arXiv subjects

Israel Martinez-Castellanos

Publications and source records attributed to Israel Martinez-Castellanos.

11 recordsLinked to original sources

arXiv2026

MeV dark matter decay forecasts for the COSI mission

The decay of dark matter (DM) is a generic prediction of many particle physics models, producing distinctive monochromatic signatures in environments such as the Galactic halo. Searches for decaying DM in the elusive 'MeV gap' have remained conspicuously under-explored, despite well-motivated models including axion-like particles, sterile neutrinos, and hidden-sector scenarios predicting signatures in this energy range. The upcoming Compton Spectrometer and Imager (COSI) telescope will help bridge this gap. Designed to probe this energy band with high spectral resolution, low background, and wide field of view, COSI is well-suited for DM line searches at the MeV scale. In this work, we make projections for COSI's 2-year sensitivity for DM decay in the Milky Way halo using dedicated all-sky simulations incorporating COSI's predicted instrument response. We conclude that COSI will improve the sensitivity to the DM lifetime by more than an order of magnitude over the DM mass range 0.4 to 10 MeV.

astro-ph.CO↗

arXiv2026

Predictions for Short Gamma-Ray Burst Detections by COSI

The Compton Spectrometer and Imager (COSI) is a soft gamma-ray telescope scheduled to launch in 2027. COSI will trigger onboard the spacecraft in response to short-duration transients, including gamma-ray bursts, and it will promptly disseminate alerts which include localizations to the community. This will enable rapid follow-up observations of the afterglow and associations with transients identified by other instruments. By simulating COSI observations of short gamma-ray bursts (sGRBs) and processing them with the trigger algorithm which will run onboard the spacecraft, we estimate the sensitivity of COSI to sGRBs. We then convolve this with a simulated set of sGRBs representing the intrinsic population in the universe to predict the rate that will be detected by COSI. We find that COSI is expected to trigger on $\sim$47-89 sGRBs per year with $\sim$7-10 localized to within 2.5°. With the gravitational wave sensitivity anticipated in the O5 run, we predict 0-3 joint detections with gravitational waves.

astro-ph.HE↗

arXiv2026

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

In the multimessenger era, the association of high-energy transients with their $γ$-ray counterparts can be obscured by magnetized large-scale structure surrounding the source. We analyze models for delayed off-axis $γ$-rays, arising from the injection of ultrahigh-energy cosmic rays ($E\gtrsim 10^{18}$ eV) and $γ$-rays ($\varepsilon_γ\gtrsim10^{18}$ eV), and very-high-energy $γ$-rays ($\varepsilon_γ\gtrsim 100$ 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 $γ$-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 $γ$-ray emission in pre-burst LAT data, while J1913.2+1901 coincides with a 664.6 GeV $γ$-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.

astro-ph.HE↗

arXiv2026

Detectability of Polarized Gamma-ray Emission from Blazar Flares with COSI

We investigate the detectability of polarized gamma-ray emission from blazar flares with the Compton Spectrometer and Imager (COSI). Using 17 years of Fermi Large Area Telescope observations, we analyze light curves for 1413 blazars and identify a maximum of 787 sources with flaring episodes through Bayesian block analysis. For each flare, we estimate the minimum detectable polarization MDP99 in the COSI energy band (0.2-5 MeV) using instrument response functions under a range of spectral assumptions and background conditions. Under baseline background levels (1 counts/s), and assuming that blazar flare statistics in the MeV band are comparable to those observed at GeV energies, we find that COSI can realistically detect polarization in up to ~6 flares with MDP99<50% over its two-year prime mission depending on different spectral and flare identification assumptions, with only a few most powerful ones reaching MDP99<20%. These expectations are shown to improve when shorter intervals around bright peaks within long flares are considered. We provide a ranked list of the most promising targets, finding that flat-spectrum radio quasars dominate the population of polarization-detectable events. Through its continuous all-sky monitoring in the largely unexplored MeV band, COSI will open a new observational window on blazar variability and deliver the first direct measurements of MeV polarization, offering unique insights into jet geometry and high-energy emission processes.

astro-ph.HE↗

arXiv2025

Enhancing Compton telescope imaging with maximum a posteriori estimation: a modified Richardson-Lucy algorithm for the Compton Spectrometer and Imager

We present a modified Richardson-Lucy (RL) algorithm tailored for image reconstruction in MeV gamma-ray observations, focusing on its application to the upcoming Compton Spectrometer and Imager (COSI) mission. Our method addresses key challenges in MeV gamma-ray astronomy by incorporating Bayesian priors for sparseness and smoothness while optimizing background components simultaneously. We introduce a novel sparsity term suitable for Poisson-sampled data in addition to a smoothness prior, allowing for flexible reconstruction of both point sources and extended emission. The performance of the algorithm is evaluated using simulated three-month COSI observations of gamma-ray lines of $^{44}$Ti (1.157 MeV), $^{26}$Al (1.809 MeV), and positron annihilation (0.511 MeV), respectively, representing various spatial features. Our results demonstrate significant improvements over conventional RL methods, particularly in suppressing artificial structures in point source reconstructions and retaining diffuse spatial structures. This work represents an important step towards establishing a robust data analysis for studying nucleosynthesis, positron annihilation, and other high-energy phenomena in our Galaxy.

astro-ph.IM↗

arXiv2023

The Compton Spectrometer and Imager

The Compton Spectrometer and Imager (COSI) is a NASA Small Explorer (SMEX) satellite mission in development with a planned launch in 2027. COSI is a wide-field gamma-ray telescope designed to survey the entire sky at 0.2-5 MeV. It provides imaging, spectroscopy, and polarimetry of astrophysical sources, and its germanium detectors provide excellent energy resolution for emission line measurements. Science goals for COSI include studies of 0.511 MeV emission from antimatter annihilation in the Galaxy, mapping radioactive elements from nucleosynthesis, determining emission mechanisms and source geometries with polarization measurements, and detecting and localizing multimessenger sources. The instantaneous field of view for the germanium detectors is >25% of the sky, and they are surrounded on the sides and bottom by active shields, providing background rejection as well as allowing for detection of gamma-ray bursts and other gamma-ray flares over most of the sky. In the following, we provide an overview of the COSI mission, including the science, the technical design, and the project status.

astro-ph.HE↗

arXiv2023

The cosipy library: COSI's high-level analysis software

The Compton Spectrometer and Imager (COSI) is a selected Small Explorer (SMEX) mission launching in 2027. It consists of a large field-of-view Compton telescope that will probe with increased sensitivity the under-explored MeV gamma-ray sky (0.2-5 MeV). We will present the current status of cosipy, a Python library that will perform spectral and polarization fits, image deconvolution, and all high-level analysis tasks required by COSI's broad science goals: uncovering the origin of the Galactic positrons, mapping the sites of Galactic nucleosynthesis, improving our models of the jet and emission mechanism of gamma-ray bursts (GRBs) and active galactic nuclei (AGNs), and detecting and localizing gravitational wave and neutrino sources. The cosipy library builds on the experience gained during the COSI balloon campaigns and will bring the analysis of data in the Compton regime to a modern open-source likelihood-based code, capable of performing coherent joint fits with other instruments using the Multi-Mission Maximum Likelihood framework (3ML). In this contribution, we will also discuss our plans to receive feedback from the community by having yearly software releases accompanied by publicly-available data challenges.

astro-ph.HE↗

arXiv2022

The All-sky Medium Energy Gamma-ray Observatory eXplorer (AMEGO-X) Mission Concept

The All-sky Medium Energy Gamma-ray Observatory eXplorer (AMEGO-X) is designed to identify and characterize gamma rays from extreme explosions and accelerators. The main science themes include: supermassive black holes and their connections to neutrinos and cosmic rays; binary neutron star mergers and the relativistic jets they produce; cosmic ray particle acceleration sources including Galactic supernovae; and continuous monitoring of other astrophysical events and sources over the full sky in this important energy range. AMEGO-X will probe the medium energy gamma-ray band using a single instrument with sensitivity up to an order of magnitude greater than previous telescopes in the energy range 100 keV to 1 GeV that can be only realized in space. During its three-year baseline mission, AMEGO-X will observe nearly the entire sky every two orbits, building up a sensitive all-sky map of gamma-ray sources and emission. AMEGO-X was submitted in the recent 2021 NASA MIDEX Announcement of Opportunity.

astro-ph.IM↗

arXiv2022

The Future of Gamma-Ray Experiments in the MeV-EeV Range

Gamma-rays, the most energetic photons, carry information from the far reaches of extragalactic space with minimal interaction or loss of information. They bring messages about particle acceleration in environments so extreme they cannot be reproduced on earth for a closer look. Gamma-ray astrophysics is so complementary with collider work that particle physicists and astroparticle physicists are often one in the same. Gamma-ray instruments, especially the Fermi Gamma-ray Space Telescope, have been pivotal in major multi-messenger discoveries over the past decade. There is presently a great deal of interest and scientific expertise available to push forward new technologies, to plan and build space- and ground-based gamma-ray facilities, and to build multi-messenger networks with gamma rays at their core. It is therefore concerning that before the community comes together for planning exercises again, much of that infrastructure could be lost to a lack of long-term planning for support of gamma-ray astrophysics. Gamma-rays with energies from the MeV to the EeV band are therefore central to multiwavelength and multi-messenger studies to everything from astroparticle physics with compact objects, to dark matter studies with diffuse large scale structure. These goals and new discoveries have generated a wave of new gamma-ray facility proposals and programs. This paper highlights new and proposed gamma-ray technologies and facilities that have each been designed to address specific needs in the measurement of extreme astrophysical sources that probe some of the most pressing questions in fundamental physics for the next decade. The proposed instrumentation would also address the priorities laid out in the recent Astro2020 Decadal Survey, a complementary study by the astrophysics community that provides opportunities also relevant to Snowmass.

astro-ph.HE↗

arXiv2019

Search for very-high-energy gamma-ray counterparts of gravitational waves with HAWC

The High-Altitude Water Cherenkov Observatory (HAWC) is a large field of view (~2 sr) continuously operating experiment sensitive to very-high energy (VHE) gamma rays (~0.3-100 TeV). These characteristics make it well suited for observing or constraining the VHE emission of rapid transients such as some gravitational waves progenitors. Of special interest are the events at low redshift where the attenuation due to the extragalactic background light is minimal. This is the case for binary neutron star mergers in the horizon of the LIGO and Virgo experiments, for which HAWC can either detect or place constraining limits on events occurring in our field of view. We report on our search for counterparts of the gravitational waves detected by LIGO and Virgo.

astro-ph.HE↗

arXiv2017

Monitoring the TeV sky on hours long timescales with HAWC

The High-Altitude Water Cherenkov Observatory (HAWC) is a large field of view ($\sim$2sr) instrument sensitive to very-high energy gamma rays ($\sim$0.5-100TeV). It is located in central Mexico (19$^\circ$N) and has a high duty cycle ($\sim$95%). These characteristics allow it to continuously monitor $\sim$2/3 of the sky, looking for transient events, such as flares from Active Galactic Nuclei or possibly other unknown phenomena. Presented here is an unbiased real-time monitoring on hours timescales which provides daily flux measurements for all locations in our observable sky promptly after they leave our field of view. These measurements are then used to follow known TeV sources and to perform a blind search. The alerts generated from these analyses, some of which have been made public through the Astronomer's Telegraph, can trigger small field of view instruments, enabling deep observations of sources during their high state activity which can constrain acceleration mechanisms. Additionally, these measurements allow us to respond quickly to external alerts.

astro-ph.HE↗