst afterglows

We investigate the effect that the absorption of high-energy (above 100 MeV) photons produced in GRB afterglow shocks has on the light-curves and spectra of Fermi-LAT afterglows. Afterglows produced by the interaction of a relativistic outflow with a wind-like medium peak when the blast-wave deceleration sets in, and the afterglow spectrum could be hardening before that peak, as the optical thickness to pair-formation is decreasing. In contrast, in afterglows produced in the interaction with a homogeneous medium, the optical thickness to pair-formation should increase and yield a light-curve peak when it reaches unity, followed by a fast light-curve decay, accompanied by a spectral softening. If energy is injected in the blast-wave, then the accelerated increase of the optical thickness yields a convex afterglow light-curve. Other features, such as a double-peak light-curve or a broad hump, can arise from the evolution of the optical thickness to photon-photon absorption. Fast decays and convex light-curves are seen in a few LAT afterglows, but the expected spectral softening is rarely seen in (and difficult to measure with) LAT observations. Furthermore, for the effects of photon-photon attenuation to shape the high-energy afterglow light-curve without attenuating it too much, the ejecta initial Lorentz factor must be in a relatively narrow range (50-200), which reduces the chance of observing those effects.

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arXiv2013

An external-shock model for GRB afterglow 130427A

The complex multiwavelength emission of GRB afterglow 130427A (monitored in the radio up to 10 days, in the optical and X-ray until 50 days, and at GeV energies until 1 day) can be accounted for by a hybrid reverse-forward shock synchrotron model, with inverse-Compton emerging only above a few GeV. The high ratio of the early optical to late radio flux requires that the ambient medium is a wind and that the forward-shock synchrotron spectrum peaks in the optical at about 10 ks. The latter has two consequences: the wind must be very tenuous and the optical emission before 10 ks must arise from the reverse-shock, as suggested also by the bright optical flash that Raptor has monitored during the prompt emission phase (<100 s). The VLA radio emission is from the reverse-shock, the Swift X-ray emission is mostly from the forward-shock, but the both shocks give comparable contributions to the Fermi GeV emission. The weak wind implies a large blast-wave radius (8 t_{day}^{1/2} pc), which requires a very tenuous circumstellar medium, suggesting that the massive stellar progenitor of GRB 130427A resided in a super-bubble.

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arXiv2013

The Bright Optical flash and Afterglow from the Gamma-Ray Burst GRB 130427A

The optical light that is generated simultaneously with the x-rays and gamma-rays during a gamma-ray burst (GRB) provides clues about the nature of the explosions that occur as massive stars collapse to form black holes. We report on the bright optical flash and fading afterglow from the powerful burst GRB 130427A and present a comparison with the properties of the gamma-ray emission that show correlation of the optical and >100 MeV photon flux light curves during the first 7,000 seconds. We attribute this correlation to co-generation in an external shock. The simultaneous, multi-color, optical observations are best explained at early times by reverse shock emission generated in the relativistic burst ejecta as it collides with surrounding material and at late times by a forward shock traversing the circumburst environment. The link between optical afterglow and >100 MeV emission suggests that nearby early peaked afterglows will be the best candidates for studying GRB emission at GeV/TeV energies.

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arXiv2013

Peaks of optical and X-ray afterglow light-curves

The peaks of 30 optical afterglows and 14 X-ray light-curves display a good anticorrelation of the peak flux with the peak epoch: F_p ~ t_p^{-2.0} in the optical, F_p ~ t_p^{-1.6} in the X-ray, the distributions of the peak epochs being consistent with each other. We investigate the ability of two forward-shock models for afterglow light-curve peaks -- an observer location outside the initial jet aperture and the onset of the forward-shock deceleration -- to account for those peak correlations. For both models, the slope of the F_p - t_p relation depends only on the slope of the afterglow spectrum. We find that only a conical jet seen off-aperture and interacting with a wind-like medium can account for both the X-ray peak relation, given the average X-ray spectral slope beta_x = 1.0, and for the larger slope of the optical peak relation. However, any conclusion about the origin of the peak flux - peak epoch correlation is, at best, tentative, because the current sample of X-ray peaks is too small to allow a reliable measurement of the F_p - t_p relation slope and because more than one mechanism and/or one afterglow parameter may be driving that correlation.

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arXiv2012

The possible ubiquity of energy injection in Gamma-Ray Burst afterglows

Since its launch in 2004, the Swift satellite has monitored the X-ray afterglows of several hundred Gamma-Ray Bursts, and revealed that their X-ray light-curves are more complex than previously thought, exhibiting up to three power-law segments. Energy injection into the relativistic blast-wave energizing the burst ambient medium has been proposed most often to be the reason for the X-ray afterglow complexity. We examine 117 light-curve breaks of 98 Swift X-ray afterglows, selected for their high-quality monitoring and well-constrained flux decay rates. Thirty percent of afterglows have a break that can be an adiabatic jet-break, in the sense that there is one variant of the forward-shock emission from a collimated outflow model that can account for both the pre- and post-break flux power-law decay indices, given the measured X-ray spectral slope. If allowance is made for a steady energy injection into the forward-shock, then another 56 percent of X-ray afterglows have a light-curve break that can be explained with a jet-break. The remaining 12 percent that are not jet-breaks, as well as the existence of two breaks in 19 afterglows (out of which only one can be a jet-break), suggest that some X-ray breaks arise from a sudden change in the rate at which energy is added to the blast-wave, and it may well be that a larger fraction of X-ray light-curve breaks are generated by that mechanism. To test the above two mechanisms for afterglow light-curve breaks, we derive comprehensive analytical results for the dynamics of outflows undergoing energy injection and for their light-curves, including closure relations for inverse-Compton afterglows and for the emission from spreading jets interacting with an wind-like ambient medium.

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arXiv2012

GRB 091029: At the limit of the fireball scenario

Using high-quality, broad-band afterglow data for GRB 091029, we test the validity of the forward-shock model for gamma-ray burst afterglows. We used multi-wavelength (NIR to X-ray) follow-up observations obtained with the GROND, BOOTES-3/YA and Stardome optical ground-based telescopes, and the UVOT and the XRT onboard the Swift satellite. To explain the almost totally decoupled light curves in the X-ray and optical/NIR domains, a two-component outflow is proposed. Several models are tested, including continuous energy injection, components with different electron energy indices and components in two different stages of spectral evolution. Only the last model can explain both the decoupled light curves with asynchronous peaks and the peculiar SED evolution. However, this model has so many unknown free parameters that we are unable to reliably confirm or disprove its validity, making the afterglow of GRB 091029 difficult to explain in the framework of the simplest fireball model.

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arXiv2012

Broadband study of GRB 091127: a sub-energetic burst at higher redshift?

GRB 091127 is a bright gamma-ray burst (GRB) detected by Swift at a redshift z=0.49 and associated with SN 2009nz. We present the broadband analysis of the GRB prompt and afterglow emission and study its high-energy properties in the context of the GRB/SN association. While the high luminosity of the prompt emission and standard afterglow behavior are typical of cosmological long GRBs, its low energy release, soft spectrum and unusual spectral lag connect this GRB to the class of sub-energetic bursts. We discuss the suppression of high-energy emission in this burst, and investigate whether this behavior could be connected with the sub-energetic nature of the explosion.

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arXiv2011

The Swift/Fermi GRB 080928 from 1 eV to 150 keV

We present the results of a comprehensive study of the gamma-ray burst 080928 and of its afterglow. GRB 080928 was a long burst detected by Swift/BAT and Fermi/GBM. It is one of the exceptional cases where optical emission had already been detected when the GRB itself was still radiating in the gamma-ray band. For nearly 100 seconds simultaneous optical, X-ray and gamma-ray data provide a coverage of the spectral energy distribution of the transient source from about 1 eV to 150 keV. In particular, we show that the SED during the main prompt emission phase agrees with synchrotron radiation. We constructed the optical/near-infrared light curve and the spectral energy distribution based on Swift/UVOT, ROTSE-IIIa (Australia), and GROND (La Silla) data and compared it to the X-ray light curve retrieved from the Swift/XRT repository. We show that its bumpy shape can be modeled by multiple energy-injections into the forward shock.Furthermore, we investigate whether the temporal and spectral evolution of the tail emission of the first strong flare seen in the early X-ray light curve can be explained by large-angle emission (LAE). We find that a nonstandard LAE model is required to explain the observations. Finally, we report on the results of our search for the GRB host galaxy, for which only a deep upper limit can be provided.

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arXiv2011

Optical afterglows of Gamma-Ray Bursts: peaks, plateaus, and possibilities

The optical light-curves of GRB afterglows display either peaks or plateaus. We identify 16 afterglows of the former type, 17 of the latter, and 4 with broad peaks, that could be of either type. The optical energy release of these two classes is similar and is correlated with the GRB output, the correlation being stronger for peaky afterglows, which suggests that the burst and afterglow emissions of peaky afterglows are from the same relativistic ejecta and that the optical emission of afterglows with plateaus arises more often from ejecta that did not produce the burst emission. Consequently, we propose that peaky optical afterglows are from impulsive ejecta releases and that plateau optical afterglows originate from long-lived engines, the break in the optical light-curve (peak or plateau end) marking the onset of the entire outflow deceleration. In the peak luminosity--peak time plane, the distribution of peaky afterglows displays an edge with L_p \propto t_p^{-3}, which we attribute to variations (among afterglows) in the ambient medium density. The fluxes and epochs of optical plateau breaks follow a L_b \propto t_b^{-1} anticorrelation. Sixty percent of 25 afterglows that were well-monitored in the optical and X-rays show light-curves with comparable power-law decays indices and achromatic breaks. The other 40 percent display three types of decoupled behaviours: i) chromatic optical light-curve breaks (perhaps due to the peak of the synchrotron spectrum crossing the optical), ii) X-ray flux decays faster than in the optical (suggesting that the X-ray emission is from local inverse-Compton scattering), and iii) chromatic X-ray light-curve breaks (indicating that the X-ray emission is from external up-scattering).

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arXiv2011

GRB 090510: a short burst from a massive star ?

GRB afterglow 090510 is (so far) the best-monitored afterglow in the optical, X-ray, and above 100 MeV, measurements covering 2-3 decades in time at each frequency. Owing to its power-law temporal decay and power-law spectrum, it seems very likely that the highest energy emission is from the forward-shock energizing the ambient medium (the standard blast-wave model for GRB afterglows), the GeV flux and its decay rate being consistent with that model's expectations. However, the synchrotron emission from a collimated outflow (the standard jet model) has difficulties in accounting for the lower-energy afterglow emission, where a simultaneous break occurs at 2 ks in the optical and X-ray light-curves, but with the optical flux decay (before and after the break) being much slower than in the X-rays (at same time). The measured X-ray and GeV fluxes are incompatible with the higher-energy afterglow emission being from same spectral component as the lower-energy afterglow emission, which suggests a synchrotron self-Compton model for this afterglow. Cessation of energy injection in the blast-wave and an ambient medium with a wind-like n ~ r^{-2} density can explain all features of the optical and X-ray light-curves of GRB afterglow 090510. Such an ambient medium radial structure is incompatible with this short-GRB originating from the merger of two compact stars.

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arXiv2008

Gamma-Ray Burst afterglows: theory and observations

I discuss some theoretical expectations for the synchrotron emission from a relativistic blast-wave interacting with the ambient medium, as a model for GRB afterglows, and compare them with observations. An afterglow flux evolving as a power-law in time, a bright optical flash during/after the burst, and a light-curve break due to a tight ejecta collimation are the major predictions that were confirmed observationally, but it should be recognized that light-curve decay indices are not correlated with the spectral slopes (as would be expected), optical flashes are quite rare, and jet-breaks harder to find in Swift X-ray afterglows. The slowing of the early optical flux decay rate is accompanied by a spectral evolution, indicating that the emission from ejecta (energized by the reverse shock) is dominant in the optical over that from the forward shock (which energizes the ambient medium) only up to 1 ks. However, a long-lived reverse shock is required to account for the slow radio flux decays observed in many afterglows after ~10 day. X-ray light-curve plateaus could be due to variations in the average energy-per-solid-angle of the blast-wave, confirming to two other anticipated features of GRB outflows: energy injection and angular structure. The latter is also the more likely origin of the fast-rises seen in some optical light-curves. To account for the existence of both chromatic and achromatic afterglow light-curve breaks, the overall picture must be even more complex and include a new mechanism that dominates occasionally the emission from the blast-wave: either late internal shocks or scattering (bulk and/or inverse-Compton) of the blast-wave emission by an outflow interior to it.

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