arXiv subjects

Nestor Mirabal

Publications and source records attributed to Nestor Mirabal.

18 recordsLinked to original sources

arXiv2025

Search for synchrotron pair echo emission following KM3-230213A

The KM3NeT Collaboration has recently reported the detection of an extraordinary ultra-high-energy neutrino event with an energy of 220 PeV. Ultrahigh energy neutrinos and gamma-rays are co-produced in ultrahigh energy cosmic-ray interactions. If a UHE neutrino was produced within the large-scale structure around the source where it was accelerated, gamma-ray emission may be expected via the synchrotron pair echo mechanism. Here, we develop the synchrotron pair echo model in the specific context of the KM3NeT neutrino. Motivated by the fact that the synchrotron pair echo signal is expected to peak in the GeV - TeV band, and that the signal may appear as a dim, transient source, we investigate the data collected by the Large Area Telescope (LAT) on-board the \textit{Fermi} Gamma-ray Space Telescope for transient and sub-threshold gamma-ray sources in the vicinity of the KM3NeT neutrino. We find three sub-threshold sources with TS $\gtrsim 16$ within $3.5^{\circ}$ of the neutrino event not included in any existing \textit{Fermi}-LAT catalogs, but note that none of the identified sub-threshold sources seem to be compelling candidates for synchrotron pair echo emission.

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arXiv2025

Modeling the imprints of large-scale magnetized structures on $\gamma$-rays from extragalactic transients

In the multimessenger era, the association of high-energy transients with their $\gamma$-ray counterparts can be obscured by magnetized large-scale structure surrounding the source. We analyze models for delayed off-axis $\gamma$-rays, arising from the injection of ultrahigh-energy cosmic rays ($E\gtrsim 10^{18}$ eV) and $\gamma$-rays ($\varepsilon_\gamma\gtrsim10^{18}$ eV), and very-high-energy $\gamma$-rays ($\varepsilon_\gamma\gtrsim 100$ GeV) from the transient. As a representative case, we consider the brightest gamma-ray burst, GRB~221009A, embedded in a magnetized structure. We examine charged-particle deflection in electromagnetic cascades, followed by propagation of secondaries through the intergalactic medium, and derive the time delays and angular offsets of the resulting $\gamma$-rays. To test the models, we analyze 1.25 years of post-event \textit{Fermi} Large Area Telescope (LAT) data and construct a 1 GeV -- 1 TeV test statistic (TS) map within $10^\circ$ of the burst localization. We find two sub-threshold peaks with TS$\geq9$. The more significant peak, J1911.8+2044, shows $\gamma$-ray emission in pre-burst LAT data, while J1913.2+1901 coincides with a 664.6 GeV $\gamma$-ray recorded $\sim191.9$ days after the trigger at $\approx 0^{\circ}.75$ from the GRB. The corresponding flux upper limit provides a fiducial value for constraining the parameter space in our models and estimating the source energy requirement for delayed off-axis signals. Although a magnetized structure can reproduce the angular offset of this sub-threshold feature, the resulting time delay disfavors an origin in this GRB.

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arXiv2023

Spatial coincidence between ultra-high energy cosmic rays and TeV gamma rays in the direction of GRB 980425/SN 1998bw

Gamma-ray bursts (GRBs) have long been suspected as possible ultra-high energy cosmic ray (UHECR) accelerators. In this brief note, I report that GRB 980425/SN 1998bw falls within the region of interest (ROI) with the highest significance in an all-sky blind search for magnetically-induced effects in the arrival directions of UHECRs conducted by the Pierre Auger Collaboration with events detected up to to 2018 August 31. There is also report in the literature of delayed TeV emission in archival Fermi-LAT observations from the direction of GRB 980425/SN 1998bw. The combined probability that two distinct cosmic ray acceleration signatures in two different multimessenger experiments may appear at the same spatial location by chance is estimated to be between $1.62 \times 10^{-3}$ and 0.0157.

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arXiv2023

Delayed teraelectronvolt emission from GRB 980425/SN 1998bw and the origin of ultra-high-energy cosmic rays

The origin of ultra-high-energy cosmic rays (UHECRs, E $> 10^{18}$ eV) is one of the great mysteries of modern astrophysics. It has been suggested that UHECRs could be accelerated in gamma-ray bursts (GRBs) and engine-driven supernovae (SNe). Here we report the discovery of a 1.4 teraelectronvolt (TeV) photon offset 0.97$^{\circ}$ from the site of the nearby (36.9 megaparsecs) GRB 980425/SN 1998bw explosion. The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope detected the TeV emission on 17 November 2018, more than 20 years after the original GRB 980425/SN 1998bw trigger. TeV detections at high Galactic latitudes by the LAT are extremely rare, with an average of 6 events per year. We propose that the delayed TeV emission is consistent with ultra-high-energy cosmic rays and/or electron-positron pairs from GRB 980425/SN 1998bw being deflected by the intergalactic magnetic field (IGMF) and subsequently cascading into secondary gamma rays. Based on the arrival time delay of the TeV emission, we estimate an IGMF strength of order $B \simeq 10^{-12}$-$10^{-13}$ Gauss. This result supports the possibility of UHECR acceleration in GRB 980425/SN 1998bw and suggests that most detected UHECRs are produced in local GRB/SNe within 200 Mpc. In addition, secondary photons from UHECRs out to 0.9-31 Gpc may also offer an explanation for extragalactic background photons with energies $\geq 1$ TeV detected by the Fermi LAT.

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arXiv2023

Revealing ultra-high-energy cosmic ray acceleration with multi-messenger observations of the nearby GRB 980425/SN 1998bw

The origin of ultra-high energy cosmic rays (UHECRs) is one of the most mystifying issues in astroparticle physics. It has been suggested that gamma-ray bursts (GRBs) are excellent acceleration sites for cosmic rays. The propagation of UHECRs from the GRB host galaxy to the Earth should generate delayed secondary photons and neutrinos. Here we present a dedicated search for delayed UHECR and neutrino emission centered around the position of nearby GRB 980425/SN 1998bw. Located at a distance of 36.9 Mpc, GRB 980425/SN 1998bw is well within the Greisen-Zatsepin-Kuzmin (GZK) distance horizon. We find no evidence for UHECR or neutrino clustering around the GRB 980425/SN 1998bw position between 2004 and 2020. Under ideal propagation conditions, we propose that it might be possible to detect an excess from delayed UHECRs around GRB 980425/SN 1998bw within the next 100 years if the intergalactic magnetic field (IGMF) strength is $B \leq 3 \times 10^{-13}$ G.

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arXiv2022

Secondary GeV-TeV emission from ultra-high-energy cosmic rays accelerated by GRB 221009A

The origin of ultra-high-energy cosmic rays (UHECRs) remains elusive. Gamma-ray bursts (GRBs) are among the best candidates able to meet the stringent energy requirements needed for particle acceleration to such high energies. If UHECRs were accelerated by the central engine of GRB 221009A, it might be possible to detect secondary photons and neutrinos as the UHECRs travel from the source to the Earth. Here we attempt to interpret some of the early publicly available data connected to this burst. If the reported early GeV-TeV detection was produced by secondary emission from UHECRs it probably indicates that UHECRs reached energies $> 10^{21}$ eV and that GRB 221009A exploded inside a magnetic void with intergalactic magnetic field (IGMF) strength $B \leq 3 \times 10^{-16}$ G. In order to understand the entire energy deposition mechanism, we propose to search existing and future Fermi-LAT data for secondary emission arriving over larger spatial scales and longer time-scales. This strategy might help clarify the origin of UHECRs, constrain the intergalactic magnetic field (IGMF) strength along this line of sight and start to quantify the fraction of magnetic voids around GRBs.

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arXiv2022

The Fourth Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope -- Data Release 3

An incremental version of the fourth catalog of active galactic nuclei (AGNs) detected by the Fermi-Large Area Telescope is presented. This version (4LAC-DR3) derives from the third data release of the 4FGL catalog based on 12 years of E>50 MeV gamma-ray data, where the spectral parameters, spectral energy distributions (SEDs), yearly light curves, and associations have been updated for all sources. The new reported AGNs include 587 blazar candidates and four radio galaxies. We describe the properties of the new sample and outline changes affecting the previously published one. We also introduce two new parameters in this release, namely the peak energy of the SED high-energy component and the corresponding flux. These parameters allow an assessment of the Compton dominance, the ratio of the Inverse-Compton to the synchrotron peak luminosities, without relying on X-ray data.

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arXiv2022

Incremental Fermi Large Area Telescope Fourth Source Catalog

We present an incremental version (4FGL-DR3, for Data Release 3) of the fourth Fermi-LAT catalog of gamma-ray sources. Based on the first twelve years of science data in the energy range from 50 MeV to 1 TeV, it contains 6658 sources. The analysis improves on that used for the 4FGL catalog over eight years of data: more sources are fit with curved spectra, we introduce a more robust spectral parameterization for pulsars, and we extend the spectral points to 1 TeV. The spectral parameters, spectral energy distributions, and associations are updated for all sources. Light curves are rebuilt for all sources with 1 yr intervals (not 2 month intervals). Among the 5064 original 4FGL sources, 16 were deleted, 112 are formally below the detection threshold over 12 yr (but are kept in the list), while 74 are newly associated, 10 have an improved association, and seven associations were withdrawn. Pulsars are split explicitly between young and millisecond pulsars. Pulsars and binaries newly detected in LAT sources, as well as more than 100 newly classified blazars, are reported. We add three extended sources and 1607 new point sources, mostly just above the detection threshold, among which eight are considered identified, and 699 have a plausible counterpart at other wavelengths. We discuss degree-scale residuals to the global sky model and clusters of soft unassociated point sources close to the Galactic plane, which are possibly related to limitations of the interstellar emission model and missing extended sources.

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arXiv2021

Machine-Learned Dark Matter Subhalo Candidates in the 4FGL-DR2: Search for the Perturber of the GD-1 Stream

The detection of dark matter subhalos without a stellar component in the Galactic halo remains a challenge. We use supervised machine learning to identify high-latitude gamma-ray sources with dark matter-like spectra among unassociated gamma-ray sources in the 4FGL-DR2. Out of 843 4FGL-DR2 unassociated sources at $|b| \geq 10\mathrm{^\circ}$, we select 73 dark matter subhalo candidates. Of the 69 covered by the Neil Gehrels Swift Observatory (Swift), 17 show at least one X-ray source within the 95% LAT error ellipse and 52 where we identify no new sources. This latest inventory of dark subhalos candidates allows us to investigate the possible dark matter substructure responsible for the perturbation in the GD-1 stellar stream. In particular, we examine the possibility that the alleged GD-1 dark subhalo may appear as a 4FGL-DR2 gamma-ray source from dark matter annihilation into Standard Model particles.

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arXiv2019

Unidentified Gamma-ray Sources as Targets for Indirect Dark Matter Detection with the Fermi-Large Area Telescope

One of the predictions of the $Λ$CDM cosmological framework is the hierarchical formation of structure, giving rise to dark matter (DM) halos and subhalos. When the latter are massive enough they retain gas (i.e., baryons) and become visible. This is the case of the dwarf satellite galaxies in the Milky Way (MW). Below a certain mass, halos may not accumulate significant amounts of baryons and remain completely dark. However, if DM particles are Weakly Interacting Massive Particles (WIMPs), we expect them to annihilate in subhalos, producing gamma rays which can be detected with the Fermi satellite. Using the three most recent point-source Fermi Large Area Telescope (LAT) catalogs (3FGL, 2FHL and 3FHL), we search for DM subhalo candidates among the unidentified sources, i.e., sources with no firm association to a known astrophysical object. We apply several selection criteria based on the expected properties of the DM-induced emission from subhalos, which allow us to significantly reduce the list of potential candidates. Then, by characterizing the minimum detection flux of the instrument and comparing our sample to predictions from the Via Lactea II (VL-II) N-body cosmological simulation, we place conservative and robust constraints on the $\langleσv\rangle-m_{DM}$ parameter space. For annihilation via the $τ^+τ^-$ channel, we put an upper limit of $4\times 10^{-26}~(5\times 10^{-25})~cm^3~s^{-1}$ for a mass of 10 (100) GeV. A critical improvement over previous treatments is the repopulation we made to include low-mass subhalos below the VL-II mass resolution. With more advanced subhalo candidate filtering the sensitivity reach of our method can potentially improve these constraints by a factor 3 (2) for $τ^+τ^-$ ($b \bar{b}$) channel.

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arXiv2018

Millisecond pulsar origin of the Galactic center excess and extended gamma-ray emission from Andromeda - a closer look

A new measurement of a spatially extended gamma-ray signal from the center of the Andromeda galaxy (M31) has been recently published by the Fermi-LAT collaboration, reporting that the emission broadly resembles the so-called Galactic center excess (GCE) of the Milky Way (MW). At the same time, evidence is accumulating on a millisecond pulsar (MSPs) origin for the GCE. These elements prompt us to compare the mentioned observations with what is, perhaps, the simplest model for an MSP population, solely obtained by rescaling of the MSP luminosity function determined in the local MW disk via the respective stellar mass of the systems. It is remarkable that without free fitting parameters, this model can account for both the energetics and the morphology of the GCE within uncertainties. For M31, the estimated luminosity due to primordial MSPs is expected to contribute only about a quarter of the detected emission, although a dominant contribution cannot be excluded given the large uncertainties. If correct, the model predicts that the M31 disk emission due to MSP is not far below the present upper bound. We also discuss a few refinements of this simple model. In particular, we use the correlation between globular cluster gamma-ray luminosity and stellar encounter rate to gauge the dynamical MSP formation in the bulge. This component is expected to contribute to the GCE only at a level $\lesssim 5\%$, but it may be of some importance in explaining the signal's morphology in the inner region of the Galaxy. We also comment on some effects which may lead to violations of the simple scaling used, on alternative models, and on future perspectives for improved diagnostics.

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arXiv2016

Searches for correlation between UHECR events and high-energy gamma-ray Fermi-LAT data

The astrophysical sources responsible for ultra high-energy cosmic rays (UHECRs) continue to be one of the most intriguing mysteries in astrophysics. We present a comprehensive search for correlations between high-energy ($\gtrapprox 1$ GeV) gamma-ray events from the Fermi Large Area Telescope (LAT) and UHECRs ($\gtrapprox 60$ EeV) detected by the Telescope Array and the Pierre Auger Observatory. We perform two separate searches. First, we conduct a standard cross-correlation analysis between the arrival directions of 148 UHECRs and 360 gamma-ray sources in the Second Catalog of Hard Fermi-LAT sources (2FHL). Second, we search for a possible correlation between UHECR directions and unresolved Fermi-LAT gamma-ray emission. For the latter, we use three different methods: a stacking technique with both a model-dependent and model-independent background estimate, and a cross-correlation function analysis. We also test for statistically significant excesses in gamma rays from signal regions centered on Cen A and the Telescope Array hotspot. No significant correlation is found in any of the analyses performed, except a weak ($\lessapprox 2σ$) hint of signal with the correlation function method on scales $\sim 1^\circ$. Upper limits on the flux of possible power-law gamma-ray sources of UHECRs are derived.

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arXiv2016

Sensitivity Projections for Dark Matter Searches with the Fermi Large Area Telescope

The nature of dark matter is a longstanding enigma of physics; it may consist of particles beyond the Standard Model that are still elusive to experiments. Among indirect search techniques, which look for stable products from the annihilation or decay of dark matter particles, or from axions coupling to high-energy photons, observations of the $γ$-ray sky have come to prominence over the last few years, because of the excellent sensitivity of the Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope mission. The LAT energy range from 20 MeV to above 300 GeV is particularly well suited for searching for products of the interactions of dark matter particles. In this report we describe methods used to search for evidence of dark matter with the LAT, and review the status of searches performed with up to six years of LAT data. We also discuss the factors that determine the sensitivities of these searches, including the magnitudes of the signals and the relevant backgrounds, considering both statistical and systematic uncertainties. We project the expected sensitivities of each search method for 10 and 15 years of LAT data taking. In particular, we find that the sensitivity of searches targeting dwarf galaxies, which provide the best limits currently, will improve faster than the square root of observing time. Current LAT limits for dwarf galaxies using six years of data reach the thermal relic level for masses up to 120 GeV for the $b\bar{b}$ annihilation channel for reasonable dark matter density profiles. With projected discoveries of additional dwarfs, these limits could extend to about 250 GeV. With as much as 15 years of LAT data these searches would be sensitive to dark matter annihilations at the thermal relic cross section for masses to greater than 400 GeV (200 GeV) in the $b\bar{b}$ ($τ^+ τ^-$) annihilation channels.

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arXiv2014

Annihilating dark matter or noise?: A statistical examination of the Fermi GeV excess around the Galactic Centre

Excess Fermi GeV emission around the Galactic Centre has been interpreted as a possible signature of annihilating dark matter. Here we analyse three aspects of this claim: its spectral cutoff, the correlation between the spectral shape of the purported signal and known noise components, and its brightness profile. Experimentally, the correlations that exist between the GeV excess and known sources of noise make it difficult to conclude that a dark matter signal claim can be made without independent direct detection confirmation. As a possible way forward, we introduce three criteria that could potentially help to validate a dark matter annihilation signal in gamma rays.

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arXiv2014

The Smith Cloud and its dark matter halo: Survival of a Galactic disc passage

The current velocity of the Smith Cloud indicates that it has undergone at least one passage of the Galactic disc. Using hydrodynamic simulations we examine the present day structure of the Smith Cloud. We find that a dark matter supported cloud is able to reproduce the observed present day neutral hydrogen mass, column density distribution and morphology. In this case the dark matter halo becomes elongated, owing to the tidal interaction with the Galactic disc. Clouds in models neglecting dark matter confinement are destroyed upon disc passage, unless the initial cloud mass is well in excess of what is observed today. We then determine integrated flux upper limits to the gamma-ray emission around such a hypothesised dark matter core in the Smith Cloud. No statistically significant core or extended gamma-ray emission are detected down to a 95% confidence level upper limit of $1.4\times10^{-10}$ ph cm$^{-2}$ s$^{-1}$ in the 1-300 GeV energy range. For the derived distance of 12.4 kpc, the Fermi upper limits set the first tentative constraints on the dark matter cross sections annihilating into $τ^{+}τ^{-}$ and $b\bar{b}$ for a high-velocity cloud.

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arXiv2013

Closing in on the Fermi Line with a New Observation Strategy

Evidence for a spectral line in the inner Galaxy has caused a great deal of excitement over the last year, mainly because of its interpretation as a possible dark matter signal. The observation has raised important questions about statistics and suspicions about systematics, especially in photons from the Earth limb. With enough additional data, we can address these concerns. In this white paper, we summarize the current observational situation and project future sensitivities, finding that the status quo is dangerously close to leaving the issue unresolved until 2015. We advocate a change in survey strategy that more than doubles the data rate in the inner Galaxy, and is relatively non-disruptive to other survey science. This strategy will clearly separate the null hypothesis from the line signal hypothesis and provide ample limb data for systematics checks by the end of 2014. The standard survey mode may not.

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arXiv2008

The Rapidly Flaring Afterglow of the Very Bright and Energetic GRB 070125

We report on multi-wavelength observations, ranging from the X-ray to radio wave bands, of the IPN-localized gamma-ray burst GRB 070125. Spectroscopic observations reveal the presence of absorption lines due to O I, Si II, and C IV, implying a likely redshift of z = 1.547. The well-sampled light curves, in particular from 0.5 to 4 days after the burst, suggest a jet break at 3.7 days, corresponding to a jet opening angle of ~7.0 degrees, and implying an intrinsic GRB energy in the 1 - 10,000 keV band of around E = (6.3 - 6.9)x 10^(51) erg (based on the fluences measured by the gamma-ray detectors of the IPN network). GRB 070125 is among the brightest afterglows observed to date. The spectral energy distribution implies a host extinction of Av < 0.9 mag. Two rebrightening episodes are observed, one with excellent time coverage, showing an increase in flux of 56% in ~8000 seconds. The evolution of the afterglow light curve is achromatic at all times. Late-time observations of the afterglow do not show evidence for emission from an underlying host galaxy or supernova. Any host galaxy would be subluminous, consistent with current GRB host-galaxy samples. Evidence for strong Mg II absorption features is not found, which is perhaps surprising in view of the relatively high redshift of this burst and the high likelihood for such features along GRB-selected lines of sight.

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arXiv2007

The Merger in Abell 576: A Line of Sight Bullet Cluster?

Using a combination of Chandra and XMM observations, we confirmed the presence of a significant velocity gradient along the NE/E-W/SW direction in the intracluster gas of the cluster Abell 576. The results are consistent with a previous ASCA SIS analysis of this cluster. The error weighted average over ACIS-S3, EPIC MOS 1 & 2 spectrometers for the maximum velocity difference is >3.3E03 km/s at the 90% confidence level, similar to the velocity limits estimated indirectly for the "bullet" cluster (1E0657-56). The probability that the velocity gradient is generated by standard random gain fluctuations with Chandra and XMM is <0.1%. The regions of maximum velocity gradient are in CCD zones that have the lowest temporal gain variations. It is unlikely that the velocity gradient is due to Hubble distance differences between projected clusters (probability<~0.01%). We mapped the distribution of elemental abundance ratios across the cluster and detected a strong chemical discontinuity using the abundance ratio of silicon to iron, equivalent to a variation from 100% SN Ia iron mass fraction in the West-Northwest regions to 32% in the Eastern region. The "center" of the cluster is located at the chemical discontinuity boundary, which is inconsistent with the radially symmetric chemical gradient found in some regular clusters, but consistent with a cluster merging scenario. We predict that the velocity gradient as measured will produce a variation of the CMB temperature towards the East of the core of the cluster that will be detectable by current and near-future bolometers. The measured velocity gradient opens for the possibility that this cluster is passing through a near line-of-sight merger stage where the cores have recently crossed.

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