arXiv subjects

M. Ajello

Publications and source records attributed to M. Ajello.

At least 19 recordsLinked to original sources

arXiv2026

Profile Analysis of the Multiwavelength 2.1-year Oscillations of PG 1553+113

We investigate the morphology of the oscillation profiles of the blazar PG~1553+113 in relation to its well-known $\sim$2.1 yr periodicity. We identify individual cycles in the $\gamma$-ray, X-ray, UV, and optical light curves and characterize their temporal profiles using analytical models for single- and multi-peaked events. We find that the oscillations are generally described by a broad activity envelope with shorter-timescale substructure, showing that the $\sim$2.1 yr signal is not a strictly sinusoidal or self-similar modulation. The internal morphology varies across cycles and energy bands. This is particularly evident in X-rays, where all analyzed cycles show a strong formal preference for multi-component profiles, unlike the $\gamma$-ray band, where several cycles admit statistically comparable empirical descriptions. The contemporaneous MWL oscillations show broadly aligned activity episodes, but the timing and relative amplitudes of secondary components are not systematically repeated. This suggests that a common long-term modulation affects the broadband emission, while additional local or energy-dependent processes shape individual cycles. Such a picture is compatible with a geometric, jet-related contribution to the broad recurrent envelope, with intrinsic variability superimposed on it. We also identify new cycles with a dominant peak accompanied by weaker twin-peak-like features, similar to structures previously discussed in a supermassive black hole binary scenario for PG~1553+113. Although our results do not provide definitive evidence for this interpretation, the recurrence of comparable morphologies in newly analyzed cycles keeps this scenario viable.

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arXiv2026

Gamma-Ray Periodicity in Jetted AGN: Revisiting Periodicity Candidates with >17 years of Fermi-LAT Data

We reanalyze previously reported gamma-ray periodicity candidates in jetted active galactic nuclei using more than 17 years of Fermi-LAT observations. The updated data provide a robust test of whether earlier results correspond to persistent periodic behavior, to transient quasi-periodic oscillations (QPOs), or to fluctuations produced by stochastic variability. We apply complementary timing methods, including the Generalized Lomb-Scargle periodogram, Phase Dispersion Minimization, and Singular Spectrum Analysis, and estimate the test statistics using artificial light curves generated from different variability models, including simple and bending power-law power spectral densities, as well as ARIMA and ARFIMA autoregressive models. We find that most previously proposed candidates in literature are not confirmed when the longer baseline is considered, indicating that many reported periods were likely driven by limited temporal coverage and red-noise variability or transient QPO-like features rather than persistent periodic behavior. Only eight sources retain hints of periodic behavior at the >2sigma local test statistics. Among them, PG 1553+113 and S5 1044+71 remain the most significant cases, with local test statistics above 3sigma, and with a global significance consistent with ~0sigma (because of the large trial factor). In addition, we assess predictions from previous studies as an independent test of the proposed periods and find that some are consistent with the new observations.

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arXiv2026

Exploring the nature of Galactic unassociated sources detected by the Fermi-LAT

We investigate the nature of the unassociated sources detected by the Fermi-LAT close (|b|<10{\deg}) to the Galactic plane, representing 16% of all sources in the 4FGL-DR4 catalog. The bulk of these sources (referred to as soft Galactic unassociated sources, SGUs) exhibit properties not found in known classes of gamma-ray emitters, as confirmed by a machine-learning classification approach. In particular, these properties include a steep, curved spectrum peaking below 1 GeV and a specific Galactic-latitude distribution with both a narrow and a broad component (dubbed the spike and the shoulder, respectively). Some source clusters are highlighted. New plausible source classes are explored, but only star-forming regions are found to account for a significant fraction (at most 10%) of the unassociated population. A thorough search for counterparts to the 175 brightest sources brings out a number of plausible counterparts but does not reveal clues about the nature of the whole population. We investigate the possibility that SGUs originate from mismodeled clumps of diffuse emission. Using Monte Carlo simulations, the SGU spectra can be reproduced in this scenario under an ad hoc condition concerning the clump spatial extension. The possible connection between the SGUs and gas not accounted for by the 12CO tracer is explored using the 13CO MOPRA data but leads to inconclusive results. The origin of SGUs being related to diffuse emission remains plausible. However, a scenario whereby SGUs represent a new class of gamma-ray emitters cannot be fully excluded.

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arXiv2026

Gamma-ray signature of superluminous supernovae: Fermi-LAT GeV detection of SN 2017egm and evidence of a central engine

Superluminous supernovae (SLSNe) are a rare class of transients with peak luminosities 10-100 times greater than those of standard core-collapse supernovae (SNe). The mechanisms powering their extreme brightness remain debated, with circumstellar medium (CSM) interaction, or energy injection from a central engine like a magnetar wind nebula being the most plausible scenarios. To further constrain the underlying mechanism, we carried out a systematic search for GeV gamma-ray emission using the Fermi-LAT telescope from a sample of nearby hydrogen-poor (Type I) and hydrogen-rich (Type II) SLSNe over the past 16 years. Among the sample, only SN 2017egm shows significant gamma-ray emission, with likelihood test statistic (TS) values of 26-33 (i.e., >5$\sigma$) depending on the adopted time window. The signal arises between 50 and 160 days after explosion and is well described by a power-law spectrum with index $\Gamma=2.17 \pm 0.23$. The emission is consistent both in terms of its light curve and its spectrum, with predictions from magnetar models requiring either low nebular magnetization or faster spin-down than dipole losses. The CSM shell interaction scenario can reproduce the observed flux level but not the observed timing of the gamma-ray signal. In addition, the observed ratio, $L_{\gamma}/L_{opt} \sim 1$, is inconsistent with theoretical expectations and not in line with ratio measurements in other interacting CSM-dominated objects (e.g., novae or SNe) where this ratio is less than $10^{-2}$. Our study strongly suggests that a central engine like a magnetar plays a key role in this SLSN and could explain the bulk of the optical and gamma-ray light curves properties. Finally, simulations of 50 hours of CTAO observations indicate that a SN 2017egm-like event would be detectable up to 140 Mpc in the magnetar model but not in the CSM model due to strong gamma-gamma absorption.

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arXiv2026

Legacy analysis of Milky Way dwarf spheroidal satellite galaxies: an update

Studies of Fermi-Large Area Telescope (LAT) data coincident with dwarf spheroidal satellite galaxies (dSphs) of the Milky Way (MW) have put the most stringent constraints on models of annihilating dark matter (DM) with candidate masses in the GeV-TeV range. Recent results found the presence of small, local significance excesses from these targets, at the 2-3 sigma level. However, these excesses disagree on the predicted properties of the DM candidate, and their significance vanishes when considering the correction factors for the number of trials. In this work, we apply key improvements to the analysis of the dSphs. We use stricter cuts on the data, implement a method to adaptively model the background, and assume an updated framework for DM annihilation. We find that our improved background modeling leads to a better agreement between the model and the data. This produces an increase in the local and global significance of the dSphs excess compared to previous studies. Finally, we find that the DM properties obtained in this work are less dependent on the sample of dSphs being considered compared to previous studies, while remaining in agreement with the predictions from the Galactic center excess observed by the Fermi-LAT and the antiproton excess observed by the Alpha Magnetic Spectrometer (AMS-02). Considering our improvements, a future significant increase in the number of dwarfs may lead to a definitive confirmation or exclusion of the DM interpretation of the Galactic center excess.

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arXiv2026

A TeV-based Determination of the Local Extragalactic Background Light and its Consistency with Galaxy Counts and Direct Measurements

The extragalactic background light (EBL), the cumulative radiation from all extragalactic sources, traces galaxy formation and cosmic evolution. High-energy $\gamma$ rays attenuated via pair production with EBL photons are a powerful probe of the EBL. In this work, we use very-high-energy (VHE; $E_\gamma > 100\,\mathrm{GeV}$) $\gamma$ rays to measure the local EBL intensity and test its consistency with galaxy counts and direct measurements. Our analysis employs a sample of 268 spectra from 45 sources observed with Imaging Atmospheric Cherenkov telescopes. A model-dependent study shows seven EBL templates require only $\le 10\%$ rescaling to fit the observed $\gamma$-ray attenuation. The galaxy-count-anchored model gives the closest match. We then derive template-marginalized TeV optical depths from a representative model subset. We combine them with \textit{Fermi}-LAT GeV measurements to reconstruct the EBL at $z = 0$ using empirical and physically motivated models. The two reconstructions agree and follow the integrated galaxy light to within $2$--$3\,\mathrm{nW\,m^{-2}\,sr^{-1}}$ (typically $<25\%$) over $0.5$--$30\,\mu$m. Both are consistent with low-zodiacal-light observations, including outer solar system and dark cloud measurements. In contrast, the near-IR excess reported by IRTS and CIBER exceeds our reconstructed intensity by $3$--$5\sigma$, implying an additional $\gtrsim 5$--$10\,\mathrm{nW\,m^{-2}\,sr^{-1}}$ incompatible with the $\gamma$-ray optical depths. Combined with GeV constraints on EBL evolution to $z \simeq 4$, these TeV optical depths provide a VHE-anchored determination of the local EBL intensity. The agreement with galaxy counts and deep-space measurements indicates that known galaxy populations account for most of the optical and near-IR background, leaving limited room for an additional diffuse component.

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arXiv2026

Testing X-ray Periodicity and Long-Term Trend in PG 1553+113 via Targeted Swift-XRT Monitoring

PG~1553+113 is the blazar with the most-significantly detected periodic patter in its multiwavelength (MWL) emission, making it one of the most promising candidates for hosting a supermassive black hole binary. However, the presence of this periodic behavior in the X-ray band remains under debate, largely due to the lack of continuous monitoring. This has led to differing conclusions in previous studies. In addition, we aim to examine whether the recently identified linear long-term trends in the gamma-ray and optical bands also exist in the X-ray regime. Here, we evaluate the 2.1-year period in the X-ray light curve of PG 1553+113 using two dedicated monitoring campaigns with Swift-XRT and UVOT, guided by predictions of future oscillation phases. We also examine whether the long-term trend is present in X-rays, the potential periodic behavior of the X-ray power-law photon index, and its potential correlation to the X-ray flux. As a result, we find tentative evidence for a correlation between the predicted high-emission states in the gamma-ray band and those observed in the X-ray and UV bands. Therefore, we do not find a strong evidence of the same periodic pattern in X-ray. In addition, we find that the X-ray light curve is consistent with the presence of a long-term linear trend, in agreement with those previously reported in gamma-ray, optical, and radio. Overall, these results indicate that the X-ray emission is likely to share the same long-term behavior observed in the gamma-ray and optical bands. Nevertheless, the pronounced stochastic variability that characterizes the X-ray light curve limits our ability to draw firm conclusions regarding the presence of the periodic behavior.

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arXiv2026

Source identification for the Swift-BAT 150-month hard X-ray catalog using soft X-ray observations

We present a comprehensive catalog of 251 potential counterparts for 250 unassociated hard X-ray sources detected in the Swift Burst Alert Telescope (BAT) 150-month hard X-ray survey. Over 150 months of observation, BAT has detected 2339 sources in the 15-150 keV energy range. Among these, 344 do not have a previously identified low-energy counterpart. Our study focuses on the analysis of soft X-ray observations at energies below 10 keV, spatially overlapping with these new Swift-BAT hard X-ray sources. Such observations were taken with Chandra, Swift-XRT, eROSITA, and XMM-Newton. Within the sample of 251 potential counterparts, 94 (37 percent) are identified as active galactic nuclei and 58 (23 percent) as galaxies. The remaining 99 sources (40 percent) include pulsars, cataclysmic variables, and unclassified soft X-ray counterparts in the 0.5-10 keV band. Redshift information is available for 139 out of the 251 sources, and its distribution is in close agreement with the redshift distribution of previous BAT catalogs. We also present the results of a small optical spectroscopy campaign of 9 out of 58 galaxies. The majority of these are classified as Seyfert 2 galaxies at redshifts slightly larger than the median of the BAT AGN sample.

astro-ph.HE↗

arXiv2025

Gamma-ray Time Delay and Magnification Ratio in the Gravitationally-Lensed Blazar PKS 1830-211

We present the characterization of macrolensing properties of the gravitationally lensed system PKS 1830-211, utilizing data from the Fermi Large Area Telescope. While at gamma-rays we can not spatially resolve the lensed images, a macrolensing-induced time pattern is expected in the blazar's lightcurve, resulting from the delay between variable gamma-ray components originating from its two brightest lensed images. Compared to our previous study, here we employ high-quality lightcurves coupled with prolonged outburst activity, and improved time-series techniques. Analyzing six independent data segments, we identified a delay of 20.26 +/- 0.62 days (statistical and stochastic uncertainty), with a chance detection probability at the 2.5 x $10^{-5}$ level (post-trial). We also present a novel approach to the magnification ratio estimate based on a comparison with simulated data. Our work suggests that the gamma-ray flux ratio between the two main lens components is $\mu_{\gamma} \lesssim$ 1.8. We do not observe convincing evidence of microlensing effects, as previously claimed. The measured gamma-ray time delay is in 2-sigma tension with radio-based estimates, suggesting either distinct emission sites, underestimated radio uncertainties, or gamma-ray production in a region opaque to radio. Our study highlights the potential of well sampled lightcurves and advanced time-series techniques to distinguish true lensing-induced delays from stochastic variability. When combined with improved radio constraints and refined lens models, PKS 1830-211 and similar sources constitute promising systems for time-delay cosmography, offering new insights into both jet structure and cosmological parameters.

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arXiv2025

Distortions in Periodicity Analysis of Blazars II: The Impact of Gaps

Time series analysis is fundamental to characterizing the variability inherent in multi-wavelength emissions from blazars. However, a major observational challenge lies in the need for well-sampled, temporally uniform data, which is often hindered by irregular sampling and data gaps. These gaps can significantly affect the reliability and accuracy of methods used to probe source variability. This paper investigates the impact of such observational gaps on time series analysis of blazar emissions. To do so, we systematically evaluate how these gaps alter observed variability patterns, mask genuine periodic signals, and introduce false periodicity detections. This evaluation is conducted using both simulated and real observational data. We assess a range of widely used time series analysis methods, including the Lomb-Scargle periodogram, Phase Dispersion Minimization, and the recently proposed Singular Spectrum Analysis (SSA). Our results demonstrate a clear and significant degradation in period detection reliability when the percentage of gaps exceeds 50\%. In such cases, the period-significance relationship becomes increasingly distorted, often leading to misleading results. Among the tested methods, SSA stands out for its ability to yield consistent and robust detections despite high data incompleteness. Additionally, the analyzed methods tend to identify artificial periodicities of around one year, likely due to seasonal sampling effects, which can result in false positives if not carefully recognized. Finally, the periods detected with $\geq$3$\sigma$ confidence are unlikely to result from stochastic processes or from the presence of gaps in the analyzed time series.

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arXiv2025

The gamma-ray emission from Radio Galaxies and their contribution to the Isotropic Gamma-Ray Background

We evaluate the contribution to the Isotropic Gamma-Ray Background (IGRB) coming from Radio Galaxies (RGs), the subclass of radio-loud Active Galactic Nuclei (AGN) with the highest misalignment from the line of sight (l.o.s.). Since only a small number of RGs are detected in gamma rays compared to the largest known radio population, the correlation between radio and gamma-ray emission serves as a crucial tool to characterize the gamma-ray properties of these sources. We analyse the population of RGs using two samples. The first sample contains 26 sources individually detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope at gamma rays. The second sample contains 210 RGs for which the gamma-ray emission is not significantly detected by the LAT. We use a stacking analysis to characterize the average properties of the gamma-ray emission of the two samples, separately at first and then combined. We then evaluate the correlation between their gamma-ray emission and the emission from their radio core at 5 GHz, and we use it to determine their contribution to the IGRB. Due to the limited number of RGs detected at the gamma-rays, information on the gamma-ray luminosity function is limited. The correlation between the gamma-ray emission and the emission of the radio core allows us to characterize it starting from the luminosity function of the radio cores, which is modeled with greater accuracy due to the larger number of sources detected at these frequencies. We find that the diffuse emission as extrapolated from the properties of the subthreshold RGs is lower than the one inferred from detected RGs, showing that the contribution of the population of RGs to the IGRB is lower than the previous estimates and it is around the 30% level of the IGRB intensity.

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arXiv2025

Extensive Analysis of gamma-Ray Periodicity in Jetted AGN from the 4FGL Catalog Using Fermi-LAT Observations

The quest to uncover periodic patterns within the $\gamma$-ray emissions of jetted active galactic nuclei (AGN) has recently emerged as a focal point in astrophysics. One of the primary challenges has been the necessity for prolonged exposures in the $\gamma$-ray energy band. In our investigation, we leverage 12 years' worth of observations from the \textit{Fermi}-LAT to systematically explore periodicity across 1492 jetted AGN cataloged in 4FGL, representing the largest sample analyzed to date. Our analysis involves a robust pipeline employing nine distinct techniques designed to detect potential periodic emissions within their $\gamma$ rays. We note that 24 objects with previous hints of periodicity are deliberately excluded in the present work since they were reanalyzed in a dedicated paper using a similar methodology. Using this thorough approach, we do not find any evidence for periodic signals in the 1492 jetted AGN $\gamma$-ray light curves analyzed here.

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arXiv2025

Transient QPOs of Fermi-LAT blazars with Linearly Multiplicative Oscillations

We present a study on the detection and characterization of transient quasi-periodic oscillations (QPOs) in the $\gamma$-ray emission of blazars 4C +31.03, MG1 J123931+0443, and PKS 1622$-$253. Using light curves derived from \textit{Fermi} Large Area Telescope data, we investigate oscillatory patterns characterized by periodic multiplicative amplitudes that vary linearly over time. By segmenting the light curves into increasing and decreasing trends, we analyze each segment independently, allowing for precise measurements of both the periodicity and long-term variations. To interpret these QPOs, we explore various theoretical scenarios that could explain their origin and underlying physical mechanisms. The variability observed in 4C~+31.03 is more consistent with a stochastic process, whereas the periods estimated for MG1~J123931+0443 and PKS~1622$-$253 align with the precessional dynamics expected from binary supermassive black hole systems. However, the current results remain tentative and do not allow for a definitive conclusion.

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arXiv2025

Transient QPOs of Fermi-LAT blazars under the Curved Jet Model

This study explores transient quasi-periodic oscillations (QPOs) in the $\gamma$-ray emission of two blazars, PMN J0531$-$4827 and PKS 1502+106, using over a decade of Fermi Large Area Telescope observations. The analysis focuses on identifying QPO signatures in their long-term light curves and interpreting the variability through a curved jet model, which predicts multiplicative oscillations with exponentially decaying amplitudes. We develop an analysis methodology to characterize the QPO and the specific properties of the amplitude of such QPOs. The findings offer insights into the dynamic processes driving relativistic jet evolution and their potential connections to underlying mechanisms, such as binary systems or other phenomena influencing the observed characteristics of these blazars.

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arXiv2025

A Systematic Search for Spectral Hardening in Blazar Flares with the Fermi-Large Area Telescope

Blazars are a subclass of active galactic nuclei (AGN) that emit non-thermal radiation through relativistic jets, characterized by rapid flux and polarization variability. High synchrotron-peaked blazars (HSPs) and extreme high synchrotron-peaked blazars (EHSPs), with synchrotron peaks exceeding $10^{15}$ Hz and $10^{17}$ Hz, respectively, are crucial for understanding the full range of blazar phenomena and testing models of jet physics. Yet, their understanding remains challenging. This work aims to systematically identify and characterize the most extreme $\gamma$-ray blazars using data from the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. The focus is on spectral hardening, where the $\gamma$-ray spectrum becomes harder at higher energies, particularly during flaring episodes. This represents the first dedicated analysis of spectral hardening across a population of EHSPs, as previous studies explored it only in individual sources. We analyze 138 blazars selected from the 4FGL-DR2 catalog with high synchrotron peak frequencies and well-sampled light curves. Flaring periods are automatically identified, and each flare is analyzed, with the significance of spectral hardening assessed through a test statistic based on the likelihood ratio of two spectral models. We identify two flaring episodes with indications of spectral hardening, in 4FGL J0238.4$-$3116 and PKS 2155$-$304, the latter detected independently by both methods but referring to the same period. These events are consistent with expectations from statistical fluctuations, suggesting that spectral hardening is a rare occurrence (< 0.1 %). These results constrain its frequency and support a smoothly varying power-law blazar emission model, motivating future multi-wavelength studies to clarify whether these rare flares reflect distinct physical processes within blazar jets.

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arXiv2025

Puzzling Variation of Gamma Rays from the Sun over the Solar Cycle Revealed with Fermi-LAT

The steady-state gamma-ray emission from the Sun is thought to consist of two emission components due to interactions with Galactic cosmic rays: (1) a hadronic disk component, and (2) a leptonic extended component peaking at the solar edge and extending into the heliosphere. The flux of these components is expected to vary with the 11-year solar cycle, being highest during solar minimum and lowest during solar maximum, as it varies with the cosmic-ray flux. No study has yet analyzed the flux variation of each component over solar cycles. In this work, we measure the temporal variations of the flux of each component over 15 years of Fermi Large Area Telescope observations and compare them with the sunspot number and Galactic cosmic-ray flux from AMS-02 near Earth. We find that the flux variation of the disk anticorrelates with the sunspot number and correlates with cosmic-ray protons, as expected, confirming its emission mechanism. In contrast, the extended component exhibits a more complex variation: despite an initial anticorrelation with the sunspot number, we find neither anticorrelation with the sunspot number nor correlation with cosmic-ray electrons over the full 15-year period. This most likely suggests that cosmic-ray transport and modulation in the inner heliosphere are unexpectedly complex and may differ for electrons and protons or, alternatively, that there is an additional, unknown component of gamma rays or cosmic rays. These findings impact space weather research and emphasize the need for close monitoring of Cycle 25 and the ongoing polarity reversal.

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arXiv2025

Search for Kink Events in Variable Fermi-LAT Blazars

This study explores the detection of Quasi-Periodic Oscillations (QPOs) in blazars as a method to identify kink events within their jets, utilizing both $\gamma$-ray and polarized light observations. Focusing on a sample of 9 blazars, we analyze $\gamma$-ray light curves to identify significant QPOs. In addition to $\gamma$-ray data, we incorporated polarized light data corresponding to the same temporal segments to cross-validate the presence of QPOs. However, the limited availability of comprehensive polarized data restricted our ability to perform a thorough analysis across all datasets. Despite these limitations, our analysis reveals a segment where QPOs in polarized light coincided with those observed in $\gamma$-rays, providing preliminary evidence supporting the kink origin of these oscillations.

astro-ph.HE↗

arXiv2025

X-ray polarization observations of NGC 2110 with IXPE

X-ray polarimetric observations from the Imaging X-ray Polarimeter Explorer (IXPE) is an excellent tool for probing the geometry and dynamics of X-ray emitting corona in active galactic nuclei (AGN). This work aims to investigate the geometry of the X-ray corona in the Seyfert 2 AGN, NGC 2110, using its first polarimetric observation with IXPE, conducted over a net exposure of 554 ks beginning on October 16, 2024. We performed a model-independent analysis of the 2-8 keV IXPE polarimetric observation to estimate the polarization properties of NGC 2110. Furthermore, we performed spectral and spectro-polarimetric analyses combining IXPE data with archival observations from NuSTAR, XMM-Newton, and Swift-XRT to derive detailed spectral and polarization properties. From the spectro-polarimetric analyses, an upper limit on the polarization degree (PD) of 7.6% (at the 99% confidence) was estimated in the 2-8 keV band. The spectro-polarimetric analysis in the 5.66-8 keV band yielded a looser upper limit of 27% at the 99 % confidence. Comparing the measured polarization properties, coronal parameters, and inclination angle of NGC 2110 with the Monte Carlo radiative transfer (MONK) simulations suggest that the current polarization measurements lack the sensitivity to place definitive constraints on the coronal geometry. The upper limits on PD, as derived from our analysis at the 99% confidence level, indicate that polarization remains undetected at a statistically significant level. Consequently, we are unable to determine whether the corona is elongated along the disk or more compact and spherical. Future observations with improved sensitivity will be crucial to breaking these degeneracies and providing deeper insight into the coronal structure of NGC 2110.

astro-ph.HE↗