Abstract
A systematic search for cyclical sources of γ-ray emission on the period range from days to years in the Fermi-LAT sky is performed. Looking for cyclical emission, the sky is binned into ...equal-area pixels and the generalized Lomb–Scargle periodogram is computed for each of these pixels. The search on the period range between 2.5 and 30 d in the Galactic plane confirms periodicities of three binaries, LSI +61°303, LS 5039 and 1FGL J1018.6−5856. The all-sky search on the period range between 30 d
and 2.5 yr confirm quasi-periodicities of three blazars, PG 1553+113, PKS 2155−304 and BL Lacertae. Evidence for quasi-periodic behaviours of four blazars, 4C +01.28, S5 0716+71, PKS 0805−07 and PKS 2052−47, is presented. Three of these blazars, 4C +01.28, PKS 0805−07 and PKS 2052−47, are located at high redshifts. These three sources are potential candidates to binary systems of supermassive black holes,
provided that major galaxy mergers are more frequent and that galaxies are more gas-rich at high redshifts.
New data about the electronic state of Tb3+ doping ions in the Tb-doped EuAl3(BO3)4 and EuGa3(BO3)4 borate single crystals have been obtained from the electron paramagnetic resonance (EPR) spectra ...measurements at X- and Q-band frequency range in the wide temperature interval. Prior to the EPR study, the X-ray diffraction (XRD) techniques the structural and elastic properties of Tb3+ doped EuAl3(BO3)4 and EuGa3(BO3)4 single crystals were determined at T = 300–1073 K. The following spin Hamiltonian parameters for Tb3+ ions were obtained: gz = 17.064 ± 0.019, Az = 6.231 ± 0.012 GHz, Δ = 7.333 ± 0.016 GHz in EuAl3(BO3)4:Tb crystals and gz = 17.664 ± 0.011, Az = 6.251 ± 0.012 GHz, Δ = 3.891 ± 0.035 GHz in EuGa3(BO3)4:Tb crystals. The temperature dependence of the EPR linewidth for Tb3+ ions in EuAl3(BO3)4:Tb and EuGa3(BO3)4:Tb crystals was described by an exponential law that is characteristic of the Aminov-Orbach processes. It was concluded that the Tb3+ ions substitute for the three-valence europium ones in EuGa3(BO3)4:Tb/EuAl3(BO3)4:Tb crystal lattice.
•The temperature variation of volume thermal expansion coefficients was determined.•The Tb3+ ions substitute for the three-valence europium.•The Aminov-Orbach relaxation process.
ABSTRACT
We present a search for Galactic transient γ-ray sources using 13 yr of the Fermi Large Area Telescope data. The search is based on a recently developed variable-size sliding-time-window ...(VSSTW) analysis and aimed at studying variable γ-ray emission from binary systems, including novae, γ-ray binaries, and microquasars. Compared to the previous search for transient sources at random positions in the sky with 11.5 yr of data, we included γ-rays with energies down to 500 MeV, increased a number of test positions, and extended the data set by adding data collected between 2020 February and 2021 July. These refinements allowed us to detect additional three novae, V1324 Sco, V5855 Sgr, V357 Mus, and one γ-ray binary, PSR B1259-63, with the VSSTW method. Our search revealed a γ-ray flare from the microquasar, Cygnus X-3, occurred in 2020. When applied to equal quarters of the data, the analysis provided us with detections of repeating signals from PSR B1259-63, LS I +61°303, PSR J2021+4026, and Cygnus X-3. While the Cygnus X-3 was bright in γ-rays in mid-2020, it was in a soft X-ray state and we found that its γ-ray emission was modulated with the orbital period.
We present new observations of Fornax A taken at ∼1 GHz with the MeerKAT telescope and at ∼6 GHz with the Sardinia Radio Telescope (SRT). The sensitive (noise ∼16
μ
Jy beam
−1
), high-resolution ...(≲10″) MeerKAT images show that the lobes of Fornax A have a double-shell morphology, where dense filaments are embedded in a diffuse and extended cocoon. We study the spectral properties of these components by combining the MeerKAT and SRT observations with archival data between 84 MHz and 217 GHz. For the first time, we show that multiple episodes of nuclear activity must have formed the extended radio lobes. The modelling of the radio spectrum suggests that the last episode of injection of relativistic particles into the lobes started ∼24 Myr ago and stopped 12 Myr ago. More recently (∼3 Myr ago), a less powerful and short (≲1 Myr) phase of nuclear activity generated the central jets. Currently, the core may be in a new active phase. It appears that Fornax A is rapidly flickering. The dense environment around Fornax A has lead to a complex recent merger history for this galaxy, including mergers spanning a range of gas contents and mass ratios, as shown by the analysis of the galaxy’s stellar- and cold-gas phases. This complex recent history may be the cause of the rapid, recurrent nuclear activity of Fornax A.
We develop a method to search for pair haloes around active galactic nuclei (AGN) through a temporal analysis of γ-ray data. The basis of our method is an analysis of the spatial distributions of ...photons coming from AGN flares and from AGN quiescent states and a further comparison of these two spatial distributions. This method can also be used for a reconstruction of a point spread function (PSF). We found no evidence for a pair halo component through this method by applying it to the Fermi-Large Area Telescope (LAT) data in the energy bands of 4.5–6, 6–10, and >10 GeV and set upper limits on the fraction of photons attributable to a pair halo component. An illustration of how to reconstruct the PSF of Fermi-LAT is given. We demonstrate that the PSF reconstructed by using this method is in good agreement with that which was obtained by using the γ-ray data taken by LAT in the direction of the Crab pulsar and nebula.
Emission from active galactic nuclei (AGNs) and from neutral pion decay are the two most natural mechanisms that could establish a galaxy cluster as a source of gamma rays in the GeV regime. We ...revisit this problem by using 52.5 months of Fermi-LAT data above 10 GeV and stacking 55 clusters from the HIFLUCGS sample of the X-ray brightest clusters. The choice of >10 GeV photons is optimal from the point of view of angular resolution, while the sample selection optimizes the chances of detecting signatures of neutral pion decay, arising from hadronic interactions of relativistic protons with an intracluster medium, which scale with the X-ray flux. In the stacked data we detected a signal for the central 0.25 deg circle at the level of 4.3σ. Evidence for a spatial extent of the signal is marginal. A subsample of cool-core clusters has a higher count rate of 1.9 ± 0.3 per cluster compared to the subsample of non-cool core clusters at 1.3 ± 0.2. Several independent arguments suggest that the contribution of AGNs to the observed signal is substantial, if not dominant. No strong support for the large contribution of pion decay was found. In terms of a limit on the relativistic proton energy density, we derived an upper limit of ≃2% relative to the gas thermal energy density, provided that the spectrum of relativistic protons is hard (s = 4.1 in dN ∝ p−sd3p). This estimate assumes that relativistic and thermal components are mixed. For softer spectra the upper limits are weaker and equal to 3% for s = 4.2, 4% for s = 4.3, and 6% for s = 4.4.
We analyse the data from the Fermi-Large Area Telescope in order to search for a ring-like γ-ray structure around the Coma cluster. The ring-like structure has recently been suggested to be detected ...with VERITAS at energies higher than 220 GeV and could possibly be associated with an accretion shock. Our analysis of the Fermi data is performed at energies >100 MeV and we find no detection of this structure in the Fermi data. We derive the 95 per cent upper limit on the flux from the region covering the proposed ring-like structure. The derived upper limit on the flux at >100 MeV cannot be incorporated with the detection of an accretion shock wave around Coma at the significance of 4.5σ by VERITAS at very high energies, if the production mechanism of the γ-ray emission generates a photon spectrum with a power index of 2 in the broad energy band. The model of γ-ray emission induced by ultrahigh-energy protons can reconcile the results of the VERITAS and Fermi observations.
ABSTRACT
One of the unsolved questions in γ-ray astronomy is whether the extragalactic γ-ray background is of the discrete-source origin. To respond to this question, one first needs to reduce the ...data by differentiating charged particles from γ-rays. This procedure is usually performed on the basis of the detector responses. In this paper, we showed that the geomagnetic shielding effect at GeV energies can, to some extent, be used for this purpose for γ-ray telescopes in a low Earth orbit. We illustrated this method by applying it to the Fermi Large Area Telescope data. To partially decompose the charge-filtered background, we examined the contribution from star-forming galaxies by implying a radio/γ-ray connection in consideration of next-generation radio surveys.
ABSTRACT
We present a systematic search for γ-ray emission from supernovae (SNe) in the Fermi Large Area Telescope (LAT) Pass 8 data. The sample of targets consists of 55,880 candidates from the Open ...Supernova Catalogue. We searched for γ rays from SNe by means of a variable-size sliding-time-window analysis. Our results confirm the presence of transient γ-ray emission from the sources of non-AGN classes, including transitional pulsars, solar flares, γ-ray bursts, novae, and the Crab Nebula, which are projected near some of these SN’s positions, and also strengthen support to the variable signal in the direction of SN iPTF14hls. The analysis is successful in finding both short (e.g. solar flares) and long (e.g. transitional pulsars) high flux states. Our search reveals two new γ-ray transient signals occurred in 2019 in the directions of optical transients that are SN candidates, AT2019bvr and AT2018iwp, with their flux increases within 6 months after the dates of SN’s discoveries. These signals are bright and their variability is at a higher statistical level than that of iPTF14hls. An exploration of archival multiwavelength observations towards their positions is necessary to establish their association with SNe or other classes of sources. Our analysis, in addition, shows a bright transient γ-ray signal at low Galactic latitudes in the direction of PSR J0205+6449. In addition, we report the results of an all-sky search for γ-ray transient sources. This provided two additional candidates to gamma-ray transient sources.
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