arXiv subjects

A. Galvan-Gamez

Publications and source records attributed to A. Galvan-Gamez.

At least 19 recordsLinked to original sources

arXiv2026

A merger within a merger: Chandra pinpoints the short GRB 230906A in a peculiar environment

We report the precise X-ray localization of GRB 230906A, a short duration ($T_{90}\sim$0.9 s) burst with no optical or radio counterpart. Deep imaging with the Hubble Space Telescope detects a faint galaxy (G$^\ast$; $F160W\simeq26$ AB mag) coincident with the sub-arcsecond X-ray position. Compared with standard GRB galaxies, its faintness, compact size and color would suggest a high redshift ($z\gtrsim$3) host. However, our observations also reveal the presence of a galaxy group at $z\!\sim$0.453, confirmed spectroscopically with VLT/MUSE, with clear signs of interactions and mergers among group members. The GRB and its putative host project onto an extended ($\approx$180 kpc) tidal tail emerging from the group's central galaxy. The probability of a chance alignment is small ($P_{cc}\!\lesssim\!4$%), we thus argue that the GRB and its galaxy G$^*$ reside within the group. Their peculiar location along the tidal debris suggests that an enhanced burst of star formation, induced by the galaxy merger, might have formed the progenitor compact binary $\lesssim$700 Myr ago. The compact binary later evolved in a neutron star merger which produced GRB 230906A and injected $r$-process material into the surrounding circumgalactic medium.

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arXiv2024

Analysis of GRB Closure Relationship in Multi-wavelengths

Gamma-ray bursts (GRBs) are intense pulses of high-energy emission associated with massive stars' death or compact objects' coalescence. Their multi-wavelength observations help verify the reliability of the standard fireball model. We analyze 14 GRBs observed contemporaneously in gamma-rays by the \textit{Fermi} Large Area Telescope (LAT), in X-rays by the \textit{Swift} Telescope, and in the optical bands by \textit{Swift} and many ground-based telescopes. We study the correlation between the spectral and temporal indices using closure relations according to the synchrotron forward-shock model in the stratified medium ($n \propto r^{-k}$) with $k$ ranging from 0 to 2.5. We find that the model without energy injection is preferred over the one with energy injection in all the investigated wavelengths. In gamma-rays, we only explored the $ν> $ max\{$ν_c,ν_m$\} (SC/FC) cooling condition (where $ν_c$ and $ν_m$ are the cooling and characteristic frequencies, namely the frequencies at the spectral break). In the X-ray and optical bands, we explored all the cooling conditions, including $ν_m < ν< ν_c$ (SC), $ν_c < ν< ν_m$ (FC), and SC/FC, and found a clear preference for SC for X-rays and SC/FC for optical. Within these cooling conditions, X-rays exhibit the highest rate of occurrence for the density profile with $k = 0$, while the optical band has the highest occurrence for $k$ = 2.5 when considering no energy injection. Although we can pinpoint a definite environment for some GRBs, we find degeneracies in other 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 $ε$, 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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arXiv2023

Off-axis Afterglow Closure Relations and Fermi-LAT Detected Gamma-Ray Bursts

Gamma-ray bursts (GRBs) are one of the most promising transient events for studying multi-wavelength observations in extreme conditions. Observation of GeV photons from bursts would provide crucial information on GRB physics, including the off-axis emission. The Second Gamma-ray Burst Catalog (2FLGC) was announced by the Fermi Large Area Telescope (Fermi-LAT) Collaboration. This catalog includes 29 bursts with photon energy higher than 10 GeV. While the synchrotron forward-shock model has well explained the afterglow data of GRBs, photon energies greater than 10 GeV are very difficult to interpret within this framework. To study the spectral and temporal indices of those bursts described in 2FLGC, Fraija et al. (2022a) proposed the closure relations (CRs) of the synchrotron self-Compton (SSC) emitted from an on-axis jet which decelerates in stellar-wind and the constant-density medium. In this paper, we extend the CRs of the SSC afterglow from an on-axis scenario to an off-axis, including the synchrotron afterglow radiation that seems off-axis. In order to investigate the spectral and temporal index evolution of those bursts reported in 2FLGC, we consider the hydrodynamical evolution with energy injection in the adiabatic and radiative regime for an electron distribution with a spectral index of $1<p<2$ and $2 < p$. The results show that the most likely scenario for synchrotron emission corresponds to the stellar wind whether or not there is energy injection and that the most likely scenario for SSC emission corresponds to the constant density when there is no energy injection and to the stellar wind when there is energy injection.

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arXiv2023

Galactic Gamma-Ray Diffuse Emission at TeV energies with HAWC Data

The Galactic gamma-ray diffuse emission (GDE) is emitted by cosmic rays (CRs), ultra-relativistic protons and electrons, interacting with gas and electromagnetic radiation fields in the interstellar medium. Here we present the analysis of TeV diffuse emission from a region of the Galactic Plane over the range in longitude of $l\in[43^\circ,73^\circ]$, using data collected with the High Altitude Water Cherenkov (HAWC) detector. Spectral, longitudinal and latitudinal distributions of the TeV diffuse emission are shown. The radiation spectrum is compatible with the spectrum of the emission arising from a CR population with an "index" similar to that of the observed CRs. When comparing with the \texttt{DRAGON} \textit{base model}, the HAWC GDE flux is higher by about a factor of two. Unresolved sources such as pulsar wind nebulae and TeV halos could explain the excess emission. Finally, deviations of the Galactic CR flux from the locally measured CR flux may additionally explain the difference between the predicted and measured diffuse fluxes.

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arXiv2023

Closure Relations of Synchrotron Self-Compton in Afterglow stratified medium and Fermi-LAT Detected Gamma-Ray Bursts

The Second Gamma-ray Burst Catalog (2FLGC) was announced by the Fermi Large Area Telescope (Fermi-LAT) Collaboration. It includes 29 bursts with photon energy higher than 10 GeV. Gamma-ray burst (GRB) afterglow observations have been adequately explained by the classic synchrotron forward-shock model, however, photon energies greater than 10 GeV from these transient events are challenging, if not impossible, to characterize using this afterglow model. Recently, the closure relations (CRs) of the synchrotron self-Compton (SSC) forward-shock model evolving in a stellar wind and homogeneous medium was presented to analyze the evolution of the spectral and temporal indexes of those bursts reported in 2FLGC. In this work, we provide the CRs of the same afterglow model, but evolving in an intermediate density profile ($\propto {\rm r^{-k}}$) with ${\rm 0\leq k \leq2.5}$, taking into account the adiabatic/radiative regime and with/without energy injection for any value of the electron spectral index. The results show that the current model accounts for a considerable subset of GRBs that cannot be interpreted in either stellar-wind or homogeneous afterglow SSC model. The analysis indicates that the best-stratified scenario is most consistent with ${\rm k=0.5}$ for no-energy injection and ${\rm k=2.5}$ for energy injection.

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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 ($Π< 6\%$). 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

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

A Two-Zone Model as origin of Hard TeV Spectrum in Extreme BL Lacs

The emission of the so-called extreme BL Lacs poses challenges to the particle acceleration models. The hardness of their spectrum, $\lesssim 2$, in the high-energy band demands unusual parameters using the standard one-zone synchrotron self-Compton (SSC) model with a deficient magnetized plasma. Some authors use either two-zone or hadronic/lepto-hadronic models to relax these atypical values. In this work, we present a lepto-hadronic two-zone model to explain the multi-wavelength observations of the six best-known \textit{extreme} BL Lacs. The very-high-energy gamma-ray observations are described by the photo-hadronic processes in a blob close to the AGN core and by SSC and external inverse Compton-processes in an outer blob. The photo-hadronic interactions occur when accelerated protons in the inner blob interact with annihilation line photons from a sub-relativistic pair plasma. The X-ray observations are described by synchrotron radiation from the outer blob. The parameter values found from the description of the spectral energy distribution for each object with our phenomenological model are similar to each other, and lie in the typical range reported in BL Lacs.

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arXiv2021

Cosmic rays, neutrinos and GeV-TeV gamma rays from Starburst Galaxy NGC 4945

The detection of high-energy astrophysical neutrinos and ultra-high-energy cosmic rays (UHECRs) provides a new way to explore sources of cosmic rays. One of the highest energy neutrino events detected by IceCube, tagged as IC35, is close to the UHECR anisotropy region detected by Pierre Auger Observatory. The nearby starburst galaxy (SBG), NGC 4945, is close to this anisotropic region and inside the mean angular error of the IC35 event. Considering the hypernovae contribution located in the SB region of NGC 4945, which can accelerate protons up to $\sim 10^{17} \, {\rm eV}$ and inject them into the interstellar medium, we investigate the origin of this event around this starburst galaxy. We show that the interaction of these protons with the SB region's gas density could explain Fermi-LAT gamma-ray and radio observations if the magnetic field's strength in the SB region is the order of $\sim \rm mG$. Our estimated PeV neutrino events, in ten years, for this source is approximately 0.01 ($4\times10^{-4}$) if a proton spectral index of 2.4 (2.7) is considered, which would demonstrate that IC35 is not produced in the central region of this SBG. Additionally, we consider the superwind region of NGC 4945 and show that protons can hardly be accelerated in it up to UHEs.

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arXiv2021

HAWC observations of the acceleration of very-high-energy cosmic rays in the Cygnus Cocoon

Cosmic rays with energies up to a few PeV are known to be accelerated within the Milky Way. Traditionally, it has been presumed that supernova remnants were the main source of very-high-energy cosmic rays but theoretically it is difficult to get protons to PeV energies and observationally there simply is no evidence to support the remnants as sources of hadrons with energies above a few tens of TeV. One possible source of protons with those energies is the Galactic Center region. Here we report observations of 1-100 TeV gamma rays coming from the 'Cygnus Cocoon', which is a superbubble surrounding a region of OB2 massive star formation. These gamma rays are likely produced by 10-1000 TeV freshly accelerated CRs originating from the enclosed star forming region Cygnus OB2. Hitherto it was not known that such regions could accelerate particles to these energies. The measured flux is likely originated by hadronic interactions. The spectral shape and the emission profile of the Cocoon changes from GeV to TeV energies, which reveals the transport of cosmic particles and historical activity in the superbubble.

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arXiv2021

HAWC Search for High-Mass Microquasars

Microquasars with high-mass companion stars are promising very-high-energy (VHE; 0.1-100 TeV) gamma-ray emitters, but their behaviors above 10 TeV are poorly known. Using the High Altitude Water Cherenkov (HAWC) observatory, we search for excess gamma-ray emission coincident with the positions of known high-mass microquasars (HMMQs). No significant emission is observed for LS 5039, Cygnus X-1, Cygnus X-3, and SS 433 with 1,523 days of HAWC data. We set the most stringent limit above 10 TeV obtained to date on each individual source. Under the assumption that HMMQs produce gamma rays via a common mechanism, we have performed source-stacking searches, considering two different scenarios: I) gamma-ray luminosity is a fraction $ε_γ$ of the microquasar jet luminosity, and II) very-high-energy gamma rays are produced by relativistic electrons up-scattering the radiation field of the companion star in a magnetic field $B$. We obtain $ε_γ< 5.4\times 10^{-6}$ for scenario I, which tightly constrains models that suggest observable high-energy neutrino emission by HMMQs. In the case of scenario II, the non-detection of VHE gamma rays yields a strong magnetic field, which challenges synchrotron radiation as the dominant mechanism of the microquasar emission between 10 keV and 10 MeV.

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arXiv2020

Description of atypical bursts seen slightly off-axis

The detection of gravitational waves together with their electromagnetic counterpart, in the gamma-ray burst GRB 170817A, marked a new era of multi-messenger astronomy. Several theoretical models have been proposed to explain the atypical behavior of this event. Recently, it was shown that the multi-wavelength afterglow of GRB 170817A was consistent with a synchrotron forward-shock model when the outflow was viewed off-axis, decelerated in a uniform medium and parametrized through a power-law velocity distribution. Motivated by the upper limits on the very-high-energy emission, and the stratified medium in the close vicinity of a binary neutron star merger proposed to explain the gamma-ray flux in the short GRB 150101B, we extend the mechanism proposed to explain GRB 170817A to a more general scenario deriving the synchrotron self-Compton (SSC) and synchrotron forward-shock model when the off-axis outflow is decelerated in a uniform and stratified circumburst density. As particular cases, we show that the delayed and long-lasting afterglow emission observed in GRB 080503, GRB140903A, GRB 150101B, and GRB 160821B could be interpreted by a similar scenario to the one used to describe GRB 170817A. In addition, we show that the proposed scenario agrees with the MAGIC, Fermi-LAT and H.E.S.S upper limits on gamma-ray emission from GRB 160821B and GRB 170817A.

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arXiv2020

On the origin of the multi-GeV photons from the closest burst with intermediate luminosity: GRB 190829A

Very-high-energy (VHE) emission is usually interpreted in the synchrotron-self Compton (SSC) scenario, and expected from the low-redshift and high-luminosity gamma-ray bursts (GRBs), as GRB 180720B and GRB 190114C. Recently, VHE emission was detected by the H.E.S.S. telescopes from one of the closest burst GRB 190829A which was associated with the supernova (SN) 2019oyw. In this paper, we present a temporal and spectral analysis from optical bands to Fermi-LAT energy range over multiple observational periods beginning just after the BAT trigger time and extending for almost three months. We show that the X-ray and optical observations are consistent with synchrotron forward-shock emission evolving between the characteristic and cooling spectral breaks during the early and late afterglow in a uniform-density medium. Modeling the light curves together with its spectral energy distribution, it is shown that the outflow expands with an initial bulk Lorentz factor of $Γ\sim 30$, which is high for a low-luminosity GRBs and low for a high-luminosity GRBs. The values of the initial bulk Lorentz factor and the isotropic equivalent energy suggest that GRB 190829A is classified as an intermediate-luminosity burst and consequently, it becomes the first burst of this class in being detected in the VHE gamma-ray band by an imaging atmospheric Cherenkov telescope, and, in turn, the first event without being simultaneously observed by the Fermi-LAT instrument. Analyzing the intermediate-luminosity bursts with $z\lesssim 0.2$ such as GRB 130702A, we show that bursts with intermediate luminosity are potential candidates to be detected in very-high energies.

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arXiv2019

Modeling the observations of GRB 180720B: From radio to sub-TeV gamma-rays

Early and late multiwavelength observations play an important role in determining the nature of the progenitor, circumburst medium, physical processes and emitting regions associated to the spectral and temporal features of bursts. GRB 180720B is a long and powerful burst detected by a large number of observatories in multiwavelenths that range from radio bands to sub-TeV gamma-rays. The simultaneous multiwavelength observations were presented over multiple periods of time beginning just after the trigger time and extending for more than 30 days. The temporal and spectral analysis of Fermi LAT observations suggests that it presents similar characteristics to other bursts detected by this instrument. Coupled with X-ray and optical observations, the standard external-shock model in a homogeneous medium is favored by this analysis. The X-ray flare is consistent with the synchrotron self-Compton (SSC) model from the reverse-shock region evolving in a thin shell and long-lived LAT, X-ray and optical data with the standard synchrotron forward-shock model. The best-fit parameters derived with the Markov chain Monte Carlo simulations indicate that the outflow is endowed with magnetic fields and that the radio observations are in the self-absorption regime. The SSC forward-shock model with our parameters can explain the LAT photons beyond the synchrotron limit as well as the emission recently reported by the HESS Collaboration.

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arXiv2019

Analysis of Fermi-LAT observations, UHECRs and neutrinos from the radio galaxy Centaurus B

Centaurus B (Cen B) is one of the closest and brightest radio-loud galaxy in the southern sky. This radio galaxy, proposed as a plausible candidate for accelerating ultra-high-energy cosmic rays (UHECRs), is near the highest-energy neutrino event reported (IC35) in the High-Energy Starting Events catalog. Pierre Auger observatory reported the highest energy comic rays during 10 years of collecting data with some of them around this source. In this paper, the analysis of the gamma-ray spectrum and the light curve above 200 MeV is presented with nine years of cumulative Fermi-LAT data around Cen B. Taking into consideration the multi-wavelength observations carried out about this radio galaxy, leptonic and hadronic scenarios are introduced in order to fit the spectral energy distribution, assuming that the gamma-ray flux is produced in a region close to the core and in the extended lobes. Using the best-fit values found, several physics properties of this radio galaxy are derived. Furthermore, a statistical analysis of the cosmic ray distribution around Cen B is performed, finding that this distribution is not different from the background at a level of significance of 5%. Considering the UHECR event associated to this source by Moskalenko et al. and extrapolating its luminosity to low energies, we do not find enough evidence to associate the highest-energy neutrino event (IC35) with this radio galaxy.

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