Fermi Large Area Telescope Data Research Articles

A multiwavelength study of the flat-spectrum radio quasar NVSS J141922−083830 covering four flaring episodes

ABSTRACT We present multiwavelength observations and a model for flat-spectrum radio quasar (FSRQ) NVSS J141922−083830, originally classified as a blazar candidate of unknown type (BCU II object) in the Third Fermi-LAT AGN Catalog. Relatively bright flares (>3 magnitudes) were observed on 2015 February 21 (MJD 57074) and 2018 September 8 (MJD 58369) in the optical band with the MASTER Global Robotic Nettelescopes. Optical spectra obtained with the Southern African Large Telescopeon 2015 March 1 (MJD 57082), during outburst, and on 2017 May 30 (MJD 57903), during quiescence, showed emission lines at 5325 Å and at ≈3630 Å that we identified as the Mg ii 2798 Å and C iii] 1909 Å lines, respectively, and hence derived a redshift $z$ = 0.903. Analysis of Fermi-Large Area Telescope (LAT) data was performed in the quiescent regime (5 yr of data) and during four prominent flaring states in 2014 February–April, 2014 October–November, 2015 February–March, and 2018 September. We present spectral and timing analysis with Fermi-LAT. We report a hardening of the gamma-ray spectrum during the last three flaring periods, with a power-law spectral index Γ = 2.0–2.1. The maximum gamma-ray flux level was observed on 2014 October 24 (MJD 56954) at (7.57 ± 1.83) × 10−7 ph cm−2 s−1. The multiwavelength spectral energy distribution (SED) during the 2015 February–March flare supports the earlier evidence of this blazar to belong to the FSRQ class. The SED can be well represented with a single-zone leptonic model with parameters typical of FSRQs, but also a hadronic origin of the high-energy emission cannot be ruled out.

FLUKA cross sections for cosmic-ray interactions with the DRAGON2 code

Secondary particles produced in spallation reactions of cosmic rays with the interstellar gas provide valuable information that allow us to investigate the injection and transport of charged particles in the Galaxy. A good understanding of the cross sections of production of these particles is crucial to correctly interpret our models, although the existing experimental data is very scarce and uncertain. We have developed a new set of cross sections, both inelastic and inclusive, computed with the FLUKA Monte Carlo nuclear code and tested its compatibility with CR data. Inelastic and inclusive cross sections have been compared to the most up-to-date data and parameterisations finding a general good agreement.Then, these cross sections have been implemented in the DRAGON2 code to characterize the spectra of CR nuclei up to Z = 26 and the secondary-to-primary ratios of B, Be and Li. Interestingly, we find that the FLUKA cross sections allow us to predict an energy-dependence of the B, Be and Li flux ratios which is compatible with AMS-02 data and to reproduce simultaneously these flux ratios with a scaling lower than 20%. Finally, we implement the cross sections of production of gamma rays, calculated with FLUKA, in the Gammasky code and compute diffuse gamma-ray sky maps and the local HI emissivity spectrum, finding a very good agreement with Fermi Large Area Telescope data.

A gamma-ray pulsar timing array constrains the nanohertz gravitational wave background.

After large galaxies merge, their central supermassive black holes are expected to form binary systems. Their orbital motion should generate a gravitational wave background (GWB) at nanohertz frequencies. Searches for this background use pulsar timing arrays, which perform long-term monitoring of millisecond pulsars at radio wavelengths. We used 12.5 years of Fermi Large Area Telescope data to form a gamma-ray pulsar timing array. Results from 35 bright gamma-ray pulsars place a 95% credible limit on the GWB characteristic strain of 1.0 × 10-14 at a frequency of 1 year-1. The sensitivity is expected to scale with tobs, the observing time span, as [Formula: see text]. This direct measurement provides an independent probe of the GWB while offering a check on radio noise models.

Diffuse GeV emission in the field of HESS J1912+101 revisited

We have analyzed 12 years of Fermi Large Area Telescope data toward the HESS J1912+101 region. With the latest source catalog and diffuse background models, a γ-ray excess is detected with a significance of ~8σ in the energy range of above 10 GeV. It has been argued that the diffuse GeV emission in the vicinity of HESS J1912+101 are from an extended pulsar wind nebula powered by PSR J1913+1011 and also that the hard GeV emission above 10 GeV stems from the shell-type supernova remnant and is connected with the TeV emissions. Different from previous works, our analysis indicates that the H2 spatial template is preferred over the other spatial templates, suggesting that the diffuse emission component spatially correlates with the dense molecular gas. This spatial correlation favors a hadronic emission scenario, although a leptonic origin cannot be ruled out. In the hadronic scenario, the parent proton spectrum can be described by a power-law function with an index of α = 2.36 ± 0.16. Above 50 GeV, there is no emission, and the upper limits reveal a spectral cutoff or break in the parent proton spectrum that can be explained as propagation effects of cosmic rays. We argue that the parent protons may come from the candidate supernova remnant HESS J1912+101 or the young massive star cluster Mc20.

Spatially Decomposed γ-Ray Features Surrounding Supernova Remnant Kes 79 and PSR J1853+0056

Abstract There have been substantial improvements on Fermi Large Area Telescope (LAT) data and analysis tools since the last analysis by Auchettl et al. on the intermediate-aged supernova remnant (SNR) Kes 79. Recent multiwavelength studies confirmed its interaction with molecular clouds. About 0.°36 north from Kes 79, a powerful pulsar, PSR J1853+0056, also deserves our attention. In this work, we analyze the 11.5 yr Fermi-LAT data to investigate the γ-ray feature in/around this complex region. Our result shows a more significant detection (∼34.8σ in 0.1–50 GeV) for this region. With ≥5 GeV data, we detect two extended sources: Src-N (the brighter one; radius ≈0.°31) concentrated at the north of the SNR while enclosing PSR J1853+0056, and Src-S (radius ≈0.°58) concentrated at the south of the SNR. Their spectra have distinct peak energies (∼1.0 GeV for Src-N and ≲0.5 GeV for Src-S), suggesting different origins for them. In our hadronic model that includes the leaked cosmic rays (CRs) from the shock-cloud collision, even with extreme values of parameters, SNR Kes 79 can by no means provide enough CRs reaching clouds at Src-N to explain the local GeV spectrum. We propose that the Src-N emission could be predominantly reproduced by a putative pulsar wind nebula powered by PSR J1853+0056. On the other hand, our same hadronic model can reproduce a majority of the GeV emission at Src-S with typical values of parameters, while the three known pulsars inside Src-S release a total power that is too low to account for half of its γ-ray emission.

Gamma-Ray Orbital Modulation of the Transitioning Millisecond Pulsar Binary XSS J12270–4859

Abstract We report on gamma-ray orbital modulation of the transitioning MSP binary XSS J12270–4859 detected in the Fermi Large Area Telescope (LAT) data. We use long-term optical data taken with the XMM-Newton OM and the Swift UltraViolet Optical Telescope to inspect radio timing solutions that are limited to relatively short time intervals and find that extrapolation of the solutions aligns well with the phasing of the optical data over 15 yr. The Fermi-LAT data folded on the timing solutions exhibit significant modulation (p = 5 × 10−6) with a gamma-ray minimum at the inferior conjunction of the pulsar. Intriguingly, the source seems to show similar modulation in both the low-mass X-ray binary and the MSP states, implying that mechanisms for gamma-ray emission in the two states are similar. We discuss these findings and their implications using an intrabinary shock scenario.

Fermi-LAT Detection of Extended Gamma-Ray Emission in the Vicinity of SNR G045.7-00.4: Evidence of Escaping Cosmic Rays Interacting with the Surrounding Molecular Clouds

Abstract We present an analysis of Fermi Large Area Telescope data of the gamma-ray emission in the vicinity of a radio supernova remnant (SNR), G045.7-00.4. To study the origin of the gamma-ray emission, we also make use of the CO survey data of Milky Way Imaging Scroll Painting to study the massive molecular gas complex that surrounds the SNR. The whole size of the gigaelectronvolt emission is significantly larger than that of the radio morphology. Above 3 GeV, the gigaelectronvolt emission is resolved into two sources: one is spatially consistent with the position of the SNR with a size comparable to that of the radio emission, and the other is located outside of the western boundary of the SNR and spatially coincident with the densest region of the surrounding molecular cloud. We suggest that the gigaelectronvolt emission of the western source may arise from cosmic rays (CRs) that have escaped the SNR and illuminated the surrounding molecular cloud. We find that the gamma-ray spectra of the western source can be consistently explained by this scenario with a total energy of ∼1050 erg in escaping CRs assuming the escape is isotropic.

Search for Very High-energy Emission from the Millisecond Pulsar PSR J0218+4232

Abstract PSR J0218+4232 is one of the most energetic millisecond pulsars known and has long been considered as one of the best candidates for very high-energy (VHE; >100 GeV) γ-ray emission. Using 11.5 yr of Fermi Large Area Telescope (LAT) data between 100 MeV and 870 GeV, and ∼90 hr of Major Atmospheric Gamma Imaging Cherenkov (MAGIC) observations in the 20 GeV to 20 TeV range, we searched for the highest energy γ-ray emission from PSR J0218+4232. Based on the analysis of the LAT data, we find evidence for pulsed emission above 25 GeV, but see no evidence for emission above 100 GeV (VHE) with MAGIC. We present the results of searches for γ-ray emission, along with theoretical modeling, to interpret the lack of VHE emission. We conclude that, based on the experimental observations and theoretical modeling, it will remain extremely challenging to detect VHE emission from PSR J0218+4232 with the current generation of Imaging Atmospheric Cherenkov Telescopes, and maybe even with future ones, such as the Cherenkov Telescope Array.

Spectral Modeling of Flares in Long-term Gamma-Ray Light Curve of PKS 0903-57

Abstract A detailed study of the BL Lacertae PKS 0903-57 has been done for the first time with 12 yr of Fermi Large Area Telescope data. We have identified two bright gamma-ray flares in 2018 and 2020. Many substructures were observed during multiple time binning of these flares. We performed a detailed temporal and spectral study on all the substructures separately. A single-zone emission model is used for time-dependent leptonic modeling of the multiwavelength spectral energy distributions. Our estimated values of variability timescale, magnetic field in the emission region, and the jet power obtained from leptonic modeling of PKS 0903-57 are presented in this work. Currently, we have a minimal number of observations in X-rays and other bands. Hence, further simultaneous multiwavelength monitoring of this source is required to have a better understanding of the physical processes occurring in the jet of the blazar PKS 0903-57.

X-Ray Emission of the γ-ray-loud Young Radio Galaxy NGC 3894

Abstract The radio source 1146+596 is hosted by the elliptical/S0 galaxy NGC 3894, with a low-luminosity active nucleus. The radio structure is compact, suggesting a very young age for the jets in the system. Recently, the source has been confirmed as a high-energy (HE; >0.1 GeV) γ-ray emitter in the most recent accumulation of Fermi Large Area Telescope data. Here we report on the analysis of the archival Chandra X-ray Observatory data for the central part of the galaxy, consisting of a single 40 ks long exposure. We have found that the core spectrum is best fitted by a combination of an ionized thermal plasma with a temperature of ≃0.8 keV, and a moderately absorbed power-law component (photon index Γ = 1.4 ± 0.4, hydrogen column density N H/1022 cm−2 = 2.4 ± 0.7). We have also detected the iron Kα line at 6.5 ± 0.1 keV, with a large equivalent width of 1.0 − 0.5 + 0.9 keV. Based on the simulations of Chandra's point-spread function, we have concluded that while the soft thermal component is extended on the scale of the galaxy host, the hard X-ray emission within the narrow photon energy range 6.0–7.0 keV originates within the unresolved core (effectively the central kiloparsec radius). The line is therefore indicative of the X-ray reflection from a cold neutral gas in the central regions of NGC 3894. We discuss the implications of our findings in the context of the X-ray Baldwin effect. NGC 3894 is the first young radio galaxy detected in HE γ-rays with the iron Kα line.

Search for Line-like and Box-shaped Spectral Features from Nearby Galaxy Clusters with 11.4 Years of Fermi Large Area Telescope Data

Abstract Sharp spectral structures in the γ-ray band are an important dark matter (DM) signature. Previously, a tentative line feature at ∼43 GeV was reported in 16 nearby galaxy clusters (GCls) with 7.1 yr of Fermi Large Area Telescope (LAT) data, whose test statistic (TS) value is ∼16.7. In this work, we search for line signals and box-shaped structures using the stacked data from those 16 GCls with 11.4 yr of Fermi-LAT P8R3 data. There is still a hint at ∼42 GeV, dominated by the radiation of the Virgo and Ophiuchus clusters. Though the TS value was high up to 21.2 in 2016 October, currently it has dropped to 13.1. Moreover, the TS value at ∼42 GeV decreases to 2.4 when the EDISP2 data are excluded from the analysis. Consequently, we do not find any statistically significant line-like signal and then set up the 95% confidence level upper limits on the thermally averaged cross section of DM annihilating into double photons. The same line search has been carried out for an alternative GCl sample from the Two Micron All-Sky Survey, but no evidence has been found. We also search for box-shaped features in those 16 baseline GCls. No signal is found, and the corresponding upper limits on the annihilation cross section are given.

Pair Cascades at the Edge of the Broad-line Region Shaping the Gamma-Ray Spectrum of 3C 279

Abstract The blazar 3C 279 emits a flux of gamma rays that is variable on timescales as short as the light-crossing time across the event horizon of its central black hole. It is commonly reported that the spectral energy distribution (SED) does not show signs of pair attenuation due to interactions of the gamma rays with ambient ultraviolet photons, concluding that the gamma rays must originate from substructures in the jet outside of the broad-line region (BLR). We address the spectral signature imprinted by atomic emission lines on the gamma-ray spectrum produced by an inverse-Compton pair cascade in the photon field of the BLR. We determine with high precision the gamma-ray SED of 3C 279 using Fermi Large Area Telescope data from MJD 58129−58150 and simulate the pair cascade spectrum for three different injection terms. Satisfactory fits to the observational data are obtained. The obtained SED shows features imprinted by pair production on atomic emission line photons due to optically thick radiation transport, but lacking further exponential attenuation expected if the emission region would lie buried deep within the BLR. The SED of 3C 279 is consistent with an inverse-Compton pair cascade spectrum without exponential external pair absorption. Our findings support the view that the gamma-ray emission in 3C 279 originates from the edge of the BLR.

Broadband High-energy Emission of the Gamma-Ray Binary System LS 5039: Spectral and Temporal Features Using NuSTAR and Fermi Observations

Abstract We report a detailed analysis of the hard X-ray and GeV gamma-ray spectra of LS 5039, one of the brightest gamma-ray binary system in the Galaxy. The Nuclear Spectroscopic Telescope Array (NuSTAR) observation covering its entire orbit in 2016 allowed us for the first time to study the orbital variability of the spectrum above 10 keV. The hard X-ray spectrum is well described with a single power-law component up to 78 keV. The X-ray flux showed a slight deviation from those observed previously with Suzaku in 2007. The fast X-ray brightening observed with Suzaku, around the inferior conjunction, was not observed in this observation. We also analyzed 11 yr of Fermi Large Area Telescope data of LS 5039. The GeV spectrum around the inferior conjunction was well described with two nonthermal components: a power law with a photon index of ∼3 and a cutoff power law with a cutoff energy of ∼2 GeV. The orbital flux variability also changed gradually around a few GeV. These results indicate that there are two emission components in the GeV band, and the dominant component above ∼1 GeV does not depend on the orbital phase. By combining these results, we update the spectral energy distribution of LS 5039 with the highest available statistics. Theoretical models proposed so far cannot explain the obtained multiwavelength spectrum, especially the emission from ∼1 to ∼400 MeV, and we discuss the possibility that particle acceleration in LS 5039 is different from the shock acceleration.

The likely counterpart to γ-ray excess from the northwest region of Arp 220

The unknown γ-ray excess in the northwest region of Arp 220 was revisited by analyzing ∼11.8 years of the Fermi Large Area Telescope (Fermi-LAT) data in this study. We found that its photon flux was approximately three times higher than that of the previous study in the 0.2–100 GeV band, and the corresponding significance level ∼ 8.15σ was approximately four times higher than before. The light curves of 15 and 45 time bins from the whole time all showed two active periods, and the variability of the second period was more significant than that of the first period. The spectral indices from the two active periods were not statistically different and were close to the range of γ-ray flat-spectrum radio quasars observed by Fermi-LAT. Because the position of CRATES J153246+234400 was consistent with the best-fit position of our analysis, we suggest that CRATES J153246+234400 is more likely a γ-ray counterpart for the variational region. For Arp 220, there was no significant variability in the γ-ray emission.

Multiwavelength investigation of the candidate Galactic PeVatron MGRO J1908+06

ABSTRACT The candidate PeVatron MGRO J1908+06, which shows a hard spectrum beyond 100 TeV, is one of the most peculiar γ-ray sources in the Galactic plane. Its complex morphology and some possible counterparts spatially related with the very high energy (VHE) emission region, preclude to distinguish between a hadronic and leptonic nature of the γ-ray emission. In this paper, we illustrate a new multiwavelength analysis of MGRO J1908+06, with the aim to shed light on its nature and the origin of its ultra-high-energy emission. We performed an analysis of the 12CO and 13CO molecular line emission demonstrating the presence of dense molecular clouds spatially correlated with the source region. We also analysed 12 yr of Fermi-Large Area Telescope (LAT) data between 10 GeV and 1 TeV finding a counterpart with a hard spectrum (Γ ∼ 1.6). Our reanalysis of XMM–Newton data allowed us to put a more stringent constraint on the X-ray flux from this source. We demonstrate that a single accelerator cannot explain the whole set of multiwavelength data, regardless of whether it accelerates protons or electrons, but a two-zone model is needed to explain the emission from MGRO J1908+06. The VHE emission seems most likely the superposition of a TeV pulsar wind nebula powered by PSR J1907+0602, in the southern region, and of the interaction between the supernova remnant G40.5−0.5 and the molecular clouds, in the northern region.

Limits on axionlike particles from Mrk 421 with 4.5-year period observations by ARGO-YBJ and Fermi-LAT

In this work, we investigate the axion-like particle (ALP)-photon oscillation effect in the spectra of the blazar Markarian 421 (Mrk 421) using 4.5 years of the Astrophysical Radiation with Ground-based Observatory at YangBaJing (ARGO-YBJ) and Fermi Large Area Telescope (Fermi-LAT) data. These data are collected during the common operation time, which cover ten activity phases of Mrk 421. No significant ALP-photon oscillation effect is confirmed. On the other hand, not all the observations of the ten phases can be individually used to set the 95% confidence level ($\rm C.L.$) constraint on the ALP parameter space. However, the constraint can be significantly improved if the analyses for the ten phases are combined. We find that the upper limit at 95% $\rm C.L.$ on the ALP-photon coupling $g_{a\gamma}$ set by the ARGO-YBJ and Fermi-LAT observations of Mrk 421 is within $\sim [2\times 10^{-11}, \ 6\times 10^{-11}] \rm \, GeV^{-1}$ for the ALP mass of $\sim 5\times 10^{-10}$ eV $\lesssim m_a \lesssim 5\times 10^{-7}\, \rm eV$.

A source of gamma rays coincident with the shell of the supernova remnant CTB 80

ABSTRACT CTB 80 (G69.0+2.7) is a relatively old (50–80 kyr) supernova remnant (SNR) with a complex radio morphology showing three extended radio arms and a radio and X-ray nebula near the location of the pulsar PSR B1951+32. We report on a study of the GeV emission in the region of CTB 80 with Fermi-Large Area Telescope data. An extended source with a size of 1.3°, matching the size of the infrared shell associated to the SNR, was discovered. The GeV emission, detected up to an energy of ∼20 GeV, is more significant at the location of the northern radio arm where previous observations imply that the SNR shock is interacting with ambient material. Both hadronic and leptonic scenarios can reproduce the multiwavelength data reasonably well. The hadronic cosmic ray energy density required is considerably larger than the local Galactic value and the gamma-ray leptonic emission is mainly due to bremsstrahlung interactions. We conclude that GeV particles are still trapped or accelerated by the SNR producing the observed high-energy emission when interacting with ambient material.

Comparing the galactic bulge and galactic disk millisecond pulsars

The Galactic Center Excess (GCE) is an extended gamma-ray source in the central region of the Galaxy found in Fermi Large Area Telescope (Fermi-LAT) data. One of the leading explanations for the GCE is an unresolved population of millisecond pulsars (MSPs) in the Galactic bulge. Due to differing star formation histories it is expected that the MSPs in the Galactic bulge are older and therefore dimmer than those in the Galactic disk. Additionally, correlations between the spectral parameters of the MSPs and the spin-down rate of the corresponding neutron stars have been observed. This implies that the bulge MSPs may be spectrally different from the disk MSPs. We perform detailed modelling of the MSPs from formation until observation. Although we confirm the correlations, we do not find they are sufficiently large to significantly differentiate the spectra of the bulge MSPs and disk MSPs when the uncertainties are accounted for. Our results demonstrate that the population of MSPs that can explain the gamma-ray signal from the resolved MSPs in the Galactic disk and the unresolved MSPs in the boxy bulge and nuclear bulge can consistently be described as arising from a common evolutionary trajectory for some subset of astrophysical sources common to all these different environments. We do not require that there is anything unusual about inner Galaxy MSPs to explain the GCE. Additionally, we use a more accurate geometry for the distribution of bulge MSPs and incorporate dispersion measure estimates of the MSPs' distances. We find that the elongated boxy bulge morphology means that some the bulge MSPs are closer to us and so easier to resolve. We identify three resolved MSPs that have significant probabilities of belonging to the bulge population.

Bayesian mixture modelling of the high-energy photon counts collected by the Fermi Large Area Telescope

Identifying as yet undetected high-energy sources in the [Formula: see text]-ray sky is one of the declared objectives of the Fermi Large Area Telescope (LAT) Collaboration. We develop a Bayesian mixture model which is capable of disentangling the high-energy extra-galactic sources present in a given sky region from the pervasive background radiation. We achieve this by combining two model components. The first component models the emission activity of the single sources and incorporates the instrument response function of the Fermi [Formula: see text]-ray space telescope. The second component reliably reflects the current knowledge of the physical phenomena which underlie the [Formula: see text]-ray background. The model parameters are estimated using a reversible jump MCMC algorithm, which simultaneously returns the number of detected sources, their locations and relative intensities, and the background component. Our proposal is illustrated using a sample of the Fermi LAT data. In the analysed sky region, our model correctly identifies 116 sources out of the 132 present. The detection rate and the estimated directions and intensities of the identified sources are largely unaffected by the number of detected sources.

Evidence of Cosmic-Ray Excess from Local Giant Molecular Clouds

Abstract We report the analysis of the Fermi-Large Area Telescope data from six nearby giant molecular clouds (MCs) belonging to the Gould Belt and the Aquila Rift regions. The high statistical γ-ray spectra above 3 GeV well described by power laws make it possible to derive precise estimates of the cosmic-ray (CR) distribution in the MCs. The comparison of γ-ray spectra of Taurus, Orion A, and Orion B clouds with the model expected from Alpha Magnetic Spectrometer (AMS-02) CR measurements confirms these clouds as passive clouds, immersed in an AMS-02-like CR spectrum. A similar comparison of Aquila Rift, Rho Oph, and Cepheus spectra yields significant deviation in both spectral indices and absolute fluxes, which can imply an additional acceleration of CRs throughout the entire clouds. Besides, the theoretical modeling of the excess γ-ray spectrum of these clouds, assuming π 0-decay interaction of CRs in the cloud, gives a considerable amount of an enhanced CR energy density and it shows a significant deviation in spectral shapes compared to the average AMS-02 CR spectrum between 30 GeV and 10 TeV. We suggest that this variation in the CR spectrum of Cepheus could be accounted for by an efficient acceleration in the shocks of winds of OB associations, while in Rho Oph, similar acceleration can be provided by multiple T-Tauri stars populated in the whole cloud. In the case of Aquila Rift, the excess in absolute CR flux could be related to an additional acceleration of CRs by supernova remnants or propagation effects in the cloud.