arXiv subjects

Alberto Domínguez

Publications and source records attributed to Alberto Domínguez.

At least 19 recordsLinked to original sources

arXiv2026

The Quasar Main Sequence of Super-Eddington NLSy1 Galaxies

The quasar main sequence, or Eigenvector 1 (EV1), describes the optical diversity of active galactic nuclei (AGN), with Narrow-Line Seyfert 1 (NLSy1) galaxies anchoring the high-accretion end. Recent discoveries of overly massive black holes in the early Universe highlight the need to study low-redshift, low-mass super-Eddington accretors as analogs of rapid black hole growth. We map a population of 18,749 NLSy1 galaxies identified in the Dark Energy Spectroscopic Instrument Data Release 1 (DESI DR1) onto the EV1 plane to determine whether they represent a distinct population of super-accretors. We compare the spectral properties of the DESI DR1 NLSy1 sample with the SDSS DR17 NLSy1 catalog. We extract key parameters, including the broad H-beta full width at half maximum (FWHM) and Fe II strength (R4570). To evaluate their accretion states, we derive single-epoch virial black hole masses using an Fe II strength-dependent scaling relation and an Eddington rate-dependent fundamental plane, both of which confirm their high accretion rates. The DESI DR1 NLSy1 population shows a consistent shift toward the extreme end of the EV1 parameter space, possessing strong Fe II emission (median R4570 = 0.97 +/- 0.36), similar to the SDSS sample (median R4570 = 0.97 +/- 0.34). Furthermore, considering both calibrations, DESI and SDSS sources harbor low-mass black holes (median log black hole mass ~6.81-6.85). About 50% of the NLSy1 galaxies exhibit super-Eddington accretion (log Eddington ratio > 0). The unprecedented sensitivity of DESI has unveiled a large population of low-mass, super-Eddington accreting sources. These intriguing EV1 objects struggle to process luminous accretion flows, naturally producing the observed intense Fe II emission. This unique sample provides a rich statistical dataset of low-redshift super-Eddington accretors for understanding early-Universe black hole growth.

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arXiv2026

Constraining the gamma-ray efficiency of LINER outflows with Fermi-LAT and MEGARA

Low-Ionization Nuclear Emission-line Regions (LINERs) commonly host ionized gas outflows, but their role as high-energy particle accelerators remains debated, particularly following the very-high-energy $γ$-ray detection of NGC 4278, which implied extreme radiative efficiencies. We empirically determine the $γ$-ray radiative efficiency of a local sample of LINERs to test whether their high-energy emission can be powered by extended ionized outflows or requires compact nuclear jets. We combine spatially resolved optical integral-field kinematics from the Multi-Espectrógrafo en GTC de Alta Resolución para Astronomía at the Gran Telescopio de Canarias, yielding ionized-outflow kinetic powers, with 17 years of Fermi-Large Area Telescope observations to derive 0.05--500 GeV luminosities or 95% confidence upper limits. We place the sample in an optical/$γ$-ray diagram alongside archetypal starbursts and radio galaxies. We present the first empirical upper limits on the $γ$-ray radiative efficiency of LINER outflows as a population. No LINER is formally detected ($TS \geq 16$). The strongest constraint is obtained for the radio-loud LINER NGC 1052, with $η< 41%$. For the remaining sources, the Fermi-LAT limits generally lie well above the outflow kinetic powers ($η\gg 100%$), while three sources show marginal hints of emission ($9 < TS < 16$). We conclude that extended ionized outflows in LINERs are highly inefficient high-energy particle accelerators, analogous to starburst superwinds. These constraints disfavor the outflows as the sole origin of extreme $γ$-ray efficiencies and favor compact nuclear jets for the most efficient $γ$-ray-emitting LINERs.

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arXiv2026

Evidence for a bursty $γ$-ray QPO in the neutrino-associated FSRQ PKS 1424$-$418 using Fast Template Periodograms and Machine Learning

Standard frequency-domain searches for quasi-periodic oscillations (QPOs) in active galactic nuclei generally assume sinusoidal variability. However, $γ$-ray blazar emission often shows asymmetric flares and localized bursts, causing spectral leakage that can reduce the sensitivity of conventional methods to non-sinusoidal periodic signals. We analyze a blind sample of 100 high-cadence (7-day binned) Fermi-LAT blazar light curves. After baseline detrending with Singular Spectrum Analysis, we apply the Fast Template Periodogram using three template families: a sinusoid, a Gaussian burst, and an empirical asymmetric template derived from PG 1553+113. Candidate signals are then evaluated with a Random Forest classifier trained on $5\times10^4$ simulated light curves to distinguish genuine periodic phase structure from stochastic red noise. We identify nine $>3σ$ periodicity candidates. Three persistent QPOs, including PG 1553+113, are recovered by all template models, whereas six additional candidates are strongly suppressed under the standard sinusoidal assumption but become detectable with morphology-aware templates. Machine learning validation rejects five of these as lacking stable phase coherence. The remaining candidate, PKS 1424$-$418, shows a structurally stable 4.8-year periodicity with a source-specific significance of $3.76σ$ (approximately $2.4σ$ after accounting for the trial factor of the 100-source sample). These results demonstrate that morphology-aware periodograms, combined with structural machine learning validation, improve the detection of burst-dominated QPOs that are difficult to identify with traditional harmonic searches.

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arXiv2026

Geometric masking in AGN jets and its implications for unification and blazar physics

I explore the theoretical implications of the Geometric Masking scenario for AGN Unification and the blazar sequence. In PKS 2155$-$304, GeV spectral hardening appears locked to its $\sim 1.7$ yr quasi-periodic oscillation trough, suggesting high-flux states are dominated by a soft, geometrically amplified envelope that masks an underlying hard core. While this foundational mechanism builds on observations of coexisting emission regions (Madero & Domínguez 2026) and phase-locked spectral hardening (Domínguez et al. 2026), here I propose hypothetical extensions that fall outside the scope of those works. I propose that Doppler boosting preferentially enhances this soft component in closely aligned jets. This visibility bias could extend AGN Unification and introduce a geometric modulation layer to the blazar sequence. Under this speculative framework, low-flux states represent windows of geometric transparency, and misaligned systems act as naturally unmasked laboratories. Consequently, the intrinsic duty cycle of extreme acceleration in AGN jets may be higher than inferred from flux-selected observations.

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arXiv2026

Spectral Hardening Revealed by Geometric De-boosting in the Masked Jet of PKS 2155$-$304

Blazar gamma-ray variability is predominantly stochastic and well described by red-noise processes. However, a subset of sources shows quasi-periodic oscillations (QPOs) on year-long timescales, whose physical origin remains debated. In high-synchrotron-peaked (HSP) blazars, departures from a single power-law gamma-ray spectrum, manifested as high-energy upturns in the GeV band, may probe emission mechanisms and the intrinsic duty cycle. We investigate the link between the 1.7 yr gamma-ray QPO in PKS 2155-304 and an exceptional spectral hardening event identified in the Fermi-LAT HSP blazar population. We analyze 17.4 years of Fermi-LAT data using 30-day binning, applying Singular Spectrum Analysis to mitigate red-noise effects and a Moving Block Bootstrap approach to quantify the correlation between photon flux and photon index. We find a statistically significant softer-when-brighter chromatic trend, supporting a geometric origin of the flux modulation. The spectral hardening event is phase-locked to the QPO trough, implying that the hardening signature is detectable only when geometrically boosted soft emission is suppressed at the flux minimum. We propose a Geometric Masking scenario in which jet geometry regulates the visibility of acceleration processes. These results favor a two-component jet structure and suggest that spectral hardening during low-flux states, even in non-periodic sources, may reveal jet physics otherwise obscured by relativistic amplification.

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arXiv2026

A New Measurement of the Extragalactic Background Light using 15\,yr of {\it Fermi}-Large Area Telescope Data

The extragalactic background Light (EBL) from ultraviolet to infrared comprises the emission from all stars, galaxies, and actively accreting black holes in the observable Universe. A precise measurement of the EBL is critically important to probe models of star formation and galaxy evolution. The EBL can be measured via the absorption imprint left on the spectra of gamma-ray blazars. In this work, we rely on 15 years of {\it Fermi}-LAT data and 1576 blazars to measure the EBL optical depth in the $0<z<4.3$ range. We detect the EBL attenuation with $\sim23σ$ significance and measure the optical depth in 19 redshift bins, extending the coverage and improving on our previous results. This allows us to reconstruct the EBL evolution and find general consistency with recent EBL models. These results represent the most precise determination of the EBL with GeV $γ$ rays to date.

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arXiv2026

Coexistence of Chromatic Flares and an Achromatic QPO in the Gamma-ray Blazar PG 1553+113

The physical origin of quasi-periodic oscillations (QPOs) in blazars remains debated, with geometric and plasma-driven scenarios as the main competing interpretations. Discriminating between them requires probing variability beyond flux periodicity. We study the spectral evolution of the BL Lac object PG 1553+113 over its 2.2-year gamma-ray QPO cycle to constrain the mechanism driving the oscillation. In particular, we test whether the variability is chromatic (coupled to spectral changes) or achromatic (independent of spectral shape), allowing us to distinguish between plasma-driven and geometric scenarios. We analyze 17 years of Fermi-LAT data (2008-2025) with 30-day binning. To mitigate red-noise effects, we apply Singular Spectrum Analysis (SSA) to remove slow baseline trends and use a Block Bootstrap approach to quantify correlations between photon flux and photon index while preserving temporal dependence. We find a robust softer-when-brighter chromatic trend, atypical for high-synchrotron-peaked blazars such as PG 1553+113 and which, based on our analysis, physically corresponds to softer-when-flaring episodes, that persists after detrending and accounting for temporal autocorrelation. In contrast, no significant correlation is detected between the photon index and the QPO phase, indicating that the periodic modulation is effectively achromatic. The coexistence of plasma-driven chromatic flares and an achromatic QPO disfavors scenarios in which the periodicity arises from intrinsic jet processes. Instead, the results support a geometric origin for the QPO modulation, such as jet precession, where Doppler-factor variations modulate the flux without altering the intrinsic particle energy distribution.

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arXiv2026

A Catalogue of Variable Active Galactic Nuclei Based on Multi-Timescale Variability Analysis from Fermi-LAT Data

Active Galactic Nuclei (AGN) sources feature supermassive black holes that launch relativistic plasma jets. They are key $γ$-ray sources providing a unique laboratory for studying extreme particle acceleration and plasma physics. Variability in $γ$-ray emission is an important signature that may constrain the size of the emission region and the physical processes driving flares. However, current large-scale $γ$-ray catalogs, such as the Fermi-LAT 4LAC-DR3, typically characterize variability only on long timescales (yearly or 60-day), lacking necessary constraints on short-term behavior from days to weeks. To address this, we systematically characterize $γ$-ray variability in AGNs across short timescales: 3-day, 7-day (weekly), and 30-day (monthly). We present a preliminary catalogue of variable AGN based on light curves from the Fermi-LAT Light Curve Repository. We show that the variability amplitude ($σ_{\rm NXS}^{2}$) presents similar values across different timescales, potentially increasing for a subsample of sources as the observation timescale increases. This high-cadence analysis reinforces the known dichotomy between flat-spectrum radio quasars (FSRQs) and BL Lacertae objects (BL Lacs), with FSRQs consistently exhibiting stronger variability. By identifying the most luminous and variable sources at each timescale, we highlight key targets for follow-up with next-generation observatories such as the Cherenkov Telescope Array Observatory (CTAO), ASTRI Mini-Array, and the Southern Wide-field Gamma-ray Observatory (SWGO), where strong short-term variability suggests highly compact emission zones and extreme particle acceleration efficiency. This catalogue contributes to the understanding of high-energy outflows in AGN jets and provides a foundation for optimizing observational strategies through a unified variability metric across timescales.

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arXiv2025

Detection Prospects for AGNs with the Cherenkov Telescope Array

The Cherenkov Telescope Array Observatory (CTAO) will enable detailed studies of Active Galactic Nuclei (AGN) in the very-high-energy (VHE) regime, as the next-generation ground-based gamma-ray observatory, designed to enhance sensitivity and energy coverage (20 GeV -- 300 TeV) over current Imaging Atmospheric Cherenkov Telescopes (IACTs). In the context of the CTAO Science Collaboration, within the AGN Population working group, we developed a variability-based strategy to improve predictions of AGNs detectable by CTAO, using Fermi-LAT data and normalized excess variance (NXS) as a tracer of flux variability. By extrapolating from 30-day to 3-day timescales, we expanded the sample of sources with short-timescale variability estimates from 87 to 407. This approach allows us to identify flaring and distant AGNs that are promising CTAO targets. The results are being used to support the CTAO extragalactic science program and will be included in an upcoming Consortium publication for the AGN Population collaboration.

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arXiv2025

TeV Emission in NGC 4278: Shock Energetics Insufficient, Compact Origin Favored

The Large High Altitude Air Shower Observatory recently detected TeV emission from the low-luminosity active galactic nucleus (LLAGN) NGC 4278. Integral-field spectroscopy with MEGARA revealed ionized outflows on hundred-parsec scales with kinetic power $\dot E_{\rm shock} \sim 10^{38}$ erg s$^{-1}$, well below the TeV luminosity $L_γ \gtrsim 10^{41}$ erg s$^{-1}$. The implied efficiencies, $η\gtrsim 1000$, are far above unity, showing that shocks cannot account for the TeV emission. Contemporaneous Fermi-Large Area Telescope and Swift data, consistent with a compact synchrotron self-Compton jet component, favor a nuclear origin. NGC 4278 is the first low-ionization nuclear emission-line region (LINER)/LLAGN of its class detected at TeV energies, a benchmark for LINERs with outflows and a prime target for the Cherenkov Telescope Array Observatory. I conclude that the TeV photons arise from a compact nuclear accelerator, while optical outflows trace larger-scale jet--ISM interactions that coexist but do not power the TeV radiation.

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arXiv2025

Decade-long periodicity study of 2FHL blazars with historical optical data

In our recent investigation, we utilized a century's worth of archival optical data to search for a decade-long periodicity from the blazar PG 1553+113, finding a hint of a 22-yr period. Building on this foundation, the current study extends our analysis to include 10 blazars from the Fermi-Large Area Telescope Second Catalog of Hard Sources (2FHL) catalogue to uncover similar long-term periodic behaviour. To ensure the reliability of our findings, we consider the impact of observational limitations, such as temporal gaps and uneven sampling, which could potentially introduce artefacts or false periodic signals. Our analysis initially identifies decade-scale periodicity in four of these blazars (AP Librae, MKN 421, MKN 501, PG 1246+586). However, further investigation reveals that three of these are likely influenced by noise and poor sampling. The most promising candidate, approximately 51 $\pm$ 9 yr signal in MKN 421, corresponds to fewer than three full cycles and cannot be considered significant. Furthermore, global significance suggests none of the candidate periodicities meet the threshold for statistical significance. These results underscore the importance of accounting for sampling artefacts and highlight the need for robust methodologies in long-term periodicity searches.

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arXiv2024

Radio Morphology of Gamma-ray Sources: Double-Lobed Radio Sources

The extragalactic gamma-ray sky is dominated by relativistic jets aligned to the observer's line of sight, i.e., blazars. A few of their misaligned counterparts, e.g., radio galaxies, are also detected with the Fermi-Large Area Telescope (LAT) albeit in a small number ($\sim$50), indicating the crucial role played by the jet viewing angle in detecting gamma-ray emission from jets. These gamma-ray emitting misaligned active galactic nuclei (AGN) provide us with a unique opportunity to understand the high-energy emission production mechanisms from a different viewpoint than the more common blazars. With this goal in mind, we have systematically studied the radio morphology of gamma-ray emitting sources present in the fourth data release of the fourth catalog of Fermi-LAT detected gamma-ray sources to identify misaligned AGN. By utilizing the high-resolution and sensitive MHz and GHz frequency observations delivered by the Very Large Array Sky Survey, Low-Frequency Array Two-metre Sky Survey, Faint Images of the Radio Sky at Twenty-Centimeters, and Rapid ASKAP Continuum Survey, here we present a catalog of 149 gamma-ray detected misaligned AGN, thus $\sim$tripling the number of known objects of this class. Our sample includes a variety of radio morphologies, e.g., edge-darkened and edge-brightened, hybrids, wide-angle-tailed, bent jets, and giants. Since the gamma-ray emission is thought to be highly sensitive to the jet viewing angle, such an enlarged sample of gamma-ray detected misaligned radio sources will permit us to explore the origin of high-energy emission in relativistic jets and radio lobes and study AGN unification, in general.

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arXiv2024

Influence of Cosmic Voids on the propagation of TeV Gamma Rays and the Puzzle of GRB 221009A

The recent detection of gamma-ray burst GRB~221009A has attracted attention due to its record brightness and first-ever detection of $\gtrsim 10$ TeV $γ$ rays from a GRB. Despite being the second-nearest GRB ever detected, at a redshift of $z=0.151$, the distance is large enough for severe attenuation of $γ$-ray flux at these energies due to $γγ\to e^\pm$ pair production with the extragalactic background light (EBL). Here, we investigate whether the presence of cosmic voids along the line of sight can significantly impact the detectability of very-high energy (VHE, $>$ 100 GeV) gamma rays from distant sources. Notably, we find that the gamma-gamma opacity for VHE gamma rays can be reduced by approximately 10\% and up to 30\% at around 13 TeV, the highest-energy photon detected from GRB~221009A, for intervening cosmic voids along the line-of-sight with a combined radius of 110 Mpc, typically found from voids catalogs, and 250 Mpc, respectively. This reduction is substantially higher for TeV photons compared to GeV photons, attributable to the broader target photon spectrum that TeV photons interact with. This finding implies that VHE photons are more susceptible to variations in the EBL spectrum, especially in regions dominated by cosmic voids. Our study sheds light on the detection of $\gtrsim 10$ TeV gamma rays from GRB 221009A in particular, and on the detection of extragalactic VHE sources in general.

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arXiv2022

Optical spectroscopic characterization of Fermi blazar candidates of uncertain type with TNG and DOT: First Results

The classification of gamma-ray-detected blazar candidates of uncertain type (BCU) is a relevant problem in extragalactic gamma-ray astronomy. Here we report the optical spectroscopic characterization, using two 3-4~m class telescopes, Telescopio Nazionale Galileo and Devasthal Optical Telescope, of 27 BCUs detected with the Fermi Large Area Telescope. Since the identification of emission lines is easier in broad-line blazars, which usually exhibit low frequency peaked (synchrotron peak frequency $\leqslant10^{14}$ Hz) spectral energy distribution, we primarily target such BCUs. We found that 8 out of 27 sources exhibit broad emission lines in their optical spectra, 3 of them have redshifts $>$1 and the farthest one is at $z=2.55$. The optical spectra of 2 of the 19 remaining objects are dominated by the absorption spectra of the host galaxy, and there is a tentative detection of the Lyman-$α$ absorption feature in one source. The spectra of the remaining 16 objects, on the other hand, are found to be featureless.

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arXiv2021

An observational determination of the evolving extragalactic background light from the multiwavelength HST/CANDELS survey in the Fermi and CTA era

The diffuse extragalactic background light (EBL) is formed by ultraviolet (UV), optical, and infrared (IR) photons mainly produced by star formation processes over the history of the Universe, and contains essential information about galaxy evolution and cosmology. Here, we present a new determination of the evolving EBL spectral energy distribution using a novel approach purely based on galaxy data aiming to reduce current uncertainties on the higher redshifts and IR intensities. Our calculations use multiwavelength observations from the UV to the far-IR of a sample of approximately 150,000 galaxies detected up to $z\sim 6$ in the five fields of the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey (CANDELS) from the Hubble Space Telescope. This is one of the most comprehensive and deepest multi-wavelength galaxy datasets ever obtained. These unprecedented resources allow us to derive the overall EBL evolution up to $z\sim 6$ and its uncertainties. Our results agree with cosmic observables estimated from galaxy surveys and $γ$-ray attenuation such as monochromatic luminosity densities, including those in the far-IR, and star formation rate densities, also at the highest redshits. Optical depths from our EBL approximation, which will be robust at high redshifts and for $γ$ rays up to tens of TeV, will be reported in a companion paper.

astro-ph.CO↗

arXiv2020

Search for the imprint of axion-like particles in the highest-energy photons of hard $γ$-ray blazars

Axion-like particles (ALPs), predicted in theories beyond the Standard Model, can have observational effects on the transparency of the Universe to $γ$ rays in the presence of magnetic fields. In this work, we search for effects compatible with the existence of ALPs with 80 months of data from the ${\it Fermi}$ Large Area Telescope, by comparing the distributions of observed highest energy photons from sources beyond redshifts of z $\geq$ 0.1 with theoretical predictions in the presence of ALPs. We find no evidence for an increased $γ$-ray transparency due to ALPs and therefore we set limits on the ALPs parameters assuming a value of the intergalactic magnetic field strength of 1 nG. Photon-ALP couplings above $10^{-11}$ GeV$^{-1}$ are excluded for ALP masses $m_{a}$ $\lesssim3.0$ neV. These limits exclude a region of the parameter space not covered by other $γ$-ray telescopes and are compatible with constraints imposed by other experiments.

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arXiv2020

TXS 2116$-$077: A Gamma-ray Emitting Relativistic Jet Hosted in a Galaxy Merger

What triggers collimated relativistic outflows or jets, from the centers of galaxies remains a fundamental question in astrophysics. The merging of two galaxies has been proposed to realize the conditions to successfully launch and drive such jets into the intergalactic medium. However, evidences for the operation of this mechanism are scarce. Here we report the first unambiguous detection of an ongoing merger of a narrow-line Seyfert 1 galaxy, TXS 2116$-$077, hosting a closely aligned, $γ$-ray emitting relativistic jet with a Seyfert 2 galaxy at a separation of $\sim$12 kpc, using the observations taken with 8.2 m Subaru telescope. Our subsequent followup observations with 10.4 m Gran Telescopio Canarias, 4.2 m William Herschel Telescope, and Chandra X-ray observatory have provided what is likely to be the first glimpse of the merging environment hosting a closely aligned relativistic jet. Our finding that the jet is considerably younger than the merger demonstrates that jet activity can be triggered by galaxy mergers and that $γ$-ray detected narrow-line Seyfert 1 galaxies represent the beginning phase of that activity. These results also highlight the crucial role of mergers in shaping the fate of galaxies in their cosmological evolution and are consistent with recent studies focused on the host galaxy imaging of this enigmatic class of active galactic nuclei.

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arXiv2019

A new measurement of the Hubble constant and matter content of the Universe using extragalactic background light $γ$-ray attenuation

The Hubble constant $H_{0}$ and matter density $Ω_{m}$ of the Universe are measured using the latest $γ$-ray attenuation results from Fermi-LAT and Cherenkov telescopes. This methodology is based upon the fact that the extragalactic background light supplies opacity for very high energy photons via photon-photon interaction. The amount of $γ$-ray attenuation along the line of sight depends on the expansion rate and matter content of the Universe. This novel strategy results in a value of $H_{0}=67.4_{-6.2}^{+6.0}$~km~s$^{-1}$~Mpc$^{-1}$ and $Ω_{m}=0.14_{-0.07}^{+0.06}$. These estimates are independent and complementary to those based on the distance ladder, cosmic microwave background (CMB), clustering with weak lensing, and strong lensing data. We also produce a joint likelihood analysis of our results from $γ$ rays and these from more mature methodologies, excluding the CMB, yielding a combined value of $H_{0}=66.6\pm 1.6$~km~s$^{-1}$~Mpc$^{-1}$ and $Ω_{m}=0.29\pm 0.02$.

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