Fermi-LAT Source Catalog Research Articles

Revisiting γ-Ray Emission of the Supernova Remnant RCW 103

We analyze more than 15 yr of γ-ray data obtained with the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (Fermi) for the region of the young supernova remnant (SNR) RCW 103, since the nearby source 4FGL J1616.2−5054e, counterpart to HESS J1616−518 and ≃13′ away from the SNR, is determined to be extended in the more recent Fermi-LAT source catalog. Different templates for 4FGL J1616.2−5054e and RCW 103 are tested, and we find that a point source (PS) with a power-law (PL) spectrum at the southern limb of the SNR best describes the detected γ-ray emission. The photon index of the PL emission is Γ ≃ 2.31 (or α ≃ 2.4 in a log-parabola model), softer than the previously reported Γ ≃ 2.0 when the counterpart to HESS J1616−518 was considered to be a PS (which likely caused misidentification of extended emission at RCW 103). In order to produce the γ-ray emission in a hadronic scenario, we estimate that protons with an index ∼2.4 PL energy distribution are needed. These results fit with those from multiwavelength observations that have indicated the remnant at the southern limb is interacting with a molecular cloud.

Study of γ-Ray Emission from a Compact Radio Galaxy with the Fermi Large Area Telescope

The radio galaxy PKS 1007+142 is classified as a compact steep-spectrum source (CSS) and belongs to the class of young active galactic nuclei (AGNs). In this paper, we investigate the γ-ray emission from this CSS by conducting a comprehensive analysis of the 15 yr Fermi Large Area Telescope (Fermi-LAT) observation data. The Fermi-LAT latest Source Catalog, 4FGL-DR4, includes an unassociated γ-ray source, 4FGL J1010.0+1416, located at 0.°24 away from the radio position of PKS 1007+142. Using the 15 yr Fermi-LAT observation data, we reestimate the best-fit position of the γ-ray source and find that PKS 1007+142 is in close proximity to the γ-ray source and falls within its 68% error circle. Therefore, we conclude that PKS 1007+142 is the most plausible counterpart to the unassociated LAT source with detection test statistics ∼ 43.4 (∼6.6σ). PKS 1007+142 exhibits a steep power-law spectrum in the 0.1–300 GeV band, with a photon spectral index (Γ γ ) of 2.86 ± 0.17. The average flux in the considered time interval is (2.14 ± 0.34) × 10−12 erg cm−2 s−1. Comparing PKS 1007+142 with other γ-ray emitting AGNs in both the L γ –Γ γ and L γ –L 1.4 GHz planes, it shows a softer γ-ray spectrum and lower luminosity compared to other γ-ray emitting CSSs. Furthermore, the possible origins of γ-ray in PKS 1007+142 are also discussed.

Classifications of Fermi-LAT unassociated sources in multiple machine learning methods

ABSTRACT The classifications of Fermi-LAT unassociated sources are studied using multiple machine learning (ML) methods. The update data from 4FGL-DR3 are divided into high Galactic latitude (HGL, Galactic latitude |b| > 10°) and low Galactic latitude (LGL, |b| ≤ 10°) regions. In the HGL region, a voting ensemble of four binary ML classifiers achieves a 91 per cent balanced accuracy. In the LGL region, an additional Bayesian–Gaussian (BG) model with three parameters is introduced to eliminate abnormal soft spectrum active galactic nuclei (AGNs) from the training set and ML-identified AGN candidates, a voting ensemble of four ternary ML algorithms reach an 81 per cent balanced accuracy. And then, a catalogue of Fermi-LAT all-sky unassociated sources is constructed. Our classification results show that (i) there are 1037 AGN candidates and 88 pulsar candidates with a balanced accuracy of 0.918 ± 0.029 in HGL region, which are consistent with those given in previous all-sky ML approaches; and (ii) there are 290 AGN-like candidates, 135 pulsar-like candidates, and 742 other-like candidates with a balanced accuracy of 0.815 ± 0.027 in the LGL region, which are different from those in previous all-sky ML approaches. Additionally, different training sets and class weights were tested for their impact on classifier accuracy and predicted results. The findings suggest that while different training approaches can yield similar model accuracy, the predicted numbers across different categories can vary significantly. Thus, reliable evaluation of the predicted results is deemed crucial in the ML approach for Fermi-LAT unassociated sources.

Is TOL 1326–379 a Prototype of γ-Ray Emitting FR0 Radio Galaxy?

With the possible spacial association to the Fermi/LAT source 3FGL J1330.0–3818, TOL 1326–379 may be the first one that is identified as a γ-ray emitting Fanaroff–Riley type 0 radio galaxy (FR0 RG). We analyze the ∼12 yr Fermi/LAT observation data of this γ-ray source and examine its association to TOL 1326–379. We show that the γ-ray source (named as J1331.0–3818) is tentatively detected with a TS value of 28.7, 3FGL J1330.0–3818 is out of the 95% containment of J1331.0–3818, and their positions are spatially separated ∼0.°2. 4FGL J1331.3–3818 falls into the 68% containment of J1331.0–3818, suggesting that our result agrees with that reported in the Fourth Fermi LAT Source Catalog. TOL 1326–379 is out of the 95% containment of J1331.0–3818, and their positions are spatially separated ∼0.°4, indicating that the association between J1331.0–3818 and TOL 1326–379 is quite ambiguous. However, we do not find other possible potential radio or X-ray counterpart within the circle centered at J1331.0–3818 with a radius of 0.°4. The spectral energy distribution (SED) of TOL 1326–379 shows a bimodal feature as seen in the γ-ray emitting RGs. We fit the SED with the one-zone leptonic model and find that the average energy spectrum of J1331.0–3818 agrees with the model prediction. Assuming that J1331.0–3818 is an unidentified γ-ray source, we derive the upper-limit of the γ-ray flux for TOL 1326–379. It is not tight enough to exclude this possibility with the SED modeling. Based on these results, we cautiously argue that the γ-ray source J1331.0–3818 is associated with TOL 1326–379 and its jet radiation physic is similar to those γ-ray emitting RGs.

Identification of point sources in gamma rays using U-shaped convolutional neural networks and a data challenge

Context. At GeV energies, the sky is dominated by the interstellar emission from the Galaxy. With limited statistics and spatial resolution, accurately separating point sources is therefore challenging. Aims. Here we present the first application of deep learning based algorithms to automatically detect and classify point sources from gamma-ray data. For concreteness we refer to this approach as AutoSourceID. Methods. To detect point sources, we utilized U-shaped convolutional networks for image segmentation and k-means for source clustering and localization. We also explored the Centroid-Net algorithm, which is designed to find and count objects. Using two algorithms allows for a cross check of the results, while a combination of their results can be used to improve performance. The training data are based on 9.5 years of exposure from The Fermi Large Area Telescope (Fermi-LAT) and we used source properties of active galactic nuclei (AGNs) and pulsars (PSRs) from the fourth Fermi-LAT source catalog in addition to several models of background interstellar emission. The results of the localization algorithm are fed into a classification neural network that is trained to separate the three general source classes (AGNs, PSRs, and FAKE sources). Results. We compared our localization algorithms qualitatively with traditional methods and find them to have similar detection thresholds. We also demonstrate the robustness of our source localization algorithms to modifications in the interstellar emission models, which presents a clear advantage over traditional methods. The classification network is able to discriminate between the three classes with typical accuracy of ∼70%, as long as balanced data sets are used in classification training. We published online our training data sets and analysis scripts and invite the community to join the data challenge aimed to improve the localization and classification of gamma-ray point sources.

An Optical Overview of Blazars with LAMOST. II. Gamma-Ray Blazar Candidates and Updated Classifications

Abstract Blazars represent the dominant class of associated γ-ray sources detected by the Fermi Large Area Telescope (LAT). However, in the more recent release of the Fourth Fermi-LAT Point Source Catalog (4FGL), ∼25% of the sources associated with lower-energy counterparts show a multifrequency behavior similar to that of blazars, but lacks an optical spectroscopic confirmation of their nature and are therefore classified as Blazar Candidates of Uncertain Type (BCUs). A particularly challenging task in blazar studies is to classify these BCUs and, when possible to estimate their redshifts, in particular for BL Lac objects, characterized by almost featureless optical spectra with only weak emission lines. Continuing our study of blazars with Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) optical spectral data, we perform an extensive search for optical spectra available in the LAMOST Data Release 6 archive. Our aim is confirming the blazar nature of BCUs and to test if new data can allow us to get a redshift estimate for BL Lac objects that lack measurement, as well as to search for and discover changing-look blazars. We selected sources out of the 4FGL catalog, the list of targets from our follow-up spectroscopic campaign of unidentified and/or unassociated γ-ray sources, and the Roma-BZCAT multifrequency catalog of blazars, finding a total of 42 sources with available LAMOST DR6 spectra. We confirmed the blazar-like nature of four blazar candidates . For the remaining 37 sources we confirm their previous classification.

Identifying the 3FHL Catalog. V. Results of the CTIO-COSMOS Optical Spectroscopy Campaign 2019

Abstract As a follow-up to the optical spectroscopic campaign aimed at achieving completeness in the Third Catalog of Hard Fermi-LAT Sources (3FHL), we present here the results of a sample of 28 blazars of an uncertain type observed using the 4 m telescope at Cerro Tololo Inter-American Observatory in Chile. Out of these 28 sources, we find that 25 are BL Lacertae objects (BL Lacs) and 3 are flat-spectrum radio quasars (FSRQs). We measure redshifts or lower limits for 16 of these blazars, and it is observed that the 12 remaining blazars have featureless optical spectra. These results are part of a more extended optical spectroscopy follow-up campaign for 3FHL blazars, where, until now, 51 blazars of an uncertain type have been classified into BL Lac and FSRQ categories. Furthermore, this campaign has resulted in redshift measurements and lower limits for 15 of these sources. Our results contribute toward attaining a complete sample of blazars above 10 GeV, which then will be crucial in extending our knowledge on blazar emission mechanisms and the extragalactic background light.

A Survey of Active Galaxies at TeV Photon Energies with the HAWC Gamma-Ray Observatory

Abstract The High Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory continuously detects TeV photons and particles within its large field of view, accumulating every day a deeper exposure of two-thirds of the sky. We analyzed 1523 days of HAWC live data acquired over four and a half years, in a follow-up analysis of 138 nearby (z < 0.3) active galactic nuclei from the Third Catalog of Hard Fermi-LAT sources culminating within 40° of the zenith at Sierra Negra, the HAWC site. This search for persistent TeV emission used a maximum-likelihood analysis assuming intrinsic power-law spectra attenuated by pair production of gamma-ray photons with the extragalactic background light. HAWC clearly detects persistent emission from Mkn 421 and Mkn 501, the two brightest blazars in the TeV sky, at 65σ and 17σ level, respectively. Marginal evidence, just above the 3σ level, was found for three other known very high-energy emitters: the radio galaxy M87 and the BL Lac objects VER J0521+211 and 1ES 1215+303, the latter two at z ∼ 0.1. We find a 4.2σ evidence for collective emission from the set of 30 previously reported very high-energy sources, with Mkn 421 and Mkn 501 excluded. Upper limits are presented for the sample under the power-law assumption and in the predefined (0.5–2.0), (2.0–8.0), and (8.0–32.0) TeV energy intervals.

Optical spectroscopy of BL Lac objects: TeV candidates

ABSTRACT We investigate the spectroscopic optical properties of gamma-ray sources detected with high significance above 50 GeV in the Third Catalog of Hard Fermi-LAT Sources and that are good candidates as TeV emitters. We focus on the 91 sources that are labelled by the Fermi team as BL Lac (BLL) objects or blazar candidates of uncertain type (BCUs), are in the Northern hemisphere, and are with unknown or uncertain redshift. We report here on GTC (Gran Telescopio Canarias) spectra (in the spectral range 4100–7750 Å) of 13 BCUs and 42 BLL objects. We are able to classify the observed targets as BLL objects and each source is briefly discussed. The spectra allowed us to determine the redshift of 25 objects on the basis of emission and/or absorption lines, finding 0.05 < z < 0.91. Most of the emission lines detected are due to forbidden transition of [O iii] and [N ii]. The observed line luminosity is found to be lower than that of quasi-stellar objects (QSOs) at similar continuum and could be reconciled with the line–continuum luminosity relationship of QSOs if a significant beaming factor is assumed. Moreover, for five sources we found intervening absorption lines that allow to set a spectroscopic lower limit of the redshift. For the remaining 25 sources, for which the spectra are lineless, a lower limit to z is given, assuming that the host galaxies are giant ellipticals.

Searches for pulsar-like candidates from unidentified objects in the Third Catalog of Hard Fermi-LAT Sources with machine learning techniques

ABSTRACT We report the results of searching pulsar-like candidates from the unidentified objects in the Third Catalog of Hard Fermi-LAT Sources (3FHL). Using a machine-learning-based classification scheme with a nominal accuracy of $\sim \!98{{\, \rm per\, cent}}$, we have selected 27 pulsar-like objects from 200 unidentified 3FHL sources for an identification campaign. Using archival data, X-ray sources are found within the γ-ray error ellipses of 10 3FHL pulsar-like candidates. Within the error circles of the much better constrained X-ray positions, we have also searched for the optical/infrared counterparts and examined their spectral energy distributions. Among our shortlisted candidates, the most secure identification is the association of 3FHL J1823.3–1339 and its X-ray counterpart with the globular cluster Mercer 5. The γ-rays from the source can be contributed by a population of millisecond pulsars residing in the cluster. This makes Mercer 5 as one of the slowly growing hard γ-ray population of globular clusters with emission >10 GeV. Very recently, another candidate picked by our classification scheme, 3FHL J1405.1–6118, has been identified as a new γ-ray binary with an orbital period of 13.7 d. Our X-ray analysis with a short Chandra observation has found a possible periodic signal candidate of ∼1.4 h and a putative extended X-ray tail of ∼20 arcsec long. Spectral energy distribution of its optical/infrared counterpart conforms with a blackbody of Tbb ∼ 40 000 K and Rbb ∼ 12 R⊙ at a distance of 7.7 kpc. This is consistent with its identification as an early O star as found by infrared spectroscopy.

NGC 3894: a young radio galaxy seen by Fermi-LAT

Context. According to radiative models, radio galaxies may produce γ-ray emission from the first stages of their evolution. However, very few such galaxies have been detected by the Fermi Large Area Telescope (LAT) so far. Aims. NGC 3894 is a nearby (z = 0.0108) object that belongs to the class of compact symmetric objects (CSOs, i.e., the most compact and youngest radio galaxies), which is associated with a γ-ray counterpart in the Fourth Fermi-LAT source catalog. Here we present a study of the source in the γ-ray and radio bands aimed at investigating its high-energy emission and assess its young nature. Methods. We analyzed 10.8 years of Fermi-LAT data between 100 MeV and 300 GeV and determined the spectral and variability characteristics of the source. Multi-epoch very long baseline array (VLBA) observations between 5 and 15 GHz over a period of 35years were used to study the radio morphology of NGC 3894 and its evolution. Results. NGC 3894 is detected in γ-rays with a significance >9σ over the full period, and no significant variability has been observed in the γ-ray flux on a yearly time-scale. The spectrum is modeled with a flat power law (Γ = 2.0 ± 0.1) and a flux on the order of 2.2 × 10−9 ph cm−2 s−1. For the first time, the VLBA data allow us to constrain with high precision the apparent velocity of the jet and counter-jet side to be βapp, NW = 0.132 ± 0.004 and βapp, SE = 0.065 ± 0.003, respectively. Conclusions. Fermi-LAT and VLBA results favor the youth scenario for the inner structure of this object, with an estimated dynamical age of 59 ± 5 years. The estimated range of viewing angle (10° < θ < 21°) does not exclude a possible jet-like origin of the γ-ray emission.

An investigation on the factors affecting machine learning classifications in gamma-ray astronomy

ABSTRACTWe have investigated a number of factors that can have significant impacts on the classification performance of gamma-ray sources detected by Fermi Large Area Telescope (LAT) with machine learning techniques. We show that a framework of automatic feature selection can construct a simple model with a small set of features that yields better performance over previous results. Secondly, because of the small sample size of the training/test sets of certain classes in gamma-ray, nested re-sampling and cross-validations are suggested for quantifying the statistical fluctuations of the quoted accuracy. We have also constructed a test set by cross-matching the identified active galactic nuclei (AGNs) and the pulsars (PSRs) in the Fermi-LAT 8-yr point source catalogue (4FGL) with those unidentified sources in the previous 3rd Fermi-LAT Source Catalog (3FGL). Using this cross-matched set, we show that some features used for building classification model with the identified source can suffer from the problem of covariate shift, which can be a result of various observational effects. This can possibly hamper the actual performance when one applies such model in classifying unidentified sources. Using our framework, both AGN/PSR and young pulsar (YNG)/millisecond pulsar (MSP) classifiers are automatically updated with the new features and the enlarged training samples in 4FGL catalogue incorporated. Using a two-layer model with these updated classifiers, we have selected 20 promising MSP candidates with confidence scores $\gt 98{{\ \rm per\ cent}}$ from the unidentified sources in 4FGL catalogue that can provide inputs for a multiwavelength identification campaign.

Two-zone Emission Modeling of PKS 1510-089 during the High State of 2015

Abstract PKS 1510-089 is one of the most variable blazars in the third Fermi-LAT source catalog. During 2015, this source has shown four flares identified as flares A, B, C, and D in between three quiescent states: Q1, Q2, and Q3. The multiwavelength data from Fermi-LAT, Swift-XRT/Ultraviolet/Optical Telescope, Owens Valley Radio Observatory, and Sub-millimeter array Observatory are used in our work to model these states. Different flux doubling times have been observed in different energy bands, which indicate that there could be multiple emission zones. The flux doubling time from the gamma-ray and X-ray light curves are found to be 10.6 hr, 2.5 days, and the average flux doubling time in the optical/UV band is 1 day. It is possible that the gamma-ray and optical/UV emission are produced in the same region whereas X-ray emission is coming from a different region along the jet axis. We have also estimated the discrete correlation functions (DCFs) among the light curves of different energy bands to infer about their emission regions. However, our DCF analysis does not show significant correlation in different energy bands though it shows peaks in some cases at small time lags. We perform a two-zone multiwavelength time-dependent modeling with one emission zone located near the outer edge of the broad line region and another further away in the dusty/molecular torus (DT/MT) region to study this high state.

Search for Gamma-Ray Emission from Local Primordial Black Holes with the Fermi Large Area Telescope

Abstract Black holes with masses below approximately 1015 g are expected to emit gamma-rays with energies above a few tens of MeV, which can be detected by the Fermi Large Area Telescope (LAT). Although black holes with these masses cannot be formed as a result of stellar evolution, they may have formed in the early universe and are therefore called primordial black holes (PBHs). Previous searches for PBHs have focused on either short-timescale bursts or the contribution of PBHs to the isotropic gamma-ray emission. We show that, in cases of individual PBHs, the Fermi-LAT is most sensitive to PBHs with temperatures above approximately 16 GeV and masses 6 × 1011 g, which it can detect out to a distance of about 0.03 pc. These PBHs have a remaining lifetime of months to years at the start of the Fermi mission. They would appear as potentially moving point sources with gamma-ray emission that become spectrally harder and brighter with time until the PBH completely evaporates. In this paper, we develop a new algorithm to detect the proper motion of gamma-ray point sources, and apply it to 318 unassociated point sources at a high galactic latitude in the third Fermi-LAT source catalog. None of the unassociated point sources with spectra consistent with PBH evaporation show significant proper motion. Using the nondetection of PBH candidates, we derive a 99% confidence limit on the PBH evaporation rate in the vicinity of Earth, ρ ˙ PBH < 7.2 × 10 3 pc − 3 yr − 1 . This limit is similar to the limits obtained with ground-based gamma-ray observatories.

Deriving the Contribution of Blazars to the Fermi-LAT Extragalactic γ-ray Background at E > 10 GeV with Efficiency Corrections and Photon Statistics

Abstract The Fermi Large Area Telescope (LAT) Collaboration has recently released the Third Catalog of Hard Fermi-LAT Sources (3FHL), which contains 1556 sources detected above 10 GeV with seven years of Pass 8 data. Building upon the 3FHL results, we investigate the flux distribution of sources at high Galactic latitudes ( ), which are mostly blazars. We use two complementary techniques: (1) a source-detection efficiency correction method and (2) an analysis of pixel photon count statistics with the one-point probability distribution function (1pPDF). With the first method, using realistic Monte Carlo simulations of the γ-ray sky, we calculate the efficiency of the LAT to detect point sources. This enables us to find the intrinsic source-count distribution at photon fluxes down to 7.5 × 10−12 ph cm−2 s−1. With this method, we detect a flux break at (3.5 ± 0.4) × 10−11 ph cm−2 s−1 with a significance of at least 5.4σ. The power-law indexes of the source-count distribution above and below the break are 2.09 ± 0.04 and 1.07 ± 0.27, respectively. This result is confirmed with the 1pPDF method, which has a sensitivity reach of ∼10−11 ph cm−2 s−1. Integrating the derived source-count distribution above the sensitivity of our analysis, we find that (42 ± 8)% of the extragalactic γ-ray background originates from blazars.

An Indication of Anisotropy in Arrival Directions of Ultra-high-energy Cosmic Rays through Comparison to the Flux Pattern of Extragalactic Gamma-Ray Sources*

Abstract A new analysis of the data set from the Pierre Auger Observatory provides evidence for anisotropy in the arrival directions of ultra-high-energy cosmic rays on an intermediate angular scale, which is indicative of excess arrivals from strong, nearby sources. The data consist of 5514 events above with zenith angles up to 80° recorded before 2017 April 30. Sky models have been created for two distinct populations of extragalactic gamma-ray emitters: active galactic nuclei from the second catalog of hard Fermi-LAT sources (2FHL) and starburst galaxies from a sample that was examined with Fermi-LAT. Flux-limited samples, which include all types of galaxies from the Swift-BAT and 2MASS surveys, have been investigated for comparison. The sky model of cosmic-ray density constructed using each catalog has two free parameters, the fraction of events correlating with astrophysical objects, and an angular scale characterizing the clustering of cosmic rays around extragalactic sources. A maximum-likelihood ratio test is used to evaluate the best values of these parameters and to quantify the strength of each model by contrast with isotropy. It is found that the starburst model fits the data better than the hypothesis of isotropy with a statistical significance of 4.0σ, the highest value of the test statistic being for energies above . The three alternative models are favored against isotropy with 2.7σ–3.2σ significance. The origin of the indicated deviation from isotropy is examined and prospects for more sensitive future studies are discussed.

Exploring the connection between radio and GeV-TeV γ-ray emission in the 1FHL and 2FHL AGN samples

The Fermi Large Area Telescope (LAT) revealed that blazars, representing the most extreme radio-loud active galactic nuclei (AGN) population, dominate the census of the gamma-ray sky, and a significant correlation was found between radio and gamma-ray emission in the 0.1-100 GeV energy range. However, the possible connection between radio and very high energy (VHE, E>0.1 TeV) emission still remains elusive, owing to the lack of a homogeneous coverage of the VHE sky. The main goal of this work is to quantify and assess the significance of a possible connection between the radio emission on parsec scale measured by the very long baseline interferometry (VLBI) and GeV-TeV gamma-ray emission in blazars, which is a central issue for understanding the blazar physics and the emission processes. We investigate the radio VLBI and high energy gamma-ray emission by using two large and unbiased AGN samples extracted from the first and second Fermi-LAT catalogs of hard gamma-ray sources detected above 10 GeV (1FHL) and 50 GeV (2FHL). For comparison, we perform the same correlation analysis by using the 0.1-300 GeV gamma-ray energy flux provided by the third Fermi-LAT source catalog. We find that the correlation strength and significance depend on the gamma-ray energy range with a different behavior among the blazar sub-classes. Overall, the radio and gamma-ray emission above 10 GeV turns out to be uncorrelated for the full samples and for all of the blazar sub-classes with the exception of high synchrotron peaked (HSP) objects, which show a strong and significant correlation. On the contrary, when 0.1-300 GeV gamma-ray energies are considered, a strong and significant correlation is found for the full blazar sample as well as for all of the blazar sub-classes. We interpret and explain this correlation behavior within the framework of the blazar spectral energy distribution properties.

3FHL: The Third Catalog of Hard Fermi-LAT Sources

Abstract We present a catalog of sources detected above 10 GeV by the Fermi Large Area Telescope (LAT) in the first 7 years of data using the Pass 8 event-level analysis. This is the Third Catalog of Hard Fermi-LAT Sources (3FHL), containing 1556 objects characterized in the 10 GeV–2 TeV energy range. The sensitivity and angular resolution are improved by factors of 3 and 2 relative to the previous LAT catalog at the same energies (1FHL). The vast majority of detected sources (79%) are associated with extragalactic counterparts at other wavelengths, including 16 sources located at very high redshift (z > 2). Of the sources, 8% have Galactic counterparts and 13% are unassociated (or associated with a source of unknown nature). The high-latitude sky and the Galactic plane are observed with a flux sensitivity of 4.4 to 9.5 × 10−11 ph cm−2 s−1, respectively (this is approximately 0.5% and 1% of the Crab Nebula flux above 10 GeV). The catalog includes 214 new γ-ray sources. The substantial increase in the number of photons (more than 4 times relative to 1FHL and 10 times to 2FHL) also allows us to measure significant spectral curvature for 32 sources and find flux variability for 163 of them. Furthermore, we estimate that for the same flux limit of 10−12 erg cm−2 s−1, the energy range above 10 GeV has twice as many sources as the range above 50 GeV, highlighting the importance, for future Cherenkov telescopes, of lowering the energy threshold as much as possible.

Searches for correlation between UHECR events and high-energy gamma-ray Fermi-LAT data

The astrophysical sources responsible for ultra high-energy cosmic rays (UHECRs) continue to be one of the most intriguing mysteries in astrophysics. We present a comprehensive search for correlations between high-energy (≳ 1 GeV) gamma-ray events from the Fermi Large Area Telescope (LAT) and UHECRs (≳ 60 EeV) detected by the Telescope Array and the Pierre Auger Observatory. We perform two separate searches. First, we conduct a standard cross-correlation analysis between the arrival directions of 148 UHECRs and 360 gamma-ray sources in the Second Catalog of Hard Fermi-LAT sources (2FHL). Second, we search for a possible correlation between UHECR directions and unresolved Fermi-LAT gamma-ray emission. For the latter, we use three different methods: a stacking technique with both a model-dependent and model-independent background estimate, and a cross-correlation function analysis. We also test for statistically significant excesses in gamma rays from signal regions centered on Cen A and the Telescope Array hotspot. No significant correlation is found in any of the analyses performed, except a weak (≲ 2σ) hint of signal with the correlation function method on scales ∼ 1°. Upper limits on the flux of possible power-law gamma-ray sources of UHECRs are derived.

Angular correlation between IceCube high-energy starting events and starburst sources

Starburst galaxies and star-forming regions in the Milkyway, with high rate of supernova activities, are candidate sources of high-energy neutrinos. Using a gamma-ray selected sample of these sources we perform statistical analysis of their angular correlation with the four-year sample of high-energy starting events (HESE), detected by the IceCube Neutrino Observatory. We find that the two samples (starburst galaxies and local star-forming regions) are correlated with cosmic neutrinos at ∼ (2–3)σ (pre-trial) significance level, when the full HESE sample with deposited energy ≳ 20 TeV is considered. However when we consider the HESE sample with deposited energy ≳ 60 TeV, which is almost free of atmospheric neutrino and muon backgrounds, the significance of correlation decreased drastically. We perform a similar study for Galactic sources in the 2nd Catalog of Hard Fermi-LAT Sources (2FHL, >50 GeV) catalog as well, obtaining ∼ (2–3)σ (pre-trial) correlation, however the significance of correlation increases with higher cutoff energy in the HESE sample for this case. We also fit available gamma-ray data from these sources using a pp interaction model and calculate expected neutrino fluxes. We find that the expected neutrino fluxes for most of the sources are at least an order of magnitude lower than the fluxes required to produce the HESE neutrinos from these sources. This puts the starburst sources being the origin of the IceCube HESE neutrinos in question.