arXiv subjects

P. Veres

Publications and source records attributed to P. Veres.

At least 19 recordsLinked to original sources

arXiv2025

Fastest or Significant: A Systematic Framework for Validating Global Minimum Variability Timescale Measurements of Gamma-ray Bursts

The minimum variability timescale (MVT) is a key observable used to probe the central engines of Gamma-Ray Bursts (GRBs) by constraining the emission region size and the outflow Lorentz factor. However, its interpretation is often ambiguous: statistical noise and analysis choices can bias measurements, making it difficult to distinguish genuine source variability from artifacts. Here we perform a comprehensive suite of simulations to establish a quantitative framework for validating Haar-based MVT measurements. We show that in multi--component light curves, the MVT returns the most statistically significant structure in the interval, which is not necessarily the fastest intrinsic timescale, and can therefore converge to intermediate values. Reliability is found to depend jointly on the MVT value and its signal-to-noise ratio ($\mathrm{SNR}_{\mathrm{MVT}}$), with shorter intrinsic timescales requiring proportionally higher $\mathrm{SNR}_{\mathrm{MVT}}$ to be resolved. We use this relation to define an empirical MVT Validation Curve, and provide a practical workflow to classify measurements as robust detections or upper limits. Applying this procedure to a sample of Fermi-GBM bursts shows that several published MVT values are better interpreted as upper limits. These results provide a path toward standardizing MVT analyses and highlight the caution required when inferring physical constraints from a single MVT measurement in complex events.

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arXiv2025

GRB 241105A: A test case for GRB classification and rapid r-process nucleosynthesis channels

Gamma-ray bursts (GRBs) offer a powerful window to probe the progenitor systems responsible for the formation of heavy elements through the rapid neutron capture (r-) process, thanks to their exceptional luminosity, which allows them to be observed across vast cosmic distances. GRB 241105A, observed at a redshift of z = 2.681, features a short initial spike (1.5 s) and a prolonged weak emission lasting about 64 s, positioning it as a candidate for a compact binary merger and potentially marking it as the most distant merger-driven GRB observed to date. However, the emerging ambiguity in GRB classification necessitates further investigation into the burst's true nature. Prompt emission analyses, such as hardness ratio, spectral lag, and minimum variability timescales, yield mixed classifications, while machine learning-based clustering places GRB 241105A near both long-duration mergers and collapsar GRBs. We conducted observations using the James Webb Space Telescope (JWST) to search for a potential supernova counterpart. Although no conclusive evidence was found for a supernova, the host galaxy's properties derived from the JWST observations suggest active star formation with low metallicity, and a sub-kpc offset of the afterglow from the host, which appears broadly consistent with a collapsar origin. Nevertheless, a compact binary merger origin cannot be ruled out, as the burst may plausibly arise from a fast progenitor channel. This would have important implications for heavy element enrichment in the early Universe.

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arXiv2025

Fermi-GBM Observations of GRB 230307A: An Exceptionally Bright Long-Duration Gamma-ray Burst with an Associated Kilonova

On March 7th, 2023 the \textit{Fermi} Gamma-ray Burst Monitor observed the second highest fluence gamma-ray burst (GRB) ever, GRB~230307A. With a duration beyond 100~s, GRB~230307A contains a multitude of rapidly-varying peaks, and was so bright it caused instrumental effects in the GBM detectors. The high fluence of this burst, (6.02 $\pm$ 0.02)$\times$10$^{-3}$ erg cm$^{-2}$, prompted rapid follow-up across the electro magnetic spectrum including the discovery of an associated kilonova. GRB~230307A is one of a few long GRBs with an associated compact merger origin. Three main temporal regions of interest are identified for fine time-resolution spectral analysis: triggering pulse, main emission, and late emission, and the parameter evolution is traced across these regions. The high flux of the burst allowed for the statistical preference of a more complex, physically-motivated model, the Double Smoothly Broken Power Law, over typical spectral fitting functions for GRBs. From this model the evolution of the parameters was found to be in accordance with those expected for synchrotron radiation in the fast-cooling regime. Additionally, it was found that the flux experiences a steep decline in late time intervals, a feature which is often attributed to high-latitude emission, which follows the dissipation episodes. Furthermore, GRB~230307A was found to have one of the highest inferred bulk Lorentz factors of $\Gamma = 1600$. GRB~230307A is a noteworthy burst in terms of flux alone, but additionally provides a unique insight into the possible temporal and spectral characteristics of a new long merger class of GRBs.

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arXiv2024

Synchrotron self-Compton in a radiative-adiabatic fireball scenario: Modelling the multiwavelength observations in some Fermi/LAT bursts

Energetic GeV photons expected from the closest and the most energetic Gamma-ray bursts (GRBs) provide an unique opportunity to study the very-high-energy emission as well as the possible correlations with lower energy bands in realistic GRB afterglow models. In the standard GRB afterglow model, the relativistic homogeneous shock is usually considered to be fully adiabatic, however, it could be partially radiative. Based on the external forward-shock scenario in both stellar wind and constant-density medium. We present a radiative-adiabatic analytical model of the synchrotron self-Compton (SSC) and synchrotron processes considering an electron energy distribution with a power-law index of 1 < p < 2 and 2 $\leq$ p. We show that the SSC scenario plays a relevant role in the radiative parameter $\epsilon$, leading to a prolonged evolution during the slow cooling regime. In a particular case, we derive the Fermi/LAT light curves together with the photons with energies $\geq$ 100 MeV in a sample of nine bursts from the second Fermi/LAT GRB catalog that exhibited temporal and spectral indices with $\geq$ 1.5 and $\approx$ 2, respectively. These events can hardly be described with closure relations of the standard synchrotron afterglow model, and also exhibit energetic photons above the synchrotron limit. We have modeled the multi-wavelength observations of our sample to constrain the microphysical parameters, the circumburst density, the bulk Lorentz factor and the mechanism responsible for explaining the energetic GeV photons.

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arXiv2024

Non-detection of Neutrinos from the BOAT: Improved Constraints on the Parameters of GRB 221009A

The IceCube neutrino observatory detects the diffuse astrophysical neutrino background with high significance, but the contribution of different classes of sources is not established. Because of their non-thermal spectrum, gamma-ray bursts (GRBs) are prime particle acceleration sites and one of the candidate classes for significant neutrino production. Exhaustive searches, based on stacking analysis of GRBs however could not establish the link between neutrinos and GRBs. Gamma-ray burst GRB 221009A had the highest time integrated gamma-ray flux of any detected GRB so far. The total fluence exceeds the sum of all Fermi Gamma-ray Burst Monitor (GBM) detected GRBs by a factor of two. Because it happened relatively nearby, it is one of the most favorable events for neutrino production from GRBs yet no neutrinos were detected. We calculate neutrino fluxes for this GRB in the TeV-PeV range using the most accurate, time-resolved spectral data covering the brightest intervals. We place limits on the physical parameters (Lorentz factor, baryon loading or emission radius) of the burst that are better by a factor of 2 compared to previous limits. The neutrino non-detection indicates a bulk Lorentz factor greater than 500 and possibly even 800, consistent with other observations.

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arXiv2024

Polarization Measurement of Gamma-ray Bursts with Fermi-GBM: The Case of GRB 180720B

To achieve confident non-zero polarization measurements for gamma-ray bursts (GRBs) we need sensitive polarimeters and bright GRBs. Here we report on the polarimetric analysis of the bright GRB 180720B using the \Fermi Gamma-ray Burst Monitor (GBM). We rely on the detection of photons that scattered off Earth's atmosphere and into GBM from this burst. Polarized gamma-rays will exhibit a characteristic pattern when scattering off the atmosphere that differs from an unpolarized beam. We compare the measured photon counts in the GBM detectors with extensive simulations of polarized beams to derive the most probable polarization degree (PD) and angle (PA). For the entire GRB, we find PD$=72^{+24}_{-30}\% ~(1\sigma)$ and PA$=91^{+11}_{-9}$ deg ($1\sigma$, equatorial frame). Interestingly, the PA value is broadly consistent with an early optical PA measurement by the Kanata telescope, starting shortly after the end of the prompt emission. The consistency of PAs lends support for this method. The relatively high polarization degree (albeit with large uncertainties) agrees with similar past measurements suggesting that some GRBs might be highly polarized. This will be confirmed or refuted by the upcoming dedicated GRB polarimeters.

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arXiv2024

High-resolution imaging of the evolving bipolar outflows in symbiotic novae: The case of the RSOphiuchi 2021 nova outburst

The recurrent and symbiotic nova RS Ophiuchi (RSOph) underwent a new outburst phase during August 2021, about 15 years after the last event that occurred in 2006. This outburst represents the first nova event ever detected at very high energies (VHE, E>100\,GeV), and a whole set of coordinated multiwavelength observations were triggered by this event. The main goals of this work are to characterize the evolving morphology of the expanding bipolar ejecta with high accuracy and to determine the physical conditions of the surrounding medium in which they propagate. By means of high-resolution very long baseline interferometry (VLBI) radio observations, we monitored RSOph with the European VLBI Network (EVN) and e-MERLIN at 1.6 and 5\,GHz during multiple epochs from 14 to 65 days after the explosion. We reveal an evolving source structure consisting of a central and compact core and two elongated bipolar outflows expanding on opposite sides of the core in the east-west direction. The ejecta angular separation with time is consistent with a linear expansion with an average projected speed of $\sim7000$ km s$^{-1}$. We find clear evidence of a radial dependence of the density along the density enhancement on the orbital plane (DEOP), going from 1$\times$10$^7$ ~cm$^{-3}$ close to the central binary to 9$\times$10$^5$~cm$^{-3}$ at $\sim400$~AU. Thanks to the accurate source astrometric position provided by \textit{Gaia} DR3, in this work we draw a detailed scenario of the geometry and physics of the RSOph evolving source structure after the most recent nova event. We conclude that most of the mass lost by the red giant companion goes into the DEOP, for which we estimate a total mass of $6.4 \times 10^{-6} ~~\mathrm{M_\odot}$, and into the circumstellar region, while only a small fraction (about one-tenth) is accreted by the white dwarf.

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arXiv2023

A Joint Fermi-GBM and Swift-BAT Analysis of Gravitational-Wave Candidates from the Third Gravitational-wave Observing Run

We present Fermi Gamma-ray Burst Monitor (Fermi-GBM) and Swift Burst Alert Telescope (Swift-BAT) searches for gamma-ray/X-ray counterparts to gravitational wave (GW) candidate events identified during the third observing run of the Advanced LIGO and Advanced Virgo detectors. Using Fermi-GBM on-board triggers and sub-threshold gamma-ray burst (GRB) candidates found in the Fermi-GBM ground analyses, the Targeted Search and the Untargeted Search, we investigate whether there are any coincident GRBs associated with the GWs. We also search the Swift-BAT rate data around the GW times to determine whether a GRB counterpart is present. No counterparts are found. Using both the Fermi-GBM Targeted Search and the Swift-BAT search, we calculate flux upper limits and present joint upper limits on the gamma-ray luminosity of each GW. Given these limits, we constrain theoretical models for the emission of gamma-rays from binary black hole mergers.

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arXiv2023

Extreme Variability in a Long Duration Gamma-ray Burst Associated with a Kilonova

The recent discovery of a kilonova from the long duration gamma-ray burst, GRB 211211A, challenges classification schemes based on temporal information alone. Gamma-ray properties of GRB 211211A reveal an extreme event, which stands out among both short and long GRBs. We find very short variations (few ms) in the lightcurve of GRB 211211A and estimate ~1000 for the Lorentz factor of the outflow. We discuss the relevance of the short variations in identifying similar long GRBs resulting from compact mergers. Our findings indicate that in future gravitational wave follow-up campaigns, some long duration GRBs should be treated as possible strong gravitational wave counterparts.

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arXiv2023

Fermi-GBM Discovery of GRB 221009A: An Extraordinarily Bright GRB from Onset to Afterglow

We report the discovery of GRB 221009A, the highest flux gamma-ray burst ever observed by the Fermi Gamma-ray Burst Monitor (GBM). This GRB has continuous prompt emission lasting more than 600 seconds which smoothly transitions to afterglow visible in the GBM energy range (8 keV--40 MeV), and total energetics higher than any other burst in the GBM sample. By using a variety of new and existing analysis techniques we probe the spectral and temporal evolution of GRB 221009A. We find no emission prior to the GBM trigger time (t0; 2022 October 9 at 13:16:59.99 UTC), indicating that this is the time of prompt emission onset. The triggering pulse exhibits distinct spectral and temporal properties suggestive of the thermal, photospheric emission of shock-breakout, with significant emission up to $\sim$15 MeV. We characterize the onset of external shock at t0+600 s and find evidence of a plateau region in the early-afterglow phase which transitions to a slope consistent with Swift-XRT afterglow measurements. We place the total energetics of GRB 221009A in context with the rest of the GBM sample and find that this GRB has the highest total isotropic-equivalent energy ($\textrm{E}_{\gamma,\textrm{iso}}=1.0\times10^{55}$ erg) and second highest isotropic-equivalent luminosity ($\textrm{L}_{\gamma,\textrm{iso}}=9.9\times10^{53}$ erg/s) based on redshift of z = 0.151. These extreme energetics are what allowed us to observe the continuously emitting central engine of GBM from the beginning of the prompt emission phase through the onset of early afterglow.

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arXiv2023

Deciphering the unusual stellar progenitor of GRB 210704A

GRB~210704A is a burst of intermediate duration ($T_{90} \sim 1-4$~s) followed by a fading afterglow and an optical excess that peaked about 7 days after the explosion. Its properties, and in particular those of the excess, do not easily fit into the well established classification scheme of GRBs as being long or short, leaving the nature of its progenitor uncertain. We present multi-wavelength observations of the GRB and its counterpart, observed up to 160 days after the burst. In order to decipher the nature of the progenitor system, we present a detailed analysis of the GRB high-energy properties (duration, spectral lag, and Amati correlation), its environment, and late-time optical excess. We discuss three possible scenarios: a neutron star merger, a collapsing massive star, and an atypical explosion possibly hosted in a cluster of galaxies. We find that traditional kilonova and supernova models do not match well the properties of the optical excess, leaving us with the intriguing suggestion that this event was an exotic high-energy merger.

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arXiv2022

Polarization From A Radially Stratified Off-Axis GRB Outflow

While the dominant radiation mechanism gamma-ray bursts (GRBs) remains a question of debate, synchrotron emission is one of the foremost candidates to describe the multi-wavelength afterglow observations. As such, it is expected that GRBs should present some degree of polarization across their evolution - presenting a feasible means of probing these bursts' energetic and angular properties. Although obtaining polarization data is difficult due to the inherent complexities regarding GRB observations, advances are being made, and theoretical modeling of synchrotron polarization is now more relevant than ever. In this manuscript, we present the polarization for a fiduciary model where the synchrotron forward-shock emission evolving in the radiative-adiabatic regime is described by a radially stratified off-axis outflow. This is parameterized with a power-law velocity distribution and decelerated in a constant-density and wind-like external environment. We apply this theoretical polarization model for selected bursts presenting evidence of off-axis afterglow emission, including the nearest orphan GRB candidates observed by the Neil Gehrels Swift Observatory and a few Gravitational Wave (GWs) events that could generate electromagnetic emission. In the case of GRB 170817A, we require the available polarimetric upper limits in radio wavelengths to constrain its magnetic field geometry.

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arXiv2022

Afterglow Polarization from Off-Axis GRB Jets

As we further our studies on Gamma-ray bursts (GRBs), both on theoretical models and observational tools, more and more options begin to open for exploration of its physical properties. As transient events primarily dominated by synchrotron radiation, it is expected that the synchrotron photons emitted by GRBs should present some degree of polarization throughout the evolution of the burst. Whereas observing this polarization can still be challenging due to the constraints on observational tools, especially for short GRBs, it is paramount that the groundwork is laid for the day we have abundant data. In this work, we present a polarization model linked with an off-axis spreading top-hat jet synchrotron scenario in a stratified environment with a density profile $n(r)\propto r^ {-k}$. We present this model's expected temporal polarization evolution for a realistic set of afterglow parameters constrained within the values observed in the GRB literature for four degrees of stratification $k=0,1,1.5 {\rm \, and\,} 2$ and two magnetic field configurations with high extreme anisotropy. We apply this model and predict polarization from a set of GRBs exhibiting off-axis afterglow emission. In particular, for GRB 170817A, we use the available polarimetric upper limits to rule out the possibility of a extremely anisotropic configuration for the magnetic field.

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arXiv2022

Exploring the Early Afterglow Polarization of GRB 190829A

The GRB 190829A has been widely studied due to its nature and the high energy emission presented. Due to the detection of a very-high-energy component by the High Energy Stereoscopic System and the event's atypically middling luminosity, it has been categorized in a select, limited group of bursts bordering classic GRBs and nearby sub-energetic events. Given the range of models utilized to adequately characterize the afterglow of this burst, it has proven challenging to identify the most probable explanation. Nevertheless, the detection of polarization data provided by the MASTER collaboration has added a new aspect to GRB 190829A that permits us to attempt to explore this degeneracy. In this paper, we present a polarization model coupled with a synchrotron forward-shock model -- a component in all models used to describe GRB 190829A's afterglow -- in order to fit the polarization's temporal evolution with the existing upper limits ($\Pi < 6\%$). We find that the polarization generated from an on-axis emission is favored for strongly anisotropic magnetic field ratios, while an off-axis scenario cannot be fully ruled out when a more isotropic framework is taken into account.

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arXiv2022

Radio interferometric imaging of RS Oph bipolar ejecta for the 2021 nova outburst

The recurrent nova and symbiotic binary RS Oph erupted again in August 2021 for its eighth known outburst. As part of a multi-epoch and frequency campaign, we observed RS Oph 34 days after the outburst at 5 GHz with the European VLBI Network (EVN). The radio image is elongated over the east-west direction for a total extension of about 90 mas (or about 240 AU at the Gaia DR3 distance d=2.68 [-0.15/+0.17] kpc), and shows a bright and compact central component coincident with the Gaia astrometric position, and two lobes east and west of it, expanding perpendicular to the orbital plane. By comparing with the evolution of emission-line profiles on optical spectra, we found the leading edge of the lobes to be expanding at 7550 km/s, and i=54 deg as the orbital inclination of the binary. The 2021 radio structure is remarkably similar to that observed following the 2006 eruption. The obscuring role of the density enhancement on the orbital plane (DEOP) is discussed in connection to the time-dependent visibility of the receding lobe in the background to the DEOP, and the origin of the triple-peaked profiles is traced to the ring structure formed by the nova ejecta impacting the DEOP.

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arXiv2022

GRB Afterglow of the Sub-relativistic Materials with Energy Injection

Sub-relativistic materials launched during the merger of binary compact objects and the core-collapse of massive stars acquire velocity structures when expanding in a stratified environment. The remnant (either a spinning magnetized neutron star (NS) or a central black hole) from the compact-object or core-collapse could additionally inject energy into the afterglow via spin-down luminosity or/and by accreting fall-back material, producing a refreshed shock, modifying the dynamics, and leading to rich radiation signatures at distinct timescales and energy bands with contrasting intensities. We derive the synchrotron light curves evolving in a stratified environment when a power-law velocity distribution parametrizes the energy of the shock, and the remnant continuously injects energy into the blastwave. As the most relevant case, we describe the latest multi-wavelength afterglow observations ($\gtrsim 900$ days) of the GW170817/GRB 170817A event via a synchrotron afterglow model with energy injection of a sub-relativistic material. The features of the remnant and the synchrotron emission of the sub-relativistic material are consistent with a spinning magnetized NS and the faster "blue" kilonova afterglow, respectively. Using the multi-band observations of some short-bursts with evidence of kilonova, we provide constraints on the expected afterglow emission.

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arXiv2022

Modeling Gamma-ray burst Afterglow observations with an Off-axis Jet emission

Gamma-ray bursts (GRBs) are fascinating extragalactic objects. They represent a fantastic opportunity to investigate unique properties not exhibited in other sources. Multi-wavelength afterglow observations from some short- and long-duration GRBs reveal an atypical long-lasting emission that evolves differently from the canonical afterglow light curves favoring the off-axis emission. We present an analytical synchrotron afterglow scenario, and the hydrodynamical evolution of an off-axis top-hat jet decelerated in a stratified surrounding environment. The analytical synchrotron afterglow model is shown during the coasting, deceleration (off- and on-axis emission), and the post-jet-break decay phases, and the hydrodynamical evolution is computed by numerical simulations showing the time evolution of the Doppler factor, the half-opening angle, the bulk Lorentz factor, and the deceleration radius. We show that numerical simulations are in good agreement with those derived with our analytical approach. We apply the current synchrotron model and describe successfully the delayed non-thermal emission observed in a sample of long and short GRBs with evidence of off-axis emission. Furthermore, we provide constraints on the possible afterglow emission by requiring the multi-wavelength upper limits derived for the closest Swift-detected GRBs and promising gravitational-wave events.

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arXiv2021

Fermi-GBM Observations of GRB 210812A: Signatures of a Million Solar Mass Gravitational Lens

Observing gravitationally lensed objects in the time domain is difficult, and well-observed time-varying sources are rare. Lensed gamma-ray bursts (GRBs) offer improved timing precision to this class of objects complementing observations of quasars and supernovae. The rate of lensed GRBs is highly uncertain, approximately 1 in 1000. The Gamma-ray Burst Monitor (GBM) onboard the Fermi Gamma-ray Space Telescope has observed more than 3000 GRBs making it an ideal instrument to uncover lensed bursts. Here we present observations of GRB 210812A showing two emission episodes, separated by 33.3 s, and with flux ratio of about 4.5. An exhaustive temporal and spectral analysis shows that the two emission episodes have the same pulse and spectral shape, which poses challenges to GRB models. We report multiple lines of evidence for a gravitational lens origin. In particular, modeling the lightcurve using nested sampling we uncover strong evidence in favor of the lensing scenario. Assuming a point mass lens, the mass of the lensing object is about 1 million solar masses. High-resolution radio imaging is needed for future lens candidates to derive tighter constraints.

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