arXiv subjects

Justin D. Finke

Publications and source records attributed to Justin D. Finke.

At least 37 records · Page 2Linked to original sources

arXiv2016

Multiband Observations of the Quasar PKS 2326-502 during Active and Quiescent Gamma-Ray States in 2010-2012

Quasi-simultaneous observations of the Flat Spectrum Radio Quasar PKS 2326-502 were carried out in the gamma-ray, X-ray, UV, optical, near-infrared, and radio bands. Thanks to these observations we are able to characterize the spectral energy distribution of the source during two flaring and one quiescent gamma-ray states. These data were used to constrain one-zone leptonic models of the spectral energy distributions of each flare and investigate the physical conditions giving rise to them. While modeling one flare only required changes to the electron spectrum, the other flare needed changes in both the electron spectrum and the size of the emitting region with respect to the quiescent state. These results are consistent with an emerging pattern of two broad classes of flaring states seen in blazars.

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arXiv2016

External Compton Scattering in Blazar Jets and the Location of the Gamma-Ray Emitting Region

I study the location of the $γ$-ray emission in blazar jets by creating a Compton-scattering approximation valid for all anisotropic radiation fields in the Thomson through Klein-Nishina regimes, which is highly accurate and can speed up numerical calculations by up to a factor $\sim10$. I apply this approximation to synchrotron self-Compton, and external Compton-scattering of photons from the accretion disk, broad-line region (BLR), and dust torus. I use a stratified BLR model and include detailed Compton-scattering calculations of a spherical and flattened BLR. I create two dust torus models, one where the torus is an annulus, and one where it is an extended disk. I present detailed calculations of the photoabsorption optical depth using my detailed BLR and dust torus models, including the full angle dependence. I apply these calculations to the emission from a relativistically moving blob traveling through these radiation fields. The ratio of $γ$-ray to optical flux produces a predictable pattern that could help locate the $γ$-ray emission region. I show that the bright flare from 3C 454.3 in 2010 November detected by the Fermi Large Area Telescope is unlikely to originate from a single blob inside the BLR since it moves outside the BLR in a time shorter than the flare duration, although emission by multiple blobs inside the BLR is possible; and $γ$-rays are unlikely to originate from outside the BLR from scattering of photons from an extended dust torus, since then the cooling timescale would be too long to explain the observed short variability.

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arXiv2016

Time-Dependent Electron Acceleration in Blazar Transients: X-Ray Time Lags and Spectral Formation

Electromagnetic radiation from blazar jets often displays strong variability, extending from radio to $γ$-ray frequencies. In a few cases, this variability has been characterized using Fourier time lags, such as those detected in the X-rays from Mrk~421 using BeppoSAX. The lack of a theoretical framework to interpret the data has motivated us to develop a new model for the formation of the X-ray spectrum and the time lags in blazar jets based on a transport equation including terms describing stochastic Fermi acceleration, synchrotron losses, shock acceleration, adiabatic expansion, and spatial diffusion. We derive the exact solution for the Fourier transform of the electron distribution, and use it to compute the Fourier transform of the synchrotron radiation spectrum and the associated X-ray time lags. The same theoretical framework is also used to compute the peak flare X-ray spectrum, assuming that a steady-state electron distribution is achieved during the peak of the flare. The model parameters are constrained by comparing the theoretical predictions with the observational data for Mrk~421. The resulting integrated model yields, for the first time, a complete first-principles physical explanation for both the formation of the observed time lags and the shape of the peak flare X-ray spectrum. It also yields direct estimates of the strength of the shock and the stochastic MHD wave acceleration components in the Mrk~421 jet.

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arXiv2015

Constraints on the Intergalactic Magnetic Field with Gamma-Ray Observations of Blazars

Distant BL Lacertae objects emit $γ$ rays which interact with the extragalactic background light (EBL), creating electron-positron pairs, and reducing the flux measured by ground-based imaging atmospheric Cherenkov telescopes (IACTs) at very-high energies (VHE). These pairs can Compton-scatter the cosmic microwave background, creating a $γ$-ray signature at slightly lower energies observable by the \fermi\ Large Area Telescope (LAT). This signal is strongly dependent on the intergalactic magnetic field (IGMF) strength ($B$) and its coherence length ($L_B$). We use IACT spectra taken from the literature for 5 VHE-detected BL Lac objects, and combine it with LAT spectra for these sources to constrain these IGMF parameters. Low $B$ values can be ruled out by the constraint that the cascade flux cannot exceed that observed by the LAT. High values of $B$ can be ruled out from the constraint that the EBL-deabsorbed IACT spectrum cannot be greater than the LAT spectrum extrapolated into the VHE band, unless the cascade spectrum contributes a sizable fraction of the LAT flux. We rule out low $B$ values ($B< 10^{-19}$ G for $L_B\ge1$\ Mpc) at $>5σ$ in all trials with different EBL models and data selection, except when using $>1$ GeV spectra and the lowest EBL models. We were not able to constrain high values of $B$.

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arXiv2015

Near-Equipartition Jets with Log-Parabola Electron Energy Distribution and the Blazar Spectral-Index Diagrams

Fermi-LAT analyses show that the gamma-ray photon spectral indices Gamma_gamma of a large sample of blazars correlate with the vFv peak synchrotron frequency v_s according to the relation Gamma_gamma = d - k log v_s. The same function, with different constants d and k, also describes the relationship between Gamma_gamma and peak Compton frequency v_C. This behavior is derived analytically using an equipartition blazar model with a log-parabola description of the electron energy distribution (EED). In the Thomson regime, k = k_EC = 3b/4 for external Compton processes and k = k_SSC = 9b/16 for synchrotron self-Compton (SSC) processes, where b is the log-parabola width parameter of the EED. The BL Lac object Mrk 501 is fit with a synchrotron/SSC model given by the log-parabola EED, and is best fit away from equipartition. Corrections are made to the spectral-index diagrams for a low-energy power-law EED and departures from equipartition, as constrained by absolute jet power. Analytic expressions are compared with numerical values derived from self-Compton and external Compton scattered gamma-ray spectra from Ly alpha broad-line region and IR target photons. The Gamma_gamma vs. v_s behavior in the model depends strongly on b, with progressively and predictably weaker dependences on gamma-ray detection range, variability time, and isotropic gamma-ray luminosity. Implications for blazar unification and blazars as ultra-high energy cosmic-ray sources are presented. Arguments by Ghisellini et al. (2014) that the jet power exceeds the accretion luminosity depend on the doubtful assumption that we are viewing at the Doppler angle.

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arXiv2015

Fourier Analysis of Blazar Variability: Klein-Nishina Effects and the Jet Scattering Environment

The strong variability of blazars can be characterized by power spectral densities (PSDs) and Fourier frequency-dependent time lags. In previous work, we created a new theoretical formalism for describing the PSDs and time lags produced via a combination of stochastic particle injection and emission via the synchrotron, synchrotron self-Compton, and external Compton (EC) processes. This formalism used the Thomson cross section and simple $δ$-function approximations to model the synchrotron and Compton emissivities. Here we expand upon this work, using the full Compton cross section and detailed and accurate emissivities. Our results indicate good agreement between the PSDs computed using the $δ$-function approximations and those computed using the accurate expressions, provided the observed photons are produced primarily by electrons with energies exceeding the lower limit of the injected particle population. Breaks are found in the PSDs at frequencies corresponding to the cooling timescales of the electrons primarily responsible for the observed emission, and the associated time lags are related to the difference in electron cooling timescales between the two energy channels, as expected. If the electron cooling timescales can be determined from the observed time lags and/or the observed EC PSDs, then one could in principle use the method developed here to determine the energy of the external seed photon source for EC, which is an important unsolved problem in blazar physics.

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arXiv2014

Fourier Analysis of Blazar Variability

Blazars display strong variability on multiple timescales and in multiple radiation bands. Their variability is often characterized by power spectral densities (PSDs) and time lags plotted as functions of the Fourier frequency. We develop a new theoretical model based on the analysis of the electron transport (continuity) equation, carried out in the Fourier domain. The continuity equation includes electron cooling and escape, and a derivation of the emission properties includes light travel time effects associated with a radiating blob in a relativistic jet. The model successfully reproduces the general shapes of the observed PSDs and predicts specific PSD and time lag behaviors associated with variability in the synchrotron, synchrotron self-Compton (SSC), and external Compton (EC) emission components, from sub-mm to gamma-rays. We discuss applications to BL Lacertae objects and to flat-spectrum radio quasars (FSRQs), where there are hints that some of the predicted features have already been observed. We also find that FSRQs should have steeper PSD power-law indices than BL Lac objects at Fourier frequencies < 10^{-4} Hz, in qualitative agreement with previously reported observations by the Fermi Large Area Telescope.

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arXiv2013

A Physical Model for the Revised Blazar Sequence

The blazar sequence is reflected in a correlation of the peak luminosity versus peak frequency of the synchrotron component of blazars. This correlation has been considered one of the fundamental pieces of evidence for the existence of a continuous sequence that includes low-power BL Lacertae objects through high-power flat spectrum radio quasars. A correlation between the Compton domi- nance, the ratio of the Compton to synchrotron luminosities, and the peak synchrotron frequency is another piece of evidence for the existence of the blazar sequence explored by Fossati et al. (1998). Since that time, however, it has essentially been ignored. We explore this correlation with a sample based on the second LAT AGN catalog (2LAC), and show that is is particularly important, since it is independent of redshift. We reproduce the trends in our sample with a simple model that in- cludes synchrotron and Compton cooling in the slow- and fast-cooling regimes, and angle-dependent effects.

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arXiv2012

Cosmic Ray Electron Evolution in the Supernova Remnant RX J1713.7-3946

A simple formalism to describe nonthermal electron acceleration, evolution, and radiation in supernova remnants (SNRs) is presented. The electron continuity equation is analytically solved assuming that the nonthermal electron injection power is proportional to the rate at which the kinetic energy of matter swept up in an adiabatically expanding SNR shell. We apply this model to \fermi\ and HESS data from the SNR \rxj, and find that a one-zone leptonic model with Compton-scattered cosmic microwave background (CMB) and interstellar infrared photons has difficulty providing a good fit to its spectral energy distribution, provided the source is at a distance $\sim 1\ \kpc$ from the Earth. However, the inclusion of multiple zones, as hinted at by recent {\em Chandra} observations, does provide a good fit, but requires a second zone of compact knots with magnetic fields $B\sim 16\ μ$G, comparable to shock-compressed fields found in the bulk of the remnant.

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arXiv2011

Lower limits on ultrahigh-energy cosmic ray and jet powers of TeV blazars

Lower limits on the power emitted in ultrahigh-energy cosmic rays (UHECRs), which are assumed to be protons with energy 1e17-1e20 eV, are derived for TeV blazars with the assumption that the observed TeV gamma rays are generated due to interactions of these protons with cosmic microwave photons. The limits depend on the spectrum of the injected UHECR protons. While for a -2.2 injection spectrum, the lower limits on the powers emitted in UHECRs by 1ES 0229+200, 1ES 1101-232 and 1ES 0347-121 are lower than their respective synchrotron luminosities (1e46 erg/s); in the case of 1ES 1426+428 it exceeds the corresponding synchrotron luminosity by up to an order of magnitude. The proposed Auger North Observatory should be able to detect 4e19 eV cosmic ray protons from 1ES 1426+428 within a few years of operation and test the TeV gamma-ray production model by UHECR energy losses while propagating along the line-of-sight, or constrain the intergalactic magnetic field to be larger than 1e-16 G in case of no detection. The lower limits on the apparent-isotropic jet power from accelerated 1e10-1e20 eV proton spectra in the blazar jet is of the order of the Eddington luminosity of a 1e9 solar mass black hole for a cosmic-ray injection spectrum -2.2 or harder for all blazars considered except for 1ES 1426+428. In the case of the latter the apparent-isotropic jet power exceeds the Eddington luminosity by an order of magnitude. For an injection spectrum softer than -2.2, as is required to fit the observed cosmic-ray data above 1e17-1e18 eV, the Eddington luminosity is exceeded by the lower limits on the jet power for all blazars considered.

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arXiv2011

Time Delay of Cascade Radiation for TeV Blazars and the Measurement of the Intergalactic Magnetic Field

Recent claims that the strength B_IGMF of the intergalactic magnetic field (IGMF) is >~ 1e-15 G are based on upper limits to the expected cascade flux in the GeV band produced by blazar TeV photons absorbed by the extragalactic background light. This limit depends on an assumption that the mean blazar TeV flux remains constant on timescales >~2 (B_ IGMF/1e-18 G)^2 / (E/{10 GeV})^2 yr for an IGMF coherence length ~ 1 Mpc, where E is the measured photon energy. Restricting TeV activity of 1ES 0229+200 to ~3 -- 4 years during which the source has been observed leads to a more robust lower limit of B_IGMF >~ 1e-18 G, which can be larger by an order of magnitude if the intrinsic source flux above ~5 -- 10 TeV from 1ES 0229+200 is strong.

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arXiv2010

Synchrotron radiation from ultra-high energy protons and the Fermi observations of GRB 080916C

Fermi gamma-ray telescope data of GRB 080916C with ~1e55 erg in apparent isotropic gamma-ray energy, show a several second delay between the rise of 100 MeV - GeV radiation compared with keV - MeV radiation. Here we show that synchrotron radiation from cosmic ray protons accelerated in GRBs, delayed by the proton synchrotron cooling timescale in a jet of magnetically-dominated shocked plasma moving at highly relativistic speeds with bulk Lorentz factor Gamma ~ 500, could explain this result. A second generation electron synchrotron component from attenuated proton synchrotron radiation makes enhanced soft X-ray to MeV gamma-ray emission. Long GRBs with narrow, energetic jets accelerating particles to ultra-high energies could explain the Auger observations of UHE cosmic rays from sources within 100 Mpc for nano-Gauss intergalactic magnetic fields. The total energy requirements in a proton synchrotron model are proportional to Gamma^(16/3). This model for GRB 080916C is only plausible if Gamma ~< 500 and the jet opening angle is ~ 1 degree.

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arXiv2010

The Gamma-Ray View of the Extragalactic Background Light

The Extragalactic Background Light (EBL) from the infrared (IR) through the ultraviolet (UV) is dominated by emission from stars, either directly or through absorption and reradiation by dust. It can thus give information on the star formation history of the universe. However, it is difficult to measure directly due to foreground radiation fields from the Galaxy and solar system. Gamma-rays from extragalactic sources at cosmological distances (blazars and gamma-ray bursts) interact with EBL photons creating electron-positron pairs, absorbing the gamma-rays. Given the intrinsic gamma-ray spectrum of a source and its redshift, the EBL can in principle be measured. However, the intrinsic gamma-ray spectra of blazars and GRBs can vary considerably from source to source and the from the same source over short timescales. A maximum intrinsic spectrum can be assumed from theoretical grounds, to give upper limits on the EBL absorption from blazars at low redshift with very high energy (VHE) gamma-ray observations with ground-based Atmospheric Cherenkov telescopes. The Fermi-LAT observations of blazars and GRBs can probe EBL absorption at higher redshifts. The lower energy portion of the LAT spectrum of these sources is unattenuated by the EBL, so that extrapolating this to higher energies can give the maximum intrinsic spectrum for a source. Comparing this to the observed higher energy LAT spectrum will then give upper limits on the EBL absorption. For blazars which have been detected by both the Fermi-LAT and at higher energies by Cherenkov telescopes, combined LAT-VHE observations can put more stringent constraints on the low redshift EBL. These procedures above can also be reversed: for sources with an unknown redshift, a given EBL model and gamma-ray spectrum can lead to an upper limit on the source's redshift.

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arXiv2010

On the Spectral Break in the Fermi-LAT Spectrum of 3C 454.3

Fermi Gamma ray Space Telescope observations of the flat spectrum radio quasar 3C~454.3 show a spectral-index change $ΔΓ\cong 1.2\pm 0.3$ at break energy $E_{br} \approx 2.4\pm0.3$ GeV. Such a sharp break is inconsistent with a cooling electron distribution and is poorly fit with a synchrotron self-Compton model. We show that a combination of two components, namely the Compton-scattered disk and broad-line region (BLR) radiations, explains this spectral break and gives a good fit to the quasi-simultaneous radio, optical/UV, X-ray, and $γ$-ray spectral energy distribution observed in 2008 August. A sharp break can be produced independent of the emitting region's distance from the central black hole if the BLR has a gradient in density $\propto R^{-2}$, consistent with a wind model for the BLR.

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arXiv2010

A method for setting upper limits to the extragalactic background light with Fermi-LAT and TeV observations of blazars

We propose a method for setting upper limits to the extragalactic background light (EBL). Our method uses simultaneous {\em Fermi}-LAT and ground-based TeV observations of blazars and is based on the assumption that the intrinsic spectral energy distribution (SED) of TeV blazars lies below the extrapolation of the {\em Fermi}-LAT SED from GeV to TeV energies. By extrapolating the {\em Fermi}-LAT spectrum, which for TeV blazars is practically unattenuated by photon-photon pair production with EBL photons, a firm upper limit on the intrinsic SED at TeV energies is provided. The ratio of the extrapolated spectrum to the observed TeV spectrum provides upper limits to the optical depth for the propagation of the TeV photons due to pair production on the EBL, which in turn sets firm upper limits to EBL models. We demonstrate our method using simultaneous observations from {\em Fermi}-LAT and ground-based TeV telescopes of the blazars \object{PKS 2155-304} and \object{1ES 1218+304}, and show that high EBL density models are disfavored. We also discuss how our method can be optimized and how {\em Fermi} and X-ray monitoring observations of TeV blazars can guide future TeV campaigns, leading to potentially much stronger constraints on EBL models.

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arXiv2010

Modeling the Extragalactic Background Light from Stars and Dust

The extragalactic background light (EBL) from the far infrared through the visible and extending into the ultraviolet is thought to be dominated by starlight, either through direct emission or through absorption and reradiation by dust. This is the most important energy range for absorbing $\g$-rays from distant sources such as blazars and gamma-ray bursts and producing electron positron pairs. In previous work we presented EBL models in the optical through ultraviolet by consistently taking into account the star formation rate (SFR), initial mass function (IMF) and dust extinction, and treating stars on the main sequence as blackbodies. This technique is extended to include post-main sequence stars and reprocessing of starlight by dust. In our simple model, the total energy absorbed by dust is assumed to be re-emitted as three blackbodies in the infrared, one at 40 K representing warm, large dust grains, one at 70 K representing hot, small dust grains, and one at 450 K representing polycyclic aromatic hydrocarbons. We find our best fit model combining the Hopkins and Beacom SFR using the Cole et al. parameterization with the Baldry and Glazebrook IMF agrees with available luminosity density data at a variety of redshifts. Our resulting EBL energy density is quite close to the lower limits from galaxy counts and in some cases below the lower limits, and agrees fairly well with other recent EBL models shortward of about 5 $μ$m. Deabsorbing TeV $\g$-ray spectra of various blazars with our EBL model gives results consistent with simple shock acceleration theory. We also find that the universe should be optically thin to $\g$-rays with energies less than 20 GeV.

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arXiv2009

Constraints on the Extragalactic Background Light from Very High Energy Gamma-Ray Observations of Blazars

The extragalactic background light (EBL) from the infrared to the ultraviolet is difficult to measure directly, but can be constrained with a variety of methods. EBL photons absorb gamma-rays from distant blazars, allowing one to use blazar spectra from atmospheric Cherenkov telescopes (ACTs) to put upper limits on the EBL by assuming a blazar source spectrum. Here we apply a simple technique, similar to the one developed by Schroedter (2005), to the most recent very-high energy (VHE) gamma-ray observations of blazars to put upper limits on the EBL energy density. This technique is independent of the EBL model and has well-defined errors on the constraints. Our results are consistent with EBL measurements and constraints but marginally inconsistent with several EBL models.

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arXiv2009

The stellar contribution to the extra-galactic background light and absorption of high-energy gamma-rays

TeV gamma rays from distant astrophysical sources are attenuated due to electron-positron pair creation by interacting with ultraviolet/optical to infrared photons which fill the universe and are collectively known as the extra-galactic background light (EBL). We model the ~0.1-10 eV starlight component of the EBL derived from expressions for the stellar initial mass function, star formation history of the universe, and wavelength-dependent absorption of a large sample of galaxies in the local universe. These models are simultaneously fitted to the EBL data as well as to the data on the stellar luminosity density in our local universe. We find that the models with modified Salpeter A initial mass function together with Cole et al. (2001) or Hopkins and Beacom (2006) star formation history best represent available data. Since no dust emission is included, our calculated EBL models can be interpreted as the lower limits in the ~0.1-1 eV range. We present simple analytic fits to the best-fit EBL model evolving with redshift. We then proceed to calculate gamma-ray opacities, and absorption of ~10-300 GeV gamma-rays coming from different redshifts. We discuss implications of our results for the Fermi Gamma Ray Space Telescope and ground-based Air Cherenkov Telescopes.

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