Fast Variations of Gamma-Ray Emission in Blazars

The largest group of sources identified by EGRET are Blazars. This sub-class of AGN is well known to vary in flux in all energy bands on time-scales ranging from a few minutes (in the optical and X-ray bands) up to decades (radio and optical regimes). In addition to variations of the gamma-ray flux between different viewing periods, the brightest of these sources showed a few remarkable gamma-ray flares on time-scales of about one day, confirming the extension of the ``Intraday-Variability (IDV)'' phenomenon into the GeV range. We present first results of a systematic approach to study fast variability with EGRET data. This statistical approach confirms the existence of IDV even during epochs when no strong flares are detected. This provides additional constraints on the site of the gamma-ray emission and allows cross-correlation analyses with light curves obtained at other frequencies even during periods of low flux. We also find that some stronger sources have fluxes systematically above threshold even during quiescent states. Despite the low count rates this allows explicit comparisons of flare amplitudes with other energy bands.Comment: 5 pages including figures, LaTex, uses aipproc.sty, to appear in the proceedings of the 4th Compton Symposium at Williamsburg, V

Probing High Redshift Radiation Fields with Gamma-Ray Absorption

The next generation of gamma-ray telescopes may be able to observe gamma-ray blazars at high redshift, possibly out to the epoch of reionization. The spectrum of such sources should exhibit an absorption edge due to pair-production against UV photons along the line of sight. One expects a sharp drop in the number density of UV photons at the Lyman edge E_{L}. This implies that the universe becomes transparent after gamma-ray photons redshift below E (m_{e}c^2)^{2}/E_{L} 18 GeV. Thus, there is only a limited redshift interval over which GeV photons can pair produce. This implies that any observed absorption will probe radiation fields in the very early universe, regardless of the subsequent star formation history of the universe. Furthermore, measurements of differential absorption between blazars at different redshifts can cleanly isolate the opacity due to UV emissivity at high redshift. An observable absorption edge should be present for most reasonable radiation fields with sufficient energy to reionize the universe. Ly-alpha photons may provide an important component of the pair-production opacity. Observations of a number of blazars at different redshifts will thus allow us to probe the rise in comoving UV emissivity with time.Comment: ApJ accepted version, minor changes. 19 pages, 5 figure

Wormholes, Gamma Ray Bursts and the Amount of Negative Mass in the Universe

In this essay, we assume that negative mass objects can exist in the extragalactic space and analyze the consequences of their microlensing on light from distant Active Galactic Nuclei. We find that such events have very similar features to some observed Gamma Ray Bursts and use recent satellite data to set an upper bound to the amount of negative mass in the universe.Comment: Essay awarded ``Honorable Mention'' in the Gravity Foundation Research Awards, 199

The 0.1-200 keV spectrum of the blazar PKS 2005-489 during an active state

The bright BL Lac object PKS 2005-489 was observed by BeppoSAX on November 1-2, 1998, following an active X-ray state detected by RossiXTE. The source, detected between 0.1 and 200 keV, was in a very high state with a continuum well fitted by a steepening spectrum due to synchrotron emission only. Our X-ray spectrum is the flattest ever observed for this source. The different X-ray spectral slopes and fluxes, as measured by various satellites, are consistent with relatively little changes of the peak frequency of the synchrotron emission, always located below 10^{17} Hz. We discuss these results in the framework of synchrotron self-Compton models. We found that for the BeppoSAX observation, the synchrotron peak frequency is between 10^{15} and 2.5x10^{16} Hz, depending on the model assumptions.Comment: 7 pages, 4 figures, accepted for publication in A&

Multiwavelength Properties of Blazars

The multiwavelength spectra of blazars appear to be dominated by nonthermal emission from a relativistic jet oriented close to the line of sight. The recent detection of many blazars at gamma-ray energies strongly supports this scenario. High quality multiwavelength monitoring data for the brightest one or two blazars suggest the optical through X-ray continuum is synchrotron emission from an inhomogeneous jet. The gamma-rays are likely due to Compton scattering of lower energy photons, either from within the jet or from the surrounding gas. The physical properties of the jet and the way in which it is produced are still largely a mystery but are probably related in some way to accretion onto a central supermassive black hole. There is little direct observational evidence for accretion disks in blazars, although there is evidence for winds which might emanate from disks.Comment: 12 pages, 6 figures, latex file with encapsulated postscript. Invited review presented at the 1996 COSPAR Scientific Assembly (Birmingham UK 14-21 July), in session E1.4 on The Variability of AGN and its Relation to Accretion Disk Phenomenology; paper to appear in Adv. Space Scienc

Pleth variability index and fluid management practices : a multicenter service evaluation

Funding Information: NonePeer reviewedPublisher PD

Contemporaneous IUE, EUVE, and High-Energy Observations of 3C 273

We present the results of our 1994 January and 1995 January observations of the quasar 3C 273 obtained with the International Ultraviolet Explorer (IUE) and the Extreme-Ultraviolet Explorer (EUV E). These observations were part of a large multiwavelength campaign to observe 3C 273 from radio through γ-rays. Our 1995 January photometric observations with the EUV E Lexan/B Deep Survey (DS) instrument indicate strong evidence for variability, at a 99% confidence level, during the 12 day observing period. We have utilized ROSAT PSPC soft X-ray power-law models to correlate with EUV E count rates. Besides variations in the normalization level between both observations, our EUV count rates are consistent with a simple power-law model with spectral index α ~ 1.77 (Fv ~ v^-α) that can be extrapolated lated from the soft X-rays to the EUV range. The active galactic nucleus 3C 273 is an important blazar to study because in our picture it reveals the presence of both disk and relativistic beam spectral contributions

Klein-Nishina Effects in the Spectra of Non-Thermal Sources Immersed in External Radiation Fields

We study Klein-Nishina (KN) effects in the spectrum produced by a steady state, non-thermal source where rapidly accelerated electrons cool by emitting synchrotron radiation and Compton upscattering ambient photons produced outside the source. We focus on the case where the radiation density inside the source exceeds that of the magnetic field. We show that the KN reduction in the electron Compton cooling rate causes the steady-state electron spectrum to harden at energies above \gamma_{KN}, where \gamma_{KN}= 1/4\epsilon_0 and \epsilon_0=h\nu_0/m_ec^2 is the characteristic ambient photon energy. The source synchrotron spectrum thus shows a high-energy ``bump'' or excess even though the electron acceleration spectrum has no such excess. In contrast, the low-energy Compton gamma-ray spectrum shows little distortion because the electron hardening compensates for the KN decline in the scattering rate. For sufficiently high electron energies, however, Compton cooling becomes so inefficient that synchrotron cooling dominates -- an effect omitted in most previous studies. The hardening of the electron distribution thus stops, leading to a rapid decline in Compton gamma-ray emission, i.e., a strong spectral break whose location does not depend on the maximum electron energy. This break can limit the importance of Compton gamma-ray pair production on ambient photons and implies that a source's synchrotron luminosity may exceed its Compton luminosity even though the source magnetic field energy density is smaller than the ambient radiation energy density. We discuss the importance of these KN effects in blazars, micro-quasars, and pulsar binaries.Comment: 36 pages, 10 figures. MNRAS LaTeX. Abtract slightly shortened. Submitted to Monthly Notice

High Energy Gamma-Ray Emission From Blazars: EGRET Observations

We will present a summary of the observations of blazars by the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory (CGRO). EGRET has detected high energy gamma-ray emission at energies greater than 100 MeV from more that 50 blazars. These sources show inferred isotropic luminosities as large as $3\times 10^{49}$ ergs s$^{-1}$. One of the most remarkable characteristics of the EGRET observations is that the gamma-ray luminosity often dominates the bolometric power of the blazar. A few of the blazars are seen to exhibit variability on very short time-scales of one day or less. The combination of high luminosities and time variations seen in the gamma-ray data indicate that gamma-rays are an important component of the relativistic jet thought to characterize blazars. Currently most models for blazars involve a beaming scenario. In leptonic models, where electrons are the primary accelerated particles, gamma-ray emission is believed to be due to inverse Compton scattering of low energy photons, although opinions differ as to the source of the soft photons. Hardronic models involve secondary production or photomeson production followed by pair cascades, and predict associated neutrino production.Comment: 16 pages, 7 figures, style files included. Invited review paper in "Observational Evidence for Black Holes in the Universe," 1999, ed. S. K. Chakrabarti (Dordrecht: Kluwer), 215-23