arXiv subjects

Vandad Fallah Ramazani

Publications and source records attributed to Vandad Fallah Ramazani.

14 recordsLinked to original sources

arXiv2026

CAZ catalog and optical light curves of 7918 blazar-selected active galactic nuclei

Active galactic nuclei (AGN) are some of the brightest and most variable objects in the Universe. Those with relativistic jets observed at small viewing angles are blazars. Due to Doppler boosting, blazars exhibit extreme stochastic variability. While the origin of this variability is thought to be changes in the accretion flow and jet dynamics, much about blazar variability remains unknown. In this paper we use several blazar-dominated AGN samples to form a catalog of 7918 blazars and candidates -- the largest to date. We also collected source types, redshifts, peak frequencies of the spectral energy distribution, radio variability Doppler factors, and X-ray flux densities for as many sources as possible. We used all-sky surveys (CRTS, ATLAS, and ZTF, abbreviated as ``CAZ'') to extract their optical multiband flux density on a nightly basis between 2007 and 2023, and we constructed as long and as high cadence light curves as possible for as many sources as attainable. We quantified the variability of the light curves and applied the Bayesian blocks algorithm to determine their flaring periods. The CAZ catalog and light curves as well as the corresponding Bayesian blocks and flaring periods are all provided in the accompanying electronic tables, with the goal of enabling analyses involving jetted AGN variability with unprecedented sample sizes. Overall, we find (1) optical flares generally have a faster rise than decay; (2) optical brightness and variability are strongly dependent on the synchrotron peak frequency; (3) flat spectrum radio quasars and BL Lac objects have comparable optical variability and flare characteristics at the same synchrotron peak frequency; and (4) optical flare times tend to decrease while amplitudes increase with an increasing radio variability Doppler factor.

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arXiv2026

A long-term multiwavelength study of the flat spectrum radio quasar OP 313

The Flat Spectrum Radio Quasar OP 313 is a high-redshift (z = 0.997) blazar that entered an intense gamma-ray active phase from November 2023 to March 2024, as observed by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. We present a multiwavelength analysis covering 15 years of data, from August 2008 to March 2024, to contextualize this period of extreme gamma-ray activity within the long-term emission of the source. We analyzed a long-term, comprehensive, multiwavelength dataset from different facilities and projects from radio to gamma-rays. We identified the 7 most intense gamma-ray flaring periods and performed a kinematic analysis of Very Long Baseline Array (VLBA) data to determine whether new jet components emerged before or during these flares. For 2 of these flaring periods, we performed the modeling of the spectral energy distribution (SED). The VLBA-BU-BLAZAR and MOJAVE datasets reveal a new jet component appearing in both visibility datasets prior to the onset of one of the strongest gamma-ray flares. By comparing the timing of the VLBA-BU-BLAZAR knots ejection with the gamma-ray flaring periods, we constrained the setup of the SED modeling. We also found that the first gamma-ray flaring period is less Compton-dominated than the others. Our results suggest that the recent activity of OP 313 is triggered by new jet components emerging from the core and interacting with a standing shock. The γ-ray emission likely arises from dusty torus photons upscattered via Inverse Compton (IC) by relativistic jet electrons. The SED modeling indicates that this component is less dominant during the first γ-ray flaring period than the later ones.

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arXiv2025

Detection of Compton scattering in the jet of 3C 84

3C 84 is the brightest cluster galaxy in the Perseus Cluster. It is among the closest radio-loud active galaxies and among the very few that can be detected from low frequency radio up to TeV $γ$-rays. Here we report on the first X-ray polarization observation of 3C~84 with the Imaging X-ray Polarimetry Explorer, for a total of 2.2 Msec that coincides with a flare in $γ$-rays. This is the longest observation for a radio-loud active galaxy that allowed us to reach unprecedented sensitivity, leading to the detection of an X-ray polarization degree of $\rmΠ_X=4.2\pm1.3\%$ ($\sim3.2σ$ confidence) at an X-ray electric vector polarization angle of $\rm ψ_X=163^{\circ}\pm9^{\circ}$, that is aligned with the radio jet direction on the sky. Optical polarization observations show fast variability about the jet axis as well. Our results strongly favor models in which X-rays are produced by Compton scattering from relativistic electrons -- specifically Synchrotron Self-Compton -- that takes places downstream, away from the supermassive black hole.

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arXiv2025

High optical to X-ray polarization ratio reveals Compton scattering in BL Lacertae's jet

Blazars, supermassive black hole systems (SMBHs) with highly relativistic jets aligned with the line of sight, are the most powerful long-lived emitters of electromagnetic emission in the Universe. We report here on a radio to gamma-ray multiwavelength campaign on the blazar BL Lacertae with unprecedented polarimetric coverage from radio to X-ray wavelengths. The observations caught an extraordinary event on 2023 November 10-18, when the degree of linear polarization of optical synchrotron radiation reached a record value of 47.5%. In stark contrast, the Imaging X-ray Polarimetry Explorer (IXPE) found that the X-ray (Compton scattering or hadron-induced) emission was polarized at less than 7.4% (3sigma confidence level). We argue here that this observational result rules out a hadronic origin of the high energy emission, and strongly favors a leptonic (Compton scattering) origin, thereby breaking the degeneracy between hadronic and leptonic emission models for BL Lacertae and demonstrating the power of multiwavelength polarimetry to address this question. Furthermore, the multiwavelength flux and polarization variability, featuring an extremely prominent rise and decay of the optical polarization degree, is interpreted for the first time by the relaxation of a magnetic "spring" embedded in the newly injected plasma. This suggests that the plasma jet can maintain a predominant toroidal magnetic field component parsecs away from the central engine.

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arXiv2025

Optimizing the hunt for extraterrestrial high-energy neutrino counterparts

It has been a decade since the IceCube collaboration began detecting high-energy (HE) neutrinos originating from cosmic sources. Despite a few well-known individual associations and numerous phenomenological, observational, and statistical multiwavelength studies, the origin of astrophysical HE neutrinos largely remains a mystery. To date, the most convincing associations link HE neutrinos with active galactic nuclei (AGN). Consequently, many studies have attempted population-based correlation tests between HE neutrinos and specific AGN subpopulations (such as blazars). While some of associations are suggestive, no definitive population-based correlation has been established. This could result from either a lack of a population-based correlation or insufficient detection power, given the substantial atmospheric neutrino background. By leveraging blazar variability, we performed spatio-temporal blazar-neutrino correlation tests aimed at enhancing detection power by reliably incorporating temporal information into the statistical analysis. We used simulations to evaluate the detection power of our method under various test strategies. We find that: (1) with sufficiently large source samples, if 20% of astrophysical HE neutrinos originate from blazars, we should robustly observe $\sim$4$σ$ associations; (2) a counting-based test-statistic combined with a top-hat weighting scheme (rather than a Gaussian one) provides the greatest detection power; (3) applying neutrino sample cuts reduces detection power when a weighting scheme is used; (4) in top-hat-like weighting schemes, small p-values do not occur arbitrarily with an increase in HE neutrino error region size (any such occurrence is indicative of an underlying blazar-neutrino correlation).

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arXiv2024

Association of the IceCube neutrinos with blazars in the CGRaBS sample

The origin of high-energy (HE) astrophysical neutrinos has remained an elusive hot topic in the field of HE astrophysics for the past decade. Apart from a handful of individual associations, the vast majority of HE neutrinos arise from unknown sources. While there are theoretically-motivated candidate populations, such as blazars -- a subclass of AGN with jets pointed towards our line-of-sight -- they have not yet been convincingly linked to HE neutrino production. Here, we perform a spatio-temporal association analysis between a sample of blazars (from CGRaBS catalog) in the radio and optical bands and the most up-to-date IceCube HE neutrino catalog. We find that if the IceCube error regions are enlarged by 1$^\circ$ in quadrature, to account for unknown systematic errors at maximal level, a spatio-temporal correlation between the multiwavelength light curves of the CGRaBS blazars and the IceCube HE neutrinos is hinted at least at a 2.17$σ$ significance level. On the other hand, when the IceCube error regions are taken as their published values, we do not find any significant correlations. A discrepancy in the blazar-neutrino correlation strengths, when using such minimal and enlarged error region scenarios, was also obtained in a recent study by the IceCube collaboration. In our study, this difference arises because several flaring blazars -- coinciding with a neutrino arrival time -- happen to narrowly miss the published 90\%-likelihood error region of the nearest neutrino event. For all of the associations driving our most significant correlations, the flaring blazar is much less than 1$^\circ$ away from the published error regions. Therefore, our results indicate that the question of the blazar-neutrino connection is highly sensitive to the reconstruction of the neutrino error regions, whose reliability is expected to improve with the next generation of neutrino observatories.

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arXiv2022

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

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

astro-ph.HE↗

arXiv2022

Incremental Fermi Large Area Telescope Fourth Source Catalog

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

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arXiv2022

Multiwavelength variability power spectrum analysis of the blazars 3C 279 and PKS 1510-089 on multiple timescales

We present the results of variability power spectral density (PSD) analysis using multiwavelength radio to GeV\,$γ$-ray light curves covering decades/years to days/minutes timescales for the blazars 3C 279 and PKS 1510-089. The PSDs are modeled as single power-laws, and the best-fit spectral shape is derived using the `power spectral response' method. With more than ten years of data obtained with weekly/daily sampling intervals, most of the PSDs cover ~2-4 decades in temporal frequency; moreover, in the optical band, the PSDs cover ~6 decades for 3C 279 due to the availability of intranight light curves. Our main results are the following: (1) on timescales ranging from decades to days, the synchrotron and the inverse Compton spectral components, in general, exhibit red-noise (slope ~2) and flicker-noise (slope ~1) type variability, respectively; (2) the slopes of $γ$-ray variability PSDs obtained using a 3-hr integration bin and a 3-weeks total duration exhibit a range between ~1.4 and ~2.0 (mean slope = 1.60$\pm$0.70), consistent within errors with the slope on longer timescales; (3) comparisons of fractional variability indicate more power on timescales $\leq$100\, days at $γ$-ray frequencies as compared to longer wavelengths, in general (except between $γ$-ray and optical frequencies for PKS 1510$-$089); (4) the normalization of intranight optical PSDs for 3C\,279 appears to be a simple extrapolation from longer timescales, indicating a continuous (single) process driving the variability at optical wavelengths; (5) the emission at optical/infrared wavelengths may involve a combination of disk and jet processes for PKS\,1510$-$089.

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arXiv2019

MAGIC and MWL monitoring of the blazar TXS 0506+056 in the 2018/2019 season

The gamma-ray blazar TXS 0506+056, was discovered in VHE gamma-rays by the MAGIC telescopes in 2017 in a follow-up campaign of a high energy neutrino event IceCube-170922A (IC+Fermi+MAGIC++, Science 361, eaat1378 (2018)). Subsequent multivawelenght (MWL) observations and theoretical modeling in a frame of hadro-leptonic emission confirmed that this source could be a potential cosmic ray and neutrino emitter (MAGIC Collaboration, Ansoldi et al., (2018)). This is, by far, the most significant association between a high-energy neutrino and an astrophysical source emitting gamma rays and X-rays. TXS 0506+056 is a key object to help the astrophysics community to establish connections between high-energy neutrinos and astrophysical sources. Accurate and contemporaneous MWL spectral measurements are essential ingredients to achieve this goal. In the conference, we present the measurements from the MAGIC and MWL monitoring of this source, spanning the time period from November 2017 till February 2019. These include the lowest VHE gamma-ray emission state measured from this source so far as well as a flaring episode in December 2018.

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arXiv2019

MAGIC eyes to the extreme: testing the blazar emission models on EHBLs

Extreme high-energy peaked BL Lac objects (EHBLs) are blazars whose synchrotron emission peaks at exceptionally high energies, above few keV, in the hard X-ray regime. So far, only a handful of those objects has been detected at very high energy (VHE, E > 100 GeV) gamma rays by Imaging Atmospheric Cherenkov Telescopes. Very remarkably, VHE observations of some of these blazars (like 1ES 0229+200) have provided evidence of a VHE gamma-ray emission extending to several TeV, which is difficult to explain with standard, one-zone synchrotron self-Compton models usually applied to BL Lac objects. The MAGIC collaboration coordinated a multi-year, multi-wavelength observational campaign on ten targets. The MAGIC telescopes detected VHE gamma rays from four EHBLs. In this paper we focus on the source 1ES 1426+426 and its X-ray and VHE gamma-ray properties. The results of different models (synchrotron self-Compton, spine-layer, hadronic) reproducing the broadband spectral energy distribution are also presented.

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arXiv2017

Multiwavelength observations of the blazar BL Lacertae in June 2015

BL Lacertae is a blazar at the redshift of z = 0.069, eponym of the BL Lac blazar type. It is also a prototype of the low-frequency-peaked BL Lac (LBL) subclass. It was first detected in sub-TeV gamma-ray range by MAGIC in 2005. In 2015, MAGIC observations of BL Lacertae were triggered by the Fermi-LAT analysis report in the MAGIC group, and were performed during 10 individual nights between 15th and 28th June for a total of 8.6 h. The measured gamma-ray flux varied from 40\% to 10\% of the Crab nebula flux above 200 GeV in the nights from 15th to 17th June. In particular, a fast variability was found during the nights of 15th and 17th June. % with a halving time scale of (33+/-9) minutes, statistical errors only. We also performed multi-wavelength (MWL) observations in the radio, optical, UV, X-ray and gamma-ray bands, and %the MWL light curves indicate that the very high energy gamma-ray flare was a so-called orphan flare, %i.e. no simultaneous activity in other wavebands was observed. the MWL light curves indicate that no apparent simultaneous activity in other wavebands accompanying the very high energy gamma-ray flare in June 2015 like an another occurrence of an orphan very high energy flare. In this proceedings we will present the results of the campaign and discuss their implications on our understanding of the object.

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arXiv2017

Measurement of the EBL through a combined likelihood analysis of gamma-ray observations of blazars with the MAGIC telescopes

The extragalactic background light (EBL) is the radiation accumulated through the history of the Universe in the wavelength range from the ultraviolet to the far infrared. Local foregrounds make the direct measurement of the diffuse EBL notoriously difficult, while robust lower limits have been obtained by adding up the contributions of all the discrete sources resolved in deep infrared and optical galaxy observations. Gamma-ray astronomy has emerged in the past few years as a powerful tool for the study of the EBL: very-high-energy (VHE) photons traversing cosmological distances can interact with EBL photons to produce e$^+$e$^-$ pairs, resulting in an energy-dependent depletion of the gamma-ray flux of distant sources that can be used to set constraints on the EBL density. The study of the EBL is one of the key scientific programs currently carried out by the MAGIC collaboration. We present here the results of the analysis of 32 VHE spectra of 12 blazars in the redshift range 0.03 - 0.94, obtained with over 300 hours of observations with the MAGIC telescopes between 2010 and 2016. A combined likelihood maximization approach is used to evaluate the density and spectrum of the EBL most consistent with the MAGIC observations. The results are compatible with state-of-the-art EBL models, and constrain the EBL density to be roughly within $\simeq 20\%$ of the nominal value in such models. The study reveals no anomalies in gamma-ray propagation in the large optical depth regime - contrary to some claims based on meta-analyses of published VHE spectra.

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arXiv2017

Multiwavelength picture of the blazar S5 0716+714 during its brightest outburst

S5 0716+714 is a well known BL Lac object, one of the brightest and most active blazars. The discovery in the Very High Energy band (VHE, E > 100 GeV) by MAGIC happened in 2008. In January 2015 the source went through the brightest optical state ever observed, triggering MAGIC follow-up and a VHE detection with 13σ significance (ATel 6999). Rich multiwavelength coverage of the flare allowed us to construct the broad-band spectral energy distribution of S5 0716+714 during its brightest outburst. In this work we will present the preliminary analysis of MAGIC and Fermi-LAT data of the flaring activity in January and February 2015 for the HE (0.1 < HE < 300 GeV) and VHE band, together with radio (Metsähovi, OVRO, VLBA, Effelsberg), sub-millimeter (SMA), optical (Tuorla, Perkins, Steward, AZT-8+ST7, LX-200, Kanata), X-ray and UV (Swift-XRT and UVOT), in the same time-window and discuss the time variability of the multiwavelength light curves during this impressive outburst.

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