arXiv subjects

Diego F. Torres

Publications and source records attributed to Diego F. Torres.

At least 19 recordsLinked to original sources

arXiv2026

Temporal evolution of the periodic GeV signal from 4FGL J1913.2+0512 and analysis of the SS 433 / W50 lobes

SS 433 is a microquasar whose relativistic jets precess every ~162 days, providing a laboratory for jet-interstellar medium interactions. We present a comprehensive analysis of 16 years of Fermi Large Area Telescope data (August 2008-September 2024) of the SS 433/W50 field, using events in the 0.3-300 GeV range and employing pulsar gating to mitigate contamination from the bright nearby pulsar PSR J1907+0602. We detect the GeV source 4FGL J1913.2+0512 (TS = 45, where TS denotes the likelihood-ratio Test Statistic) with a power-law spectrum (photon index 2.61 +- 0.08) and confirm a GeV excess at the western lobe (TS = 17). The eastern lobe of SS 433 is hinted at with lower significance. One additional GeV excess, Fermi J1909.6+0552 (TS = 20; TS = 28 over 0.1-300 GeV), located outside the SS 433 / W50 system, is revealed after gating. Exposure-corrected Lomb-Scargle periodograms and precessional phase-folded light curves show a ~162-day modulation in 4FGL J1913.2+0512. This periodicity is prominent during the first 10 years of the mission (2008-2018) but disappears thereafter, with the phase-folded flux concentrated in precessional phases 0.0-0.5. Over the full 16-year dataset, the modulation remains detectable but with reduced significance, consistent with dilution by the later non-modulated epoch. These results indicate that the efficiency and/or geometry of gamma-ray production in the SS 433 environment evolves on multi-year timescales.

astro-ph.HE↗

arXiv2025

The dispersion in pulsar $γ$-ray efficiency

The observational efficiency of pulsars, defined as the ratio of the observationally derived isotropic-equivalent luminosity, $4πd_{obs}^2 F_{obs}$, where $F_{obs}$ is the average pulsed energy flux of a pulsar and $d_{obs}$ is its estimated distance, to its energy budget, shows a wide range of values. This dispersion is believed to be a combination of beaming effects, different geometries, and case-by-case variability of the emission mechanism efficiency, but it is not clear in what proportion. In this work we focused on the gamma-ray range and analysed the four main ingredients that likely contribute to this dispersion: the geometrical term arising from the anisotropic emission (beaming), viewing and inclination angles, the uncertainty on the pulsar distance, the uncertainty on the moment of inertia, and the intrinsic efficiency of the mechanism producing the gamma-ray emission. Estimating the expected ranges of the moment of inertia and the distance errors, and considering a geometrical and spectral model that we have recently used to fit the light curves and spectra of the entire gamma-ray pulsar population, we estimate the a priori distribution of the first three ingredients in order to obtain the a posteriori distribution of the intrinsic efficiency of the mechanism. We found the latter to peak at $\sim 5-15 \%$ and to have a dispersion of around one order of magnitude. That is, we found the intrinsic efficiency of the mechanism to be the leading factor in the observed dispersion. In addition, we found little sensitivity of these results on different distributions of the estimated pulsar distance errors, and saw that the weak, alleged correlation with the spin-down power can only explain part of the observed dispersion. This methodology can be easily applied to other geometrical models of the emission, to test the sensitivity of these results on the beaming distribution.

astro-ph.HE↗

arXiv2025

Synchro-curvature description of γ-ray light curves and spectra of pulsars: concurrent fitting

We present a concurrent fitting of spectra and light curves of the whole population of detected gamma-ray pulsars. Using a synchro-curvature model we compare our theoretical output with the observational data published in the Third Fermi Pulsar Catalog, which has significantly increased the number of known gamma-ray pulsars. Our model properly fits all the spectra and reproduces well a considerable fraction of light curves. Light curve fitting is carried out with two different techniques, whose strong points and caveats are discussed. We use a weighted reduced \{chi}^2 of light curves in time domain, and the Euclidean distance of the Fourier transform of the light curves, i.e. transforming the light curves to the frequency domain. The performance of both methods is found to be qualitatively similar, but individual best-fit solutions may differ. We also show that, in our model based on few effective parameters, the light curve fitting is basically insensitive to the timing and spectral parameters of the pulsar. Finally, we look for correlations between model and physical parameters, and recover trends found in previous studies but without any significant correlation involving geometrical parameters.

astro-ph.HE↗

arXiv2025

Probing multi-band variability and mode switching in the candidate transitional millisecond pulsar 3FGL J1544.6-1125

We present the most extensive high-time resolution multi-band campaign to date on the candidate transitional millisecond pulsar (tMSP) 3FGL J1544.6-1125 in the sub-luminous disk state, with coordinated observations from the radio to the X-ray band. While XMM-Newton and NuSTAR X-ray light curves exhibit the characteristic high- and low-mode bimodality, the source faintness prevents firm evidence for similar bimodality in the ultraviolet and near-infrared light curves, presented here for the first time. A re-analysis of archival XMM-Newton/OM data reveals an optical flare without an X-ray counterpart, likely originating from the outer accretion disk or the companion star. During our observations, no radio emission was detected, with a 3$σ$ flux density upper limit of 8 $μ$Jy at 6 GHz. While past works have already reported radio variability in the source, this limit is a factor of 3.5 below the average value measured in 2019 in similar conditions, underscoring significant radio variability despite the relatively stable X-ray flux. Simultaneous optical light curves in five filters with GTC/HiPERCAM revealed flickering and dipping activities that resemble the observed X-ray variability, along with a reddening trend at lower fluxes. The latter is consistent with discrete mass ejections that disrupt the inner flow and reduce both X-ray and optical fluxes, thereby driving the high-to-low-mode switches. This suggests a common origin for most optical and X-ray emission at the boundary region between the pulsar wind and the inner disk, as also supported by our modelling of the spectral energy distribution in the high mode. Overall, our findings reinforce the mini-pulsar nebula picture for tMSPs in the sub-luminous state and demonstrate how coordinated, high-time resolution, multi-wavelength campaigns are essential to probe the processes governing rapid mode switches in these systems.

astro-ph.HE↗

arXiv2025

Fermi-LAT and FAST observation of the gamma-ray binary HESS J0632+057

Using 15 years of data from the Fermi Large Area Telescope (Fermi-LAT), we performed a comprehensive analysis on the gamma-ray binary HESS J0632+057. Its spectrum in 0.1-300 GeV band is well described by a power law model with an index of $2.40\pm0.16$, leading to an energy flux of (5.5$\pm$1.6$)\times$ 10$^{-12}$ erg cm$^{-2}$ s$^{-1}$. The GeV Spectral Energy Distribution (SED) of HESS J0632+057 hints for a spectral turn-over between $\sim$10-100 GeV. Orbital analysis reveals a flux enhancement during the phase range of 0.2-0.4, consistent with the X-ray and TeV light curves, indicating an origin of a common particle population. We carried out six deep radio observations on HESS J0632+057 with the Five-hundred-meter Aperture Spherical Telescope (FAST), evenly distributed across its orbit, reaching a detection sensitivity of 2$μ$Jy. However, no radio pulsation was detected within these observations. The absence of radio pulsation may be attributed to the dense stellar wind environment of HESS J0632+057.

astro-ph.HE↗

arXiv2025

Upper limits on the gamma-ray emission from the microquasar V4641 Sgr

Following a recent detection of TeV radiation by the Large High Altitude Air Shower Observatory (LHAASO) and the High-Altitude Water Cherenkov Observatory (HAWC), coincident with the direction of the microquasar V4641 Sgr, we search for possible GeV emission from this source. We explored the morphology and temporal features of the source as well as two nearby unassociated point sources which could be a part of extended structure of V4641 Sgr, and compared results with corresponding X-ray and TeV emissions. The 95% confidence level upper limits for the flux from the source, assuming both point and extended source models were 5.38$\times$ 10$^{-13}$ erg cm$^{-2}$ s$^{-1}$ and 1.12$\times$ 10$^{-12}$ erg cm$^{-2}$ s$^{-1}$, respectively. Additionally, no correlation between gamma-ray light curve and X-ray outbursts was observed.

astro-ph.HE↗

arXiv2025

Quantitative exploration of the similarity of gamma-ray pulsar light curves

We introduce and apply a methodology based on dynamic time warping (DTW) to compare the whole set of gamma-ray light curves reported in the Third Fermi-Large Area Telescope Pulsar Catalogue. Our method allows us to quantitatively measure the degree of global similarity between two light curves beyond comparing indicators such as how many peaks there are, which is their separation, width, and height. Once the morphology of the light curve is showcased via background subtraction, min-max scaler normalization, and rotations are considered to take into account that phase 0 is arbitrary, the level of detail with which light curves of different pulsars appear is revealed. In many cases their similarity is striking and occurs disregarding any other timing, physical, or spectral property. In particular, some MSPs and young pulsars share detailed light curve morphology.

astro-ph.HE↗

arXiv2024

Separating repeating fast radio bursts using the minimum spanning tree as an unsupervised methodology

Fast radio bursts (FRBs) represent one of the most intriguing phenomena in modern astrophysics. However, their classification into repeaters and non-repeaters is challenging. Here, we present the application of the graph theory Minimum Spanning Tree (MST) methodology as an unsupervised classifier of repeaters and non-repeaters FRBs. By constructing MSTs based on various combinations of variables, we identify those that lead to MSTs that exhibit a localized high density of repeaters at each side of the node with the largest betweenness centrality. Comparing the separation power of this methodology against known machine learning methods, and with the random expectation results, we assess the efficiency of the MST-based approach to unravel the physical implications behind the graph pattern. We finally propose a list of potential repeater candidates derived from the analysis using the MST.

astro-ph.HE↗

arXiv2024

Analysis of the possible detection of the pulsar wind nebulae of PSR J1208-6238, J1341-6220, J1838-0537 and J1844-0346

Context. Pulsar wind nebulae (PWNe) are a source of very high energy radiation that can reach up to tera-electron volts and even peta-electron volts. Our work uses the pulsar tree, a graph theory tool recently presented to analyze the pulsar population and select candidates of interest. Aims. We aim to discover detectable PWNe. We also aim to test to what extent the pulsar tree is able to group detectable PWNe despite only considering the intrinsic properties of pulsars. Methods. We selected four pulsars as tera-electron volt PWNe candidates based on their positions in the pulsar tree. Using observed and assumed ranges of values for relevant parameters, we anticipated the possible spectral energy distributions of the PWNe of four pulsars (PSR J1208-6238, J1341-6220, J1838-0537, and J1844-0346) via a detailed time-dependent leptonic model that was already found to be appropriate for describing almost all other detected nebulae. Results. We estimated the likelihood of detection for the four candidates we studied by comparing the TeV fluxes predicted by the possible models with the sensitivities of different observatories. In doing so, we provide context for analyzing the advantages and caveats of using the pulsar tree position as a marker for properties that go beyond the intrinsic features of pulsars that are considered in producing the pulsar tree.

astro-ph.HE↗

arXiv2024

Spatio-spectral-temporal Modelling of Two Young Pulsar Wind Nebulae

Recent observations of a few young pulsar wind nebulae (PWNe) have revealed their morphologies in some detail. Given the availability of spatio-spectral-temporal data, we use our multi-zone (1D) leptonic emission code to model the PWNe associated with G29.7-0.3 (Kes 75) and G21.5-0.9 (G21.5) and obtain (by-eye) constraints on additional model parameters compared to spectral-only modelling. Kes 75 is a Galactic composite supernova remnant (SNR) with an embedded pulsar, PSR J1846-0258. X-ray studies reveal rapid expansion of Kes 75 over the past two decades. PWN G21.5 is also a composite SNR, powered by PSR J1833-1034. For Kes 75, we study a sudden plasma bulk speed increase that may be due to the magnetar-like outbursts of the central pulsar. An increase of a few percent in this speed does not result in any significant change in the model outputs. For G21.5, we investigate different diffusion coefficients and pulsar spin-down braking indices. We can reproduce the broadband spectra and X-ray surface brightness profiles for both PWNe, and the expansion rate, flux over different epochs, and X-ray photon index vs epoch and central radius for Kes 75 quite well. The latter three features are also investigated for G21.5. Despite obtaining reasonable fits overall, some discrepancies remain, pointing to further model revision. We find similar values to overlapping parameters between our 1D code and those of an independent 0D dynamical code (TIDE). Future work will incorporate spatial data from various energy wavebands to improve model constraints.

astro-ph.HE↗

arXiv2024

X-ray hardening preceding the onset of SGR 1935+2154's radio pulsar phase

Magnetars are neutron stars with extremely strong magnetic fields, frequently powering high-energy activity in X-rays. Pulsed radio emission following some X-ray outbursts have been detected (\citealt{Camilo2006,camilo2007a}), albeit its physical origin is unclear. It has long been speculated that the origin of magnetars' radio signals is different from those from canonical pulsars, although convincing evidence is still lacking. Five months after magnetar SGR 1935+2154's X-ray outburst and its associated Fast Radio Burst (FRB) 20200428, a radio pulsar phase was discovered. Here we report the discovery of X-ray spectral hardening associated with the emergence of periodic radio pulsations from SGR 1935+2154 and a detailed analysis of the properties of the radio pulses. The observations suggest that radio emission originates from the outer magnetosphere of the magnetar, and the surface heating due to the bombardment of inward-going particles from the radio emission region is responsible for the observed X-ray spectral hardening.

astro-ph.HE↗

arXiv2024

Synchro-curvature description of $γ$-ray light curves and spectra of pulsars: global properties

This work presents a methodological approach to generate realistic $γ$-ray light curves of pulsars, resembling reasonably well the observational ones observed by the Fermi-Large Area Telescope instrument, fitting at the same time their high-energy spectra. The theoretical light curves are obtained from a spectral and geometrical model of the synchro-curvature emission. Despite our model relies on a few effective physical parameters, the synthetic light curves present the same main features observed in the observational $γ$-ray light curve zoo, such as the different shapes, variety in the number of peaks, and a diversity of peak widths. The morphological features of the light curves allows us to statistically compare the observed properties. In particular, we find that the proportion on the number of peaks found in our synthetic light curves is in agreement with the observational one provided by the third Fermi-LAT pulsar catalog. We also found that the detection probability due to beaming is much higher for orthogonal rotators (approaching 100%) than for small inclination angles (less than 20%).The small variation on the synthetic skymaps generated for different pulsars indicates that the geometry dominates over timing and spectral properties in shaping the gamma-ray light curves. This means that geometrical parameters like the inclination angle can be in principle constrained by gamma-ray data alone independently on the specific properties of a pulsar. At the same time, we find that $γ$-ray spectra seen by different observers can slightly differ, opening the door to constraining the viewing angle of a particular pulsar.

astro-ph.HE↗

arXiv2023

Quantitative determination of minimum spanning tree structures: Using the pulsar tree for analyzing the appearance of new classes of pulsars

In this work, we introduce a quantitative methodology to define what is the main trunk and what are the significant branches of a minimum spanning tree (MST). We apply it to the pulsar tree, i.e. the MST of the pulsar population constructed upon a Euclidean distance over the pulsar's intrinsic properties. Our method makes use of the betweenness centrality estimator, as well as of non-parametric tests to establish the distinct character of the defined branches. Armed with these concepts, we study how the pulsar population has evolved throughout history, and analyze how to judge whether a new class of pulsars appears in new data, future surveys, or new incarnations of pulsar catalogs.

astro-ph.HE↗

arXiv2022

The Crab nebula variability at short timescales with the Cherenkov Telescope Array

Since 2009, several rapid and bright flares have been observed at high energies (>100 MeV) from the direction of the Crab Nebula. Several hypotheses have been put forward to explain this phenomenon, but the origin is still unclear. The detection of counterparts at higher energies with the next generation of Cherenkov telescopes will be determinant to constrain the underlying emission mechanisms. We aim at studying the capability of the Cherenkov Telescope Array (CTA) to explore the physics behind the flares, by performing simulations of the Crab Nebula spectral energy distribution, both in flaring and steady state, for different parameters related to the physical conditions in the nebula. In particular, we explore the data recorded by Fermi during two particular flares that occurred in 2011 and 2013. The expected GeV and TeV gamma-ray emission is derived using different radiation models. The resulting emission is convoluted with the CTA response and tested for detection, obtaining an exclusion region for the space of parameters that rule the different flare emission models. Our simulations show different scenarios that may be favourable for achieving the detection of the flares in Crab with CTA, in different regimes of energy. In particular, we find that observations with low sub-100 GeV energy threshold telescopes could provide the most model-constraining results.

astro-ph.HE↗

arXiv2022

Testing source confusion and identification capability in Cherenkov Telescope Array data

The Cherenkov Telescope Array will provide the deepest survey of the Galactic Plane performed at very-high-energy gamma-rays. Consequently, this survey will unavoidably face the challenge of source confusion, i.e., the non-unique attribution of signal to a source due to multiple overlapping sources. Among the known populations of Galactic gamma-ray sources and given their extension and number, pulsar wind nebulae (PWNe, and PWN TeV halos) will be the most affected. We aim to probe source confusion of TeV PWNe in forthcoming CTA data. For this purpose, we performed and analyzed simulations of artificially confused PWNe with CTA. As a basis for our simulations, we applied our study to TeV data collected from the H.E.S.S. Galactic Plane Survey for ten extended and two point-like firmly identified PWNe, probing various configurations of source confusion involving different projected separations, relative orientations, flux levels, and extensions among sources. Source confusion, defined here to appear when the sum of the Gaussian width of two sources is larger than the separation between their centroids, occurred in $\sim$30% of the simulations. For this sample and 0.5$°$ of average separation between sources, we found that CTA can likely resolve up to 60% of those confused sources above 500 GeV. Finally, we also considered simulations of isolated extended sources to see how well they could be matched to a library of morphological templates. The outcome of the simulations indicates a remarkable capability (more than 95% of the cases studied) to match a simulation with the correct input template in its proper orientation.

astro-ph.HE↗

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.

astro-ph.HE↗

arXiv2022

LHAASO J2226+6057 as a pulsar wind nebula

The Large High Altitude Air Shower Observatory has reported the detection of cosmic-ray sources in Milky Way that can accelerate particles up to PeV (= 10$^{15}$ eV) energies. These sources, so called ``PeVatrons'', are mostly unidentified. Several classes of sources, such as supernova remnants, pulsar wind nebula, or young stellar clusters can potentially be the counterparts of these PeVatrons. The aim of this work is to study a pulsar wind nebula interpretation of one of these PeVatrons, LHAASO J2226+6057, which has a relatively well covered multi-frequency spectrum. We have performed a leptonic, time-dependent modeling of the pulsar wind nebula (PWN) associated with PSR J2229+6114 considering a time-energy-dependent diffusion-loss equation. Injection, energy losses, as well as escape of particles were considered to balance the time-dependent lepton population. We have also included the dynamics of the PWN and the associated supernova remnant (SNR) and their interaction via the reverse shock to study the reverberation phase of the system. We have considered different values of braking index ($n$) and true age ($t_{age}$) for the fitting of the multi-wavelength (MWL) spectral energy distribution (SED) of LHAASO J2226+6057. The best-fit PWN model parameters and their 1$σ$ confidence intervals were evaluated. We have also demonstrated the impact of reverberation on the MWL SED with increasing time. Additionally, we have discussed the resultant large radius and low magnetic field associated with the PWN in question, as caveats for the possible physical connection of the pulsar as the origin of this high energy source.

astro-ph.HE↗

arXiv2022

Synchro-curvature emitting regions in high-energy pulsar models

The detected high-energy pulsars' population is growing in number, and thus, having agile and physically relevant codes to analyze it consistently is important. Here, we update our existing synchro-curvature radiation model by including a better treatment of the particle injection, particularly where the large pitch angle particles dominate the spectra, and by implementing a fast and accurate minimization technique. The latter allows a large improvement in computational cost, needed to test model enhancements and to apply the model to a larger pulsar population. We successfully fit the sample of pulsars with X-ray and $γ$-ray data. Our results indicate that, for every emitting particle, the spatial extent of their trajectory where the pitch angle is large and most of the detected X-ray radiation is produced is a small fraction of the light cylinder. We also confirm with this new approach that synchrotron radiation is not negligible for most of the gamma-ray pulsars detected. In addition, with the results obtained, we argue that J0357+3205 and J2055+2539 are MeV-pulsar candidates and are suggested for exhaustive observations in this energy band.

astro-ph.HE↗