1,721,198 research outputs found
A gamma-ray pulsar timing array constrains the nanohertz gravitational wave background
No full textAfter 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 t obs , the observing time span, as t obs − 13 / 6 . This direct measurement provides an independent probe of the GWB while offering a check on radio noise models.A gamma-ray pulsar timing array After galaxies merge, the supermassive black holes (SMBHs) at their centers are expected to form binaries that emit gravitational waves at nanohertz frequencies. Numerous SMBH binaries throughout the Universe should combine to produce a gravitational wave background. Existing searches for this signal use radio observations of pulsars as sensitive clocks and look for small shifts in the pulse timings. The Fermi-LAT Collaboration implemented a pulsar timing array using gamma rays and achieved a sensitivity close to that of the radio approaches. The results set an independent upper limit on the gravitational wave background, which is subject to different noise sources. —KTSA pulsar timing array is implemented using gamma rays, setting a limit on the nanohertz gravitational wave background
Dark matter implications of Fermi-LAT measurement of anisotropies in the diffuse gamma-ray background
No full textThe detailed origin of the diffuse gamma-ray background is still unknown. However, the contribution of unresolved sources is expected to induce small-scale anisotropies in this emission, which may provide a way to identify and constrain the properties of its contributors. Recent studies have predicted the contributions to the angular power spectrum (APS) from extragalactic and galactic dark matter (DM) annihilation or decay. The Fermi-LAT collaboration reported detection of angular power with a significance larger than 3 sigma in the energy range from 1 GeV to 10 GeV on 22 months of data (Ackermann et al., 2012 [2]). For these preliminary results the already published Fermi-LAT APS measurements (Ackermann et al., 2012 [2]) are compared to the accurate predictions for DM anisotropies from state-of-the-art cosmological simulations as presented in Fornasa et at (2013) [1] to derive constraints on different DM candidates.</p
Temporal properties of bright BGO GRBs detected by Fermi
No full textWe present results of an analysis of a sample of bright Gamma-Ray Bursts
(GRBs) detected by Fermi-GBM up to more than 1 MeV, which were collected during
six years of Fermi operations. In particular, we focus on the GRB durations
over several energy bands of the prompt emission of a subsample of bright GRBs
detected up to 10 MeV by GBM and, when possible, up to 1 GeV by Fermi-LAT, thus
expanding the Duration-Energy relationship in GRB light curves to high energies
for the first time. We find that the relationship for these energetic GRBs is
flatter than reported for other samples, suggesting that the high- and
low-energy emission mechanisms are closely related
High-energy emission from GRBs: 10 years with Fermi-LAT
Full text linkIn 2018, the Fermi mission celebrated its first decade of operation. In this
time, the Large Area Telescope (LAT) has been very successful in detecting the
high-energy emission (>100 MeV) from Gamma-Ray Bursts (GRBs). The analysis of
particularly remarkable events - such as GRB 080916C, GRB 090510 and GRB
130427A - has been presented in dedicated publications. Here we present the
results of a new systematic search for high-energy emission from the full
sample of GRBs detected in 10 years by the Fermi Gamma-Ray Burst Monitor, as
well as Swift, AGILE, Integral and IPN bursts, featuring a detection efficiency
more than 50% better than previous works, and returning 186 detections during
10 years of LAT observations. This milestone marks a vast improvement from the
35 events contained in the first LAT GRB catalog (covering the first 3 years of
Fermi operations). We assess the characteristics of the GRB population at high
energy with unprecedented sensitivity, covering aspects such as temporal
properties, energetics and spectral index of the high-energy emission. Finally,
we show how the LAT observations can be used to inform theory, in particular
the prospects for very high-energy emission
Determination of the Point-spread Function for the Fermi Large Area Telescope from On-orbit Data and Limits on Pair Halos of Active Galactic Nuclei
Full text linkThe Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope is a pair-conversion telescope designed to detect photons with energies from ≈20 MeV to >300 GeV. The pre-launch response functions of the LAT were determined through extensive Monte Carlo simulations and beam tests. The point-spread function (PSF) characterizing the angular distribution of reconstructed photons as a function of energy and geometry in the detector is determined here from two years of on-orbit data by examining the distributions of γ rays from pulsars and active galactic nuclei (AGNs). Above 3 GeV, the PSF is found to be broader than the pre-launch PSF. We checked for dependence of the PSF on the class of γ-ray source and observation epoch and found none. We also investigated several possible spatial models for pair-halo emission around BL Lac AGNs. We found no evidence for a component with spatial extension larger than the PSF and set upper limits on the amplitude of halo emission in stacked images of low- and high-redshift BL Lac AGNs and the TeV blazars 1ES0229+200 and 1ES0347?121.Fil: The Fermi-lat Collaboration. University Of Stanford. Physics Department;Fil: Ackermann, M..Fil: Cillis, Analia Nilda. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio(i); Argentin
Fermi Large Area Telescope Fourth Source Catalog
No full textWe present the fourth Fermi Large Area Telescope catalog (4FGL) of gamma-ray sources. Based on the first eight years of science data from the Fermi Gamma-ray Space Telescope mission in the energy range from 50 MeV to 1 TeV, it is the deepest yet in this energy range. Relative to the 3FGL catalog, the 4FGL catalog has twice as much exposure as well as a number of analysis improvements, including an updated model for the Galactic diffuse gamma-ray emission, and two sets of light curves (1-year and 2-month intervals). The 4FGL catalog includes 5064 sources above 4 sigma significance, for which we provide localization and spectral properties. Seventy-five sources are modeled explicitly as spatially extended, and overall 358 sources are considered as identified based on angular extent, periodicity or correlated variability observed at other wavelengths. For 1336 sources we have not found plausible counterparts at other wavelengths. More than 3130 of the identified or associated sources are active galaxies of the blazar class, and 239 are pulsars
Fermi LAT observation of quiet gamma-ray emission from the Sun and first solar flares detection
No full textWe show the latest results of Fermi-LAT observations of the quiescent Sun during the first 18 months of the mission. During this period the solar activity was at its minimum, hence the solar emission induced by cosmic rays was at its maximum. Two emission components are clearly distinguished: the point-like emission from the solar disk due to the cosmic-ray cascades in the solar atmosphere, and the extended emission due to inverse Compton scattering of cosmic ray electrons on solar photons in the heliosphere. We present the entire analysis, showing spectra and angular profiles of both components and discuss the comparison with models and future plans. Finally we report on Fermi Large Area Telescope (LAT) detection of the first solar flares and discussing the possible emission mechanisms
A gamma-ray determination of the Universe’s star formation history
No full textThe light emitted by all galaxies over the history of the Universe produces the extragalactic background light (EBL) at ultraviolet, optical, and infrared wavelengths. The EBL is a source of opacity for gamma rays via photon-photon interactions, leaving an imprint in the spectra of distant gamma-ray sources. We measured this attenuation using 739 active galaxies and one gamma-ray burst detected by the Fermi Large Area Telescope. This allowed us to reconstruct the evolution of the EBL and determine the star formation history of the Universe over 90% of cosmic time. Our star formation history is consistent with independent measurements from galaxy surveys, peaking at redshift z ~ 2. Upper limits of the EBL at the epoch of reionization suggest a turnover in the abundance of faint galaxies at z ~ 6.link_to_subscribed_fulltex
High-energy emission from a magnetar giant flare in the Sculptor galaxy
No full textMagnetars are the most highly magnetized neutron stars in the cosmos (with magnetic field 1013–1015 G). Giant flares from magnetars are rare, short-duration (about 0.1 s) bursts of hard X-rays and soft γ rays1,2. Owing to the limited sensitivity and energy coverage of previous telescopes, no magnetar giant flare has been detected at gigaelectronvolt (GeV) energies. Here, we report the discovery of GeV emission from a magnetar giant flare on 15 April 2020 (refs. 3,4 and A. J. Castro-Tirado et al., manuscript in preparation). The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope detected GeV γ rays from 19 s until 284 s after the initial detection of a signal in the megaelectronvolt (MeV) band. Our analysis shows that these γ rays are spatially associated with the nearby (3.5 megaparsecs) Sculptor galaxy and are unlikely to originate from a cosmological γ-ray burst. Thus, we infer that the γ rays originated with the magnetar giant flare in Sculptor. We suggest that the GeV signal is generated by an ultra-relativistic outflow that first radiates the prompt MeV-band photons, and then deposits its energy far from the stellar magnetosphere. After a propagation delay, the outflow interacts with environmental gas and produces shock waves that accelerate electrons to very high energies; these electrons then emit GeV γ rays as optically thin synchrotron radiation. This observation implies that a relativistic outflow is associated with the magnetar giant flare, and suggests the possibility that magnetars can power some short γ-ray bursts.link_to_subscribed_fulltex
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