The advent of Fermi is changing our understanding on the radio and gamma-ray emission in Active Galactic Nuclei. In fact, contrary to previous campaigns, Fermi mission reveals that BL Lac objects are the most abundant emitters in gamma-ray band. However, since they are relatively weak sources, most of their parsec scale structure as their multifrequency properties are poorly understood and/or not systematically investigated. Our main goal is to analyse, using a multiwavelength approach, the nuclear properties of an homogeneous sample of 42 faint BL Lacs, selected, for the first time in literature, with no constraint on their radio and gamma-ray flux densities/emission. We began asking and obtaining new VLBA observations at 8 and 15 GHz for the whole sample. We derived fundamental parameters as radio flux densities, spectral index information, and parsec scale structure. Moreover, we investigated their gamma-ray emission properties using the 2LAT Fermi results. Here, we report our preliminary results on the radio and gamma-ray properties of this sample of faint BL Lacs. In the next future, we will complete the multiwavelength analysis.
The spectral energy distribution (SED) of blazars is generally understood through the standard one-zone synchrotron self-Compton (SSC) model, where a strong correlation between X-ray and TeV gamma-ray fluxes is expected. Recently, for Mrk 421 the correlation was confirmed and interpreted within the SSC model arguing that a description of the correlation up to large flux values considerably restricts the value of the magnetic field under the assumption of an electron spectral index independent of the flux state. In this work, we extend the analysis done on Mrk 421 for other two high-energy peak…
Using a significantly enlarged \textit{Fermi}-LAT BL Lac objects (BL Lacs) sample, we construct the gamma-ray luminosity function (GLF) of BL Lacs, by the joint use of the space density distribution and source counts distribution. We use three well-studied forms of the GLF, i.e., the forms of pure density evolution (PDE), pure luminosity evolution (PLE) and luminosity-dependent density evolution (LDDE). The Markov Chain Monte Carlo (MCMC) technique is used to constrain model parameters. Our results suggest that LDDE model can give the best description for the BL Lac GLF. And the model shows th…
We have recently proposed a new simplified scenario where blazars are classified as flat-spectrum radio quasars (FSRQs) or BL Lacs according to the prescriptions of unified schemes, and to a varying combination of Doppler boosted radiation from the jet, emission from the accretion disk, the broad line region, and light from the host galaxy. Here we extend our approach, previously applied to radio and X-ray surveys, to the gamma-ray band and, through detailed Monte Carlo simulations, compare our predictions to Fermi-LAT survey data. Our simulations are in remarkable agreement with the overall o…
We study the origin of the extragalactic diffuse gamma-ray background using the data from the Fermi telescope. To estimate the background level, we count photons at high Galactic latitudes |b|>60 degrees. Subtracting photons associated to known sources and the residual cosmic ray and Galactic diffuse backgrounds, we estimate the Extragalactic Gamma-ray Background (EGB) flux. We find that the spectrum of EGB in the very-high-energy (VHE) band above 30 GeV follows the stacked spectrum of BL Lacs. LAT data reveal the positive (1+z)^k, 1<k<4 cosmological evolution of the BL Lac source population c…
The HAWC gamma ray observatory is located at the Sierra Negra Volcano in Puebla, Mexico, at an altitude of 4,100 meters. HAWC is a wide field of view array of 300 water Cherenkov detectors that are continuously surveying ~ 2sr of the sky, operating since March 2015. The large collected data sample allows HAWC to perform an unbiased monitoring of the BL Lac Mrk 421. This is the closest and brightest known extragalactic high-synchrotron-peaked BL Lac in the gamma-ray/X- ray bands and is extensively monitored by the Large Area Telescope (LAT) on-board the Fermi satellite, and the BAT and XRT inst…
Among the ~2157 unassociated sources in the third data release (DR3) of the fourth Fermi catalog, ~1200 were observed with the Neil Gehrels Swift Observatory pointed instruments. These observations yielded 238 high S/N X-ray sources within the 95% Fermi uncertainty regions. Recently, Kerby et al. employed neural networks to find blazar candidates among these 238 X-ray counterparts to the 4FGL unassociated sources and found 112 likely blazar counterpart sources. A complete sample of blazars, along with their sub-classification, is a necessary step to help understand the puzzle of the blazar seq…
High-frequency-peaked BL Lacs (HBLs) dominate the extragalactic TeV sky, with more than 50 objects detected by the current generation of TeV observatories. Still, the properties of TeV-emitting HBLs as a population are poorly understood due to biases introduced by the observing strategies of Cherenkov Telescopes, limiting our ability to estimate the potential contribution of TeV blazars to the diffuse neutrino, gamma-ray, and cosmic-ray background as well as their role in the late-stage evolution of active galactic nuclei. The VERITAS telescope array has designed a program to quantify and mini…
We report on spectroscopic observations covering most of the 475 BL Lacs in the 2nd Fermi LAT catalog of AGN. Including archival measurements (correcting several erroneous literature values) we now have spectroscopic redshifts for 44% of the BL Lacs. We establish firm lower redshift limits via intervening absorption systems and statistical lower limits via searches for host galaxies for an additional 51% of the sample leaving only 5% of the BL Lacs unconstrained. The new redshifts raise the median spectroscopic z from 0.23 to 0.33 and include redshifts as large as z=2.471. Spectroscopic redshi…
Is it significant that the intrinsic outputs of several BL Lacs are observed to level off at values of about 10^46 erg s^-1? In searching for an answer, we compare gamma-ray observations by the AGILE satellite of the BL Lac S5 0716+714 with those of Mrk 421 and Mrk 501; the former are particularly marked by intense flares up to fluxes of 2 x 10^-6 photons cm^-2 s^-1 in the 0.1-10 GeV energy range. These "dry" BL Lacs show evidence of neither thermal disk emissions nor emission lines signaling any accreting or surrounding gas; the spectral distributions of their pure non-thermal radiations are …
The emission of the so-called extreme blazars challenges the particle acceleration models. The hardness of its spectrum, $<2$, demands extreme parameters using the standard one-zone SSC model in the high energy band. Some authors use both two-zone or hadronic/leptohadronic models to relax these extreme values. In this work, we present a leptohadronic two-zone model with external radiation fields to explain the broadband emission, where the contribution of two components forms the hard-spectrum in the $γ$-rays band. The first is produced by the photopion process, where accelerated protons in an…
We present a morphological and spectral study of a sample of 99 BL Lacs using the LOFAR Two-Metre Sky Survey Second Data Release (LDR2). Extended emission has been identified at gigahertz frequencies around BL Lacs, but with LDR2 it is now possible to systematically study their morphologies at 144 MHz, where more diffuse emission is expected. LDR2 reveals the presence of extended radio structures around 66/99 of the BL Lac nuclei, with angular extents ranging up to 115 arcseconds, corresponding to spatial extents of 410 kpc. The extended emission is likely to be both unbeamed diffuse emission …
We construct a large sample of gamma-ray blazars with low-frequency radio data using the recent released TGSS AD1 catalog at 150 MHz. The radio/gamma connections of blazars are compared from 143 GHz to 150 MHz. The radio flux density at all radio frequencies shows strong correlation with gamma-ray flux for blazars, as well as for the two subclasses, FSRQs and BL Lacs. But the correlations get worse from high to low radio frequencies, which indicates that the low-frequency radio emission is the mixture of extended and core components for blazars. In addition, we find that the correlation betwee…
Aim: We have collected the most complete multi-wavelength ($6.0 - 6.0 \times 10^{-18} cm$) dataset of very high energy (VHE) $γ$-ray emitting (TeV) BL Lacs, which are the most numerous extragalactic VHE sources. Using significant correlations between different bands, we aim to identify the best TeV BL Lac candidates that can be discovered by the current and next generation of imaging air Cherenkov telescopes. Methods: We formed five datasets from lower energy data, i.e. radio, mid-infrared, optical, X-rays, and GeV $γ$-ray, and five VHE $γ$-ray datasets to perform a correlation study between d…
The synchrotron-self Compton (SSC) radiation process is widely held to provide a close representation of the double peaked spectral energy distributions from BL Lac Objects (BL Lacs), which are marked by non-thermal beamed radiations, highly variable on timescales of days or less. Their outbursts in the gamma ray relative to the optical/X rays might be surmised to be enhanced in BL Lacs as these photons are upscattered via the inverse Compton (IC) process. From the observed correlations among the spectral parameters during optical/X-ray variations we aim at predicting corresponding correlation…
We use optical data from the Palomar Transient Factory (PTF) and the Catalina Real-Time Transient Survey (CRTS) to study the variability of gamma-ray detected and non-detected objects in a large population of active galactic nuclei (AGN) selected from the Candidate Gamma-Ray Blazar Survey and Fermi Gamma-Ray Space Telescope catalogs. Our samples include 714 sources with PTF data and 1244 sources with CRTS data. We calculate the intrinsic modulation index to quantify the optical variability amplitude in these samples. We find the gamma-ray detected objects to be more variable than the non-detec…
As very high energy (VHE) photons propagate through the extragalactic background light (EBL), they interact with the soft photons and initiate electromagnetic cascades of lower energy photons and electrons. The collective intensity of a cosmological population emitting at VHEs will be attenuated at the highest energies through interactions with the EBL and enhanced at lower energies by the resulting cascade. We calculate the cascade radiation created by VHE photons produced by blazars and investigate the effects of cascades on the collective intensity of blazars and the resulting effects on th…
As very-high--energy photons propagate through the extragalactic background light (EBL), they interact with the soft photons and initiate electromagnetic cascades of lower energy photons and electrons. The collective intensity of a cosmological population emitting at very-high energies (VHE) will be attenuated at the highest energies through interactions with the EBL and enhanced at lower energies by the resulting cascade. We calculate the cascade radiation created by VHE photons produced by blazars and investigate the effects of cascades on the collective intensity of blazars and the resultin…
We present $γ$-ray, X-ray, ultraviolet, optical, and near-infrared light curves of 33 $γ$-ray bright blazars over four years that we have been monitoring since 2008 August with multiple optical, ground-based telescopes and the Swift satellite, and augmented by data from the Fermi Gamma-ray Space Telescope and other publicly available data from Swift. The sample consists of 21 flat-spectrum radio quasars (FSRQs) and 12 BL Lac objects (BL Lacs). We identify quiescent and active states of the sources based on their $γ$-ray behavior. We derive $γ$-ray, X-ray, and optical spectral indices, $α_γ$, $…
In this paper we investigate the properties of a sample of six BL Lacs by fitting their spectra using our inhomogeneous jet model with an accelerating, magnetically dominated, parabolic base, which transitions to a slowly decelerating conical jet with a geometry based on observations of M87. Our model is able to fit very well to the simultaneous multiwavelength spectra of all the BL Lacs including radio observations. We find that the BL Lacs have lower jet powers and bulk Lorentz factors than the sample of Compton-dominant blazars investigated in Paper III, consistent with the blazar sequence.…
Gamma-ray bursts (GRBs) and GeV-TeV selected radio loud Active Galactic Nuclei (AGNs) are compared based on our systematic modeling of the observed spectral energy distributions of a sample of AGNs with a single-zone leptonic model. We show that the correlation between the jet power (P_{jet}) and the prompt gamma-ray luminosity (L_{jet}) of GRBs is consistent, within the uncertainties, with the correlation between jet power and the synchrotron peak luminosity (L_{s, jet}) of flat spectrum radio quasars (FSRQs). Their radiation efficiencies (\varepsilon) are also comparable (>10% for most sourc…
BL Lacertae (BL Lac) objects are a subclass of blazar, distinguished by their featureless optical spectrum. The featureless spectrum presents a challenge in measuring the redshift of the BL Lacs. In this paper, we measure the redshift of BL Lacs using the photometric dropout technique. The space-based telescope \emph{Swift} and the ground-based SARA telescopes are employed to provide magnitudes in the $uvw2,\ uvm2,\ uvw1,\ u,\ b,\ v,\ g',\ r',\ i',\ z'$ filters. We observe 60 BL Lacs and report reliable redshift upper limits for 41 of them. We discover three new high-$z$ BL Lacs ($z>1.3$) at $…
We revisit the blazar sequence exploiting the complete, flux limited sample of blazars with known redshift detected by the Fermi satellite after 4 years of operations (the 3LAC sample). We divide the sources into gamma-ray luminosity bins, collect all the archival data for all blazars, and construct their spectral energy distribution (SED). We describe the average SED of blazars in the same luminosity bin through a very simple, completely phenomenological function consisting of two broken power laws connecting with a power law of fixed slope describing the radio emission. We do that separately…
We reconsider the emission properties of the BL Lac objects emitting in the high-energy gamma-ray band exploiting the new information in the MeV-GeV band obtained by the Large Area Telescope (LAT) onboard the Fermi Gamma-Ray Space Telescope in its first three months of operation. To this aim we construct the spectral energy distribution of all the BL Lacs revealed by LAT and of the known TeV BL Lacs not detected by LAT, also including data from the Swift satellite, and model them with a simple one-zone leptonic model. The analysis shows that the BL Lacs detected by LAT (being or not already de…
The majority of the extragalactic sources yet detected at TeV photon energies belong to the class of "high frequency peaked BL Lacs" (HBLs) that exhibit a spectral energy distribution with a lower peak in the X-ray band. Such spectra are well described in terms of a log-parabolic shape with a considerable curvature, and widely interpreted as synchrotron emission from ultrarelativistic electrons outflowing in a relativistic jet; these are expected to radiate also in gamma-rays by the inverse Compton process. Recently we have compared the X-ray spectral parameter distributions of TeV detected HB…
We utilize machine learning methods to distinguish BL Lacertae objects (BL Lac) from Flat Spectrum Radio Quasars (FSRQ) within a sample of likely X-ray blazar counterparts to Fermi 3FGL unassociated gamma-ray sources. From our previous work, we have extracted 84 sources that were classified as $\geq$ 99% likley to be blazars. We then utilize Swift$-$XRT, Fermi, and WISE (The Wide-field Infrared Survey Explorer) data together to distinguish the specific type of blazar, FSRQs or BL Lacs. Various X-ray and Gamma-ray parameters can be used to differentiate between these subclasses. These are also …
By cross-correlating an archival sample of 170 BL Lacs with 2 year \fermilat AGN sample, we have compiled a sample of 100 BL Lacs with \fermi detection (FBLs), and a sample of 70 non-\fermi BL Lacs (NFBLs). We compared various parameters of FBLs with those of NFBLs, including the redshift, the low frequency radio luminosity at 408 MHz ($L_{\rm 408MHz}$), the absolute magnitude of host galaxies ($M_{\rm host}$), the polarization fraction from NVSS survey ($P_{\rm NVSS}$), the observed arcsecond scale radio core flux at 5 GHz ($F_{\rm core}$) and jet Doppler factor; all the parameters are direct…
We use the third catalog of blazars detected by Fermi/LAT (3LAC) and gamma-ray Narrow-line Seyfert 1 Galaxies (gamma-NLSy1s) to study the blazar sequence and relationship between them. Our results are as follows: (i) There is a weak anti-correlation between synchrotron peak frequency and peak luminosity for both Fermi blazars and gamma-NLSy1s, which supports the blazar sequence. However, after Doppler correction, the inverse correlation disappeared, which suggests that anti-correlation between synchrotron peak frequency and peak luminosity is affected by the beaming effect. (ii) There is a sig…
We present statistical analyses of a large homogenous data sample of Fermi-detected blazars thoroughly studied in order to reassess the relationship between flat-spectrum radio quasars (FSRQs) and subclasses of BL Lacertae objects (BL Lacs) blazar populations. We discovered from the average values of gamma-ray and X-ray spectral indices that the sequence of distribution is indicative of the blazar orientation scheme. Analyses of FSRQs and BL Lacs data show difference in the shape of gamma-ray and X-ray indices: significant anti-correlation (r~ greater than - 0.79) exists between gamma-ray and …
BL Lac objects can be extreme in two ways: with their synchrotron emission, peaking beyond 1 keV in their spectral energy distribution, or with their gamma-ray emission, peaking at multi-TeV energies up to and beyond 10-20 TeV, like 1ES 0229+200. This second type of extreme BL Lacs --which we can name "TeV-peaked BL Lacs" is not well explained by the usual synchrotron self-Compton scenarios for BL Lacs. These sources are also important as probes for the intergalactic diffuse infrared background and cosmic magnetic fields, as well as possible sites of production of ultra-high-energy cosmic rays…
Determining redshifts for BL Lacertae (BL Lac) objects using the traditional spectroscopic method is challenging due to the absence of strong emission lines in their optical spectra. We employ the photometric dropout technique to determine redshifts for this class of blazars using the combined 13 broad-band filters from Swift-UVOT and the multi-channel imager GROND at the MPG 2.2 m telescope at ESO's La Silla Observatory. The wavelength range covered by these 13 filters extends from far ultraviolet to the near-Infrared. We report results on 40 new Fermi detected BL Lacs with the photometric re…
Fermi has provided the largest sample of gamma-ray selected blazars to date. In this work we use a uniformly selected set of 211 BL Lacertae (BL Lac) objects detected by it Fermi during its first year of operation. We have obtained redshift constraints for 206 out of the 211 BL Lacs in our sample making it the largest and most complete sample of BL Lacs available in the literature. We use this sample to determine the luminosity function of BL Lacs and its evolution with cosmic time. We find that for most BL Lac classes, the evolution is positive with a space density peaking at modest redshift …
The production site of gamma rays in blazars is closely related to their interaction with the photon fields surrounding the active galactic nucleus. In this work we discuss an indirect method that may help to unveil the presence of ambient structures in BL Lacs through the analysis of their gamma-ray spectrum. Passing through structures at different distances from the black hole, gamma rays interact with the corresponding photon fields via gamma-gamma pair production, producing absorption features in their spectral energy distribution. An interaction of the gamma-ray photons with a putative br…
We present two catalogs of radio-loud candidate blazars whose WISE mid-infrared colors are selected to be consistent with the colors of confirmed gamma-ray emitting blazars. The first catalog is the improved and expanded release of the WIBRaLS catalog presented by D'Abrusco et al. (2014): it includes sources detected in all four WISE filters, spatially cross-matched with radio source in one of three radio surveys and radio-loud based on their q22 spectral parameter. WIBRaLS2 includes 9541 sources classified as BL Lacs, FSRQs or mixed candidates based on their WISE colors. The second catalog, c…
Lately a specific kind of blazars drew the attention of the gamma-ray astronomy community: the extreme TeV BL Lacs, blazars that present an extremely energetic and hard emission at very high-energy. Explaining their features is still an open challenge, in fact the most used phenomenological models have difficulties to satisfactorily reproduce their SED. Based on a scenario we have recently proposed, we suppose that the non-thermal particles are firstly accelerated by a jet recollimation shock, which induces turbulence in the rest of the jet. Non-thermal particles are further accelerated by the…
To fully exploit the scientific potential of the Fermi mission, we initiated the F-GAMMA program. Between 2007 and 2015 it was the prime provider of complementary multi-frequency monitoring in the radio regime. We quantify the radio variability of gamma-ray blazars. We investigate its dependence on source class and examine whether the radio variability is related to the gamma-ray loudness. Finally, we assess the validity of a putative correlation between the two bands. The F-GAMMA monitored monthly a sample of about 60 sources at up to twelve radio frequencies between 2.64 and 228.39 GHz. We p…
We present an XMM-Newton catalogue of BL Lac X-ray, optical, and UV properties based on cross-correlation with the 1151 BL Lacs listed in the fifth edition of the Roma-BZCAT. We searched for the X-ray counterparts to these objects in the field of view of all pointed observations in the XMM-Newton archive over nearly 20 years of mission. The cross-correlation yields a total of 310 XMM-Newton fields which correspond to 103 different BL Lacs. We homogeneously analysed data from the three EPIC cameras (X-ray) and OM(optical/UV) using the XMM-Newton SAS software, and produced images, light curves, …
Blazars optical emission is generally dominated by relativistic jets, although the host galaxy, accretion disk and broad line region (BLR) may also contribute significantly. Disentangling their contributions has been challenging for years due to the dominance of the jet. To quantify the contributions to the spectral variability, we use the statistical technique for dimensionality reduction Non-Negative Matrix Factorization on a spectroscopic data set of 26 $γ$-ray blazars. This technique allows to model large numbers of spectra in terms of a reduced number of components.We use a priori knowled…
The two-component broad-band spectral energy distributions of blazars were suggested to form a sequence in which (1) the peak frequency of the low-energy (synchrotron) component nu_syn is anticorrelated with the synchrotron luminosity L_syn, and (2) the luminosity ratio of the high-energy (inverse Compton) to synchrotron components q = L_IC / L_syn (Compton dominance) increases with L_syn from the BL Lac objects (BL Lacs) to the flat spectrum radio quasars (FSRQs). The Compton dominance parameter is an important probe of plasma magnetisation in the blazar zones within relativistic jets. We inv…
We explain the observed multiwavelength photon spectrum of a number of BL Lac objects detected at very high energy (VHE, $E \gtrsim 30$ GeV), using a lepto-hadronic emission model. The one-zone leptonic emission is employed to fit the synchrotron peak. Subsequently, the SSC spectrum is calculated, such that it extends up to the highest energy possible for the jet parameters considered. The data points beyond this energy, and also in the entire VHE range are well explained using a hadronic emission model. The ultrahigh-energy cosmic rays (UHECRs, $E\gtrsim 0.1$ EeV) escaping from the source int…
Context. The advent of Fermi is changing our understanding on the radio and gamma-ray emission in Active Galactic Nuclei. Contrary to pre-Fermi ideas, BL Lac objects are found to be the most abundant emitters in the gamma-ray band. However, since they are relatively weak radio sources, most of their parsec-scale structure and their multi-frequency properties are poorly understood and/or have not been investigated in a systematically fashion. Aims. Our main goal is to analyze the radio and gamma-ray emission properties of a sample of 42 BL Lacs selected, for the first time in the literature, wi…
AIMS. We are studying an unbiased sample of 42 nearby (z < 0.2) BL Lacertae objects with a multi-wavelength approach. The results of VLBI observations were presented in the first paper of this series. In this paper, we study the $γ$-ray properties of the sample. METHODS. We analyse data collected by the Fermi LAT during its first 8.5 years of operation in the energy range 0.1-300 GeV. RESULTS. We reveal 23 sources with a test statistic greater than 25 (corresponding to $\sim$ 4.6-$σ$) out of 42, with 3 sources not detected in the 3LAC catalogue, and fluxes between $3.5\times 10^{-10}$ and $7.4…
We investigate the Fermi LAT gamma-ray and 15 GHz VLBA radio properties of a joint gamma-ray- and radio-selected sample of AGNs obtained during the first 11 months of the Fermi mission (2008 Aug 4 - 2009 Jul 5). Our sample contains the brightest 173 AGNs in these bands above declination -30 deg. during this period, and thus probes the full range of gamma-ray loudness (gamma-ray to radio band luminosity ratio) in the bright blazar population. The latter quantity spans at least four orders of magnitude, reflecting a wide range of spectral energy distribution (SED) parameters in the bright blazar…
Hard-TeV BL Lacs are a new type of blazars characterized by a hard intrinsic TeV spectrum, locating the peak of their gamma-ray emission in the spectral energy distribution (SED) above 2-10 TeV. Such high energies are problematic for the Compton emission, using a standard one-zone leptonic model. We study six examples of this new type of BL Lacs in the hard X-ray band with the NuSTAR satellite. Together with simultaneous observations with the SWIFT satellite, we fully constrain the peak of the synchrotron emission in their SED, and test the leptonic synchrotron self-Compton (SSC) model. We con…
We report on multiwavelength observations of the blazar PKS 0537-441 (z = 0.896) obtained from microwaves through gamma rays by SMA, REM, ATOM, Swift and Fermi during 2008 August-2010 April. Strong variability has been observed in gamma rays, with two major flaring episodes (2009 July and 2010 March) and a harder-when-brighter behaviour, quite common for FSRQs and low-synchrotron-peaked BL Lacs, in 2010 March. In the same way the SED of the source cannot be modelled by a simple synchrotron self-Compton model, as opposed to many BL Lacs, but the addition of an external Compton component of seed…
Linking {\it Fermi} blazars and radio galaxies through accretion and jet radiation mechanisms
Università di Padova（帕多瓦大学）；Istituto Nazionale di Fisica Nucleare, Sezione di Padova（意大利国家核物理研究所帕多瓦分部）；Guangzhou University（广州大学）；INAF - Istituto di Radioastronomia（意大利国家天体物理研究所射电天文台）；Astronomy Science and Technology Research Laboratory of Department of Education of Guangdong Province（广东省教育厅天文学科学与技术研究实验室）；Wuhu Vocational Technical University（芜湖职业技术学院）；Institute of Astronomy and National Astronomical Observatory（天文学研究所与国家天文观测站）
We have used the Green Bank Telescope at 350MHz to search 50 faint, unidentified Fermi Gamma-ray sources for radio pulsations. So far, these searches have resulted in the discovery of 10 millisecond pulsars, which are plausible counterparts to these unidentified Fermi sources. Here we briefly describe this survey and the characteristics of the newly discovered MSPs.
The fourth Fermi Large Area Telescope Source Catalog (4FGL) lists over 5000 gamma-ray sources with statistical significance above 4$σ$. About 23% of the sources listed in this catalog are unidentified/unassociated gamma-ray sources while ~26% of the sources are classified as blazar candidates of uncertain type (BCUs), lacking optical spectroscopic information. To probe the blazar nature of candidate counterparts of UGSs and BCUs, we started our optical spectroscopic follow up campaign in 2012, which up to date account for more than 350 observed sources. In this paper, the tenth of our campaign…
We have recently proposed a simplified scenario for blazars in which these sources are classified as flat-spectrum radio quasars or BL Lacs according to the prescriptions of unified schemes, and to a varying combination of Doppler boosted radiation from the jet, emission from the accretion disk, the broad line region, and light from the host galaxy. This scenario has been thoroughly tested through detailed Monte Carlo simulations and reproduces all the main features of existing radio, X-ray, and gamma-ray surveys. In this paper we consider the case of very high energy emission (E > 100 GeV) ex…
We study the correlation between gamma-ray and radio band radiation for 123 blazars, using the Fermi-LAT first source catalog (1FGL) and the RATAN-600 data obtained at the same period of time (within a few months). We found an apparent positive correlation for BL Lac and flat-spectrum radio quasar (FSRQ) sources from our sample through testing the value of the Pearson product-moment correlation coefficient. The BL Lac objects show higher values of the correlation coefficient than FSRQs at all frequencies, except 21.7 GHz, and at all bands, except $10-100$ GeV, typically at high confidence leve…
We derive a relation for the steepening of blazar gamma-ray spectra between the multi-GeV Fermi energy range and the TeV energy range observed by atmospheric Cerenkov telescopes. The change in spectral index is produced by two effects: (1) an intrinsic steepening, independent of redshift, owing to the properties of emission and absorption in the source, and (2) a redshift-dependent steepening produced by intergalactic pair production interactions of blazar gamma-rays with low energy photons of the intergalactic background light (IBL). Given this relation, with good enough data on the mean gamm…
The emission of very-high-energy photons (VHE, E$>$100 GeV) in active galactic nuclei (AGN) is closely connected with the production of ultra-relativistic particles. Among AGN, the subclass of extreme BL Lacertae are of particular interest because they challenge state-of-art models on how these cosmic particle accelerators operate. By cross-matching two gamma-ray catalogs (this is, 4FGL-DR2 and 2BIGB), we identified 23 high-synchrotron-peaked (HSP) blazar candidates with photometric or spectroscopic redshifts, good multi-wavelength coverage, that are possibly detectable by VHE instruments. We …
We present a determination of the distributions of gamma-ray photon flux -- the so called LogN-LogS relation -- and photon spectral index for blazars, based on the third extragalactic source catalog of the Fermi Gamma-ray Space Telescope's Large Area Telescope, and considering the photon energy range from 100 MeV to 100 GeV. The dataset consists of the 774 blazars in the so-called "Clean" sample detected with a greater than approximately seven sigma detection threshold and located above $\pm$20 deg Galactic latitude. We use non-parametric methods verified in previous works to reconstruct the i…
Blazars constitute the most enigmatic class of extragalactic gamma-ray sources, and their observational features have been ascribed to a relativistic jet closely aligned to the line of sight. They are generally divided in two main classes: the BL Lac objects (BL Lacs) and the Flat Spectrum Radio Quasars (FSRQs). In the case of BL Lacs the double bumped spectral energy distribution (SED) is generally described by the Synchrotron Self Compton (SSC) emission, while for the FSRQs it is interpreted as due to External Compton (EC) emission. Recently, we showed that in the [3.4]-[4.6]-[12] micron col…
