(Abridged) The Large Area Telescope (Fermi/LAT, hereafter LAT), the primary instrument on the Fermi Gamma-ray Space Telescope (Fermi) mission, is an imaging, wide field-of-view, high-energy gamma-ray telescope, covering the energy range from below 20 MeV to more than 300 GeV. This paper describes the LAT, its pre-flight expected performance, and summarizes the key science objectives that will be addressed. On-orbit performance will be presented in detail in a subsequent paper. The LAT is a pair-conversion telescope with a precision tracker and calorimeter, each consisting of a 4x4 array of 16 modules, a segmented anticoincidence detector that covers the tracker array, and a programmable trigger and data acquisition system. Each tracker module has a vertical stack of 18 x,y tracking planes, including two layers (x and y) of single-sided silicon strip detectors and high-Z converter material (tungsten) per tray. Every calorimeter module has 96 CsI(Tl) crystals, arranged in an 8 layer hodoscopic configuration with a total depth of 8.6 radiation lengths. The aspect ratio of the tracker (height/width) is 0.4 allowing a large field-of-view (2.4 sr). Data obtained with the LAT are intended to (i) permit rapid notification of high-energy gamma-ray bursts (GRBs) and transients and facilitate monitoring of variable sources, (ii) yield an extensive catalog of several thousand high-energy sources obtained from an all-sky survey, (iii) measure spectra from 20 MeV to more than 50 GeV for several hundred sources, (iv) localize point sources to 0.3 - 2 arc minutes, (v) map and obtain spectra of extended sources such as SNRs, molecular clouds, and nearby galaxies, (vi) measure the diffuse isotropic gamma-ray background up to TeV energies, and (vii) explore the discovery space for dark matter.

The Large Area Telescope on the Fermi Gamma-ray Space Telescope Mission

On 2017 August 17, the gravitational-wave event GW170817 was observed by the Advanced LIGO and Virgo detectors, and the gamma-ray burst (GRB) GRB 170817A was observed independently by the Fermi Gamma-ray Burst Monitor, and the Anti-Coincidence Shield for the Spectrometer for the International Gamma-Ray Astrophysics Laboratory. The probability of the near-simultaneous temporal and spatial observation of GRB 170817A and GW170817 occurring by chance is $5.0\times {10}^{-8}$. We therefore confirm binary neutron star mergers as a progenitor of short GRBs. The association of GW170817 and GRB 170817A provides new insight into fundamental physics and the origin of short GRBs. We use the observed time delay of $(+1.74\pm 0.05)\,{\rm{s}}$ between GRB 170817A and GW170817 to: (i) constrain the difference between the speed of gravity and the speed of light to be between $-3\times {10}^{-15}$ and $+7\times {10}^{-16}$ times the speed of light, (ii) place new bounds on the violation of Lorentz invariance, (iii) present a new test of the equivalence principle by constraining the Shapiro delay between gravitational and electromagnetic radiation. We also use the time delay to constrain the size and bulk Lorentz factor of the region emitting the gamma-rays. GRB 170817A is the closest short GRB with a known distance, but is between 2 and 6 orders of magnitude less energetic than other bursts with measured redshift. A new generation of gamma-ray detectors, and subthreshold searches in existing detectors, will be essential to detect similar short bursts at greater distances. Finally, we predict a joint detection rate for the Fermi Gamma-ray Burst Monitor and the Advanced LIGO and Virgo detectors of 0.1–1.4 per year during the 2018–2019 observing run and 0.3–1.7 per year at design sensitivity.

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Gravitational Waves and Gamma-Rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A

We describe new optically thin solutions for rotating accretion flows around black holes and neutron stars. These solutions are advection dominated, so that most of the viscously dissipated energy is advected radially with the flow. We model the accreting gas as a two temperature plasma and include cooling by bremsstrahlung, synchrotron, and Comptonization. We obtain electron temperatures $T_e\sim 10^{8.5}-10^{10}$K. The new solutions are present only for mass accretion rates $\dot M$ less than a critical rate $\dot M_{crit}$ which we calculate as a function of radius $R$ and viscosity parameter $\alpha$. For $\dot M<\dot M_{crit}$ we show that there are three equilibrium branches of solutions. One of the branches corresponds to a cool optically thick flow which is the well-known thin disk solution of Shakura \& Sunyaev (1973). Another branch corresponds to a hot optically thin flow, discovered originally by Shapiro, Lightman \& Eardley (1976, SLE). This solution is thermally unstable. The third branch corresponds to our new advection-dominated solution. This solution is hotter and more optically thin than the SLE solution, but is viscously and thermally stable. It is related to the ion torus model of Rees et al. (1982) and may potentially explain the hard X-ray and $\gamma$-ray emission from X-ray binaries and active galactic nuclei.

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Advection-Dominated Accretion: Underfed Black Holes and Neutron Stars

The second catalog of active galactic nuclei (AGNs) detected by the Fermi Large Area Telescope (LAT) in two years of scientific operation is presented. The second LAT AGN catalog (2LAC) includes 1017 γ-ray sources located at high Galactic latitudes (|b| > 10°) that are detected with a test statistic (TS) greater than 25 and associated statistically with AGNs. However, some of these are affected by analysis issues and some are associated with multiple AGNs. Consequently, we define a Clean Sample which includes 886 AGNs, comprising 395 BL Lacertae objects (BL Lac objects), 310 flat-spectrum radio quasars (FSRQs), 157 candidate blazars of unknown type (i.e., with broadband blazar characteristics but with no optical spectral measurement yet), 8 misaligned AGNs, 4 narrow-line Seyfert 1 (NLS1s), 10 AGNs of other types, and 2 starburst galaxies. Where possible, the blazars have been further classified based on their spectral energy distributions (SEDs) as archival radio, optical, and X-ray data permit. While almost all FSRQs have a synchrotron-peak frequency 1015 Hz. The 2LAC represents a significant improvement relative to the first LAT AGN catalog (1LAC), with 52% more associated sources. The full characterization of the newly detected sources will require more broadband data. Various properties, such as γ-ray fluxes and photon power-law spectral indices, redshifts, γ-ray luminosities, variability, and archival radio luminosities and their correlations are presented and discussed for the different blazar classes. The general trends observed in 1LAC are confirmed.

https://iopscience.iop.org/article/10.1088/0004-637X/743/2/171/pdf

The second catalog of active galactic nuclei detected by the Fermi Large Area Telescope

The physics of gamma-ray bursts & relativistic jets

We are presenting a detailed parameter study of the time-dependent electron injection and kinematics and the self-consistent radiation transport in jets of intermediate and low-frequency peaked BL Lac objects. Using a time-dependent, combined synchrotron-self-Compton and external-Compton jet model, we study the influence of variations of several essential model parameters, such as the electron injection compactness, the relative contribution of synchrotron to external soft photons to the soft photon compactness, the electron- injection spectral index, and the details of the time profiles of the electron injection episodes giving rise to flaring activity. In the analysis of our results, we focus on the expected X-ray spectral variability signatures in a region of parameter space particularly well suited to reproduce the broadband spectral energy distributions of intermediate and low-frequency peaked BL Lac objects. We demonstrate that SSC- and external-Compton dominated models for the gamma-ray emission from blazars are producing significantly different signatures in the X-ray variability, in particular in the soft X-ray light curves and the spectral hysteresis at soft X-ray energies, which can be used as a powerful diagnostic to unveil the nature of the high-energy emission from BL Lac objects.

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X-Ray Spectral Variability Signatures of Flares in BL Lacertae Objects

We investigate the dynamics and radiation from a relativistic blast wave that decelerates as it sweeps up ambient matter. The bulk kinetic energy of the blast-wave shell is converted into internal energy by the process of accreting external matter. If it takes the form of nonthermal electrons and magnetic fields, then this internal energy will be emitted as synchrotron and synchrotron self-Compton radiation. We perform analytic and numerical calculations for the deceleration and radiative processes and present time-resolved spectra throughout the evolution of the blast wave. We also examine the dependence of the burst spectra and light curves on various parameters describing the magnetic field and nonthermal electron distributions. We find that for bursts such as GRB 910503, GRB 910601, and GRB 910814, the spectral shapes of the prompt gamma-ray emission at the peaks in νFν strongly constrain the magnetic fields in these bursts to be well below (10-2) the equipartition values. These calculations are also considered in the context of the afterglow emission from the recently detected gamma-ray burst counterparts.

Synchrotron and Synchrotron Self-Compton Emission and the Blast-Wave Model of Gamma-Ray Bursts

A simple function for the spectral power P(,t)≡νL(ν) is proposed to model, with nine parameters, the spectral and temporal evolution of the observed nonthermal synchrotron power flux from gamma-ray bursts (GRBs) in the blast-wave model. Here =hν/mec2 is the observed dimensionless photonenergy, and t is the observing time. Assumptions and an issue of lack of self-consistency are spelled out. The spectra are found to be most sensitive to baryon loading, expressed in terms of the initial bulk Lorentz factor Γ0 and an equipartition term q, which is assumed to be constant in time and independent of Γ0. Expressions are given for the peak spectral power Pp(t)=P(p,t) at the photon energy =p(t) of the spectral power peak. A general rule is that the total fireball particle kinetic energy E0~Π0td, where tdΓ is the deceleration timescale, and Π0≡P(p,td)Γ is the maximum measured bolometric power output in radiation during which it is carried primarily by photons with energy 0=p(td)qΓ40. This rule governs the general behavior of fireballs with different baryon loading. Clean fireballs with small baryon loading (Γ0300) are intense, subsecond, medium-to-high-energy gamma-ray events and are difficult to detect because of dead time and sensitivity limitations of previous gamma-ray detectors like EGRET on the Compton Gamma Ray Observatory. Dirty fireballs with large baryon loading (Γ0300) produce transient emissions that are longer lasting and most luminous at X-ray energies and below, but these events are lost behind the glow of the X-ray and lower energy background radiations, except for rare serendipitous detections by pointed instruments. The correlation between hardness and duration of loaded GRB fireballs (100Γ01000) follows from this rule.

Fireball Loading and the Blast-Wave Model of Gamma-Ray Bursts

We examine the properties of the gamma-ray active galactic nuclei (AGNs) detected by EGRET. We calculate the evolution and luminosity function of the gamma-ray-loud AGNs, and we estimate the contribution this source class makes to the diffuse extragalactic gamma-ray background. These calculations are based on a sample of EGRET observations complete through the Compton Gamma-Ray Observatory observing Cycle 4. We use the Ve/Va test to measure the luminosity evolution and the nonparametric c--method to estimate the de-evolved luminosity function. We also consider inclusion in the 1 Jy radio catalog of Kuhr et al. (1981) as an additional criterion to help guard against selection effects resulting from incomplete redshift determinations for AGNs in our sample. Our results include evidence for a low-luminosity cutoff in the gamma-ray AGN luminosity distribution, a more precise estimate of the contribution of these objects to the diffuse extragalactic gamma-ray background, and constraints on the gamma-ray production mechanism based upon the statistics implied by relativistic beaming of the emission. We discuss the implications of these findings for models of AGN unification and the relationship of gamma-ray to radio emission.

The Luminosity Function of the EGRET Gamma-Ray Blazars

A simple function for the spectral power $P(\epsilon,t) \equiv \nu L(\nu) $ is proposed to model, with 9 parameters, the spectral and temporal evolution of the observed nonthermal synchrotron power flux from GRBs in the blast wave model. Here $\epsilon = h\nu/$m$_e$c$^2$ is the observed dimensionless photon energy and $t$ is the observing time. Assumptions and an issue of lack of self-consistency are spelled out. The spectra are found to be most sensitive to the baryon loading, expressed in terms of the initial bulk Lorentz factor $\Gamma_0$, and an equipartition term $q$ which is assumed to be constant in time and independent of $\Gamma_0$. Expressions are given for the peak spectral power $P_p(t) = P(\epsilon_p,t)$ at the photon energy $\epsilon = \epsilon_p(t)$ of the spectral power peak. A general rule is that the total fireball particle kinetic energy $E_0 \sim \Pi_0 t_d$, where $t_d \propto \Gamma_0^{-8/3}$ is the deceleration time scale and $\Pi_0 \equiv P(\epsilon_p,t_d) \propto \Gamma_0^{8/3}$ is the maximum measured bolometric power output in radiation, during which it is carried primarily by photons with energy ${\cal E}_0 = \epsilon_p(t_d) \propto q\Gamma_0^4$.

James Chiang

Papers

X-Ray Spectral Variability Signatures of Flares in BL Lacertae Objects

Synchrotron and Synchrotron Self-Compton Emission and the Blast-Wave Model of Gamma-Ray Bursts

Fireball Loading and the Blast-Wave Model of Gamma-Ray Bursts

The Luminosity Function of the EGRET Gamma-Ray Blazars

Fireballs Loading and the Blast Wave Model of Gamma Ray Bursts