Evidence for Intergalactic Absorption in the TeV Gamma-Ray Spectrum of Mkn 501

The recent HEGRA observations of the blazar Mkn 501 show strong curvature in the very high energy gamma-ray spectrum. Applying the gamma-ray opacity derived from an empirically based model of the intergalactic infrared background radiation field (IIRF), to these observations, we find that the intrinsic spectrum of this source is consistent with a power-law: dN/dE~ E^-alpha with alpha=2.00 +/- 0.03 over the range 500 GeV - 20 TeV. Within current synchrotron self-Compton scenarios, the fact that the TeV spectral energy distribution of Mkn 501 does not vary with luminosity, combined with the correlated, spectrally variable emission in X-rays, as observed by the BeppoSAX and RXTE instruments, also independently implies that the intrinsic spectrum must be close to alpha=2. Thus, the observed curvature in the spectrum is most easily understood as resulting from intergalactic absorption.Comment: 7 pages, 1 figure, accepted in ApJ Letters 1999 April

Rapid X-ray Variability of the BL Lacertae Object PKS 2155-304

(Abridged) We present a detailed power density spectrum and cross-correlation analysis of the X-ray light curves of the BL Lac object PKS 2155-304, observed with BeppoSAX in 1997 and 1996, aimed at exploring the rapid variability properties and the inter-band cross correlations in the X-rays. We also perform the same analysis on the (archival) X-ray light curve obtained with ASCA in 1994.Comment: 47 pages, 11 figures, AAS Latex macros V4.0, accepted for publication in the Astrophysical Journa

Spectral Evolution of PKS 2155-304 observed with BeppoSAX during an Active Gamma-ray Phase

We present the results of BeppoSAX observations of PKS 2155-304 during an intense gamma-ray flare. The source was in a high X-ray state. A temporal analysis of the data reveals a tendency of the amplitude of variations to increase with energy, and the presence of a soft lag with a timescale of the order 10^3 s. A curved continuum spectrum, with no evidence of spectral features, extends up to ~50 keV, while there is indication of a flatter component emerging at higher energies, consistent with the interpretation of the broad band spectral energy distribution (SED) as due to synchrotron self-Compton (SSC) emission from a single region. Notably, the fitting of the SED with such a model is consistent with an interpretation of the detected soft lag as due to radiative cooling, supporting the idea that radiation losses play an important role in variability. The observed shifts of the SED peaks between the lowest and highest flux levels can be accounted for by an increase of the break energy in the relativistic particle spectrum. The model predicts emission at TeV energies in good agreement with the recently reported detection.Comment: 36 pages (8 figures), Latex with AAS macros, etc), accepted for publication on Astrophysical Journa

HST and Palomar Imaging of GRB 990123: Implications for the Nature of Gamma-Ray Bursts and their Hosts

We report on HST and Palomar optical images of the field of GRB 990123, obtained on 8 and 9 February 1999. We find that the optical transient (OT) associated with GRB 990123 is located on an irregular galaxy, with magnitude V=24.20 +/- 0.15. The strong metal absorption lines seen in the spectrum of the OT, along with the low probability of a chance superposition, lead us to conclude that this galaxy is the host of the GRB. The OT is projected within the ~1'' visible stellar field of the host, nearer the edge than the center. We cannot, on this basis, rule out the galactic nucleus as the site of the GRB, since the unusual morphology of the host may be the result of an ongoing galactic merger, but our demonstration that this host galaxy has extremely blue optical to infrared colors more strongly supports an association between GRBs and star formation. We find that the OT magnitude on 1999 Feb 9.05, V = 25.45 +/- 0.15, is about 1.5 mag fainter than expected from extrapolation of the decay rate found in earlier observations. A detailed analysis of the OT light curve suggests that its fading has gone through three distinct phases: an early rapid decline (f_{nu} \propto t^{-1.6} for t < 0.1 days), a slower intermediate decline power-law decay (f_{nu} \propto t^{-1.1} for 0.1 < t < 2 days), and then a more rapid decay (at least as steep as (f_{\nu} \propto t^{-1.8} for t > 2 days). The break to steeper slope at late times may provide evidence that the optical emission from this GRB was highly beamed.Comment: Accepted for publication in Astrophysical Journal (Letters). Fourteen pages. Three encapsulated figure

Synthetic X-ray light curves of BL Lacs from relativistic hydrodynamic simulations

We present the results of relativistic hydrodynamic simulations of the collision of two dense shells in a uniform external medium, as envisaged in the internal shock model for BL Lac jets. The non-thermal radiation produced by highly energetic electrons injected at the relativistic shocks is computed following their temporal and spatial evolution. The acceleration of electrons at the relativistic shocks is parametrized using two different models and the corresponding X-ray light curves are computed. We find that the interaction time scale of the two shells is influenced by an interaction with the external medium. For the chosen parameter sets, the efficiency of the collision in converting dissipated kinetic energy into the observed X-ray radiation is of the order of one percent.Comment: 22 pages, 6 figures, accepted to A&

Time dependent gamma-ray production in the anisotropic IC e±e^\pm pair cascade initiated by electrons in active galaxies

New high energy emission features have been recently discovered by the Cherenkov telescopes from active galaxies e.g., a few minutes variability time scale of TeV emission from Mrk 501 and PKS 2155-304, sub-TeV $\gamma$-ray emission from GeV peaked blazar 3C 279, and TeV emission from two nearby active galaxies, M87 and Cen A, which jets are inclined at a relatively large angle to the line of sight. These results have put a new light on the high energy processes occurring in central parts of active galaxies stimulating more detailed studies of $\gamma$-ray emission models. Here we report the results of a detailed analysis concerning the most general version of the model for the $\gamma$-ray production by leptons injected in the jet which interact with the thermal radiation from an accretion disk (the so called {\it external inverse Compton model}). We investigate the $\gamma$-ray spectra produced in an anisotropic Inverse Compton (IC) $e^\pm$ pair cascade in the whole volume above the accretion disk. The cascade $\gamma$-ray spectra are obtained for different locations of the observer in respect to the direction of the jet. We also study the time evolution of this $\gamma$-ray emission caused by the propagation of the relativistic leptons along the jet and the delays resulting from different places of the origin of $\gamma$-rays above the accretion disk. We discuss the main features of such a cascade model assuming constant injection rate of electrons along the jet. We are investigating two models for their different maximum energies: constant value independent on the distance along the jet or limited by the synchrotron energy losses considered locally in the jet. The model is discussed in the context of blazars observed at small and large inclination angles taking as an example the parameters of the two famous sources Cen A and 3C 279.Comment: 16 pages, 18 figures, accepted for publication in MNRA

X-Ray Observations of Type Ia Supernovae with Swift: Evidence for Circumstellar Interaction for SN 2005ke

We present a study of the early (days to weeks) X-ray and UV properties of eight Type Ia supernovae (SNe Ia) which have been extensively observed with the X-Ray Telescope (XRT) and UV/Optical Telescope (UVOT) onboard Swift, ranging from 5-132 days after the outburst. SN 2005ke is tentatively detected (at a 3-3.6 sigma level of significance) in X-rays based on deep monitoring with the XRT ranging from 8 to 120 days after the outburst. The inferred X-ray luminosity [(2+/-1) x 10^{38} ergs/s; 0.3-2 keV band] is likely caused by interaction of the SN shock with circumstellar material (CSM), deposited by a stellar wind from the progenitor's companion star with a mass-loss rate of ~ 3 x 10^{-6} M_sun/yr (v_w/10 km/s). Evidence of CSM interaction in X-rays is independently confirmed by an excess of UV emission as observed with the UVOT onboard Swift, starting around 35 days after the explosion. The non-detection of SN 2005ke with Chandra 105 days after the outburst implies a rate of decline steeper than L_x \propto t^{-0.75}, consistent with the decline expected from the interaction of the SN shock with a spherically symmetric CSM (t^{-1}). None of the other seven SNe Ia is detected in X-rays or shows a UV excess, which allows us to put tight constraints on the mass-loss rates of the progenitor systems.Comment: 12 pages, 3 figures, accpeted for publication in ApJ

Hubble Space Telescope Imaging of the Optical Transient Associated with GRB970508

We report on Hubble Space Telescope (HST) observations of the optical transient (OT) discovered in the error box of the gamma-ray burst GRB970508. The object was imaged on 1997 June 2 with the Space Telescope Imaging Spectrograph (STIS) and Near-Infrared Camera and Multi-Object Spectrometer (NICMOS). The observations reveal a point-like source with R = 23.1 +- 0.2 and H = 20.6 +- 0.3, in agreement with the power-law temporal decay seen in ground-based monitoring. Unlike the case of GRB970228, no nebulosity is detected surrounding the OT of GRB970508. We set very conservative upper limits of R ~ 24.5 and H ~ 22.2 on the brightness of any underlying extended source. If this subtends a substantial fraction of an arcsecond, then the R band limit is ~25.5. In combination with Keck spectra that show Mg I absorption and [O II] emission at a redshift of z = 0.835, our observations suggest that the OT is located in a star-forming galaxy with total luminosity one order of magnitude lower than the knee of the galaxy luminosity function, L*. Such galaxies are now thought to harbor the majority of star formation at z ~ 1; therefore, these observations may provide support for a link between GRBs and star formation.Comment: 12 pages, Latex, 2 Postscript figures, to appear in The Astrophysical Journal Letter

The new model of fitting the spectral energy distributions of Mkn 421 and Mkn 501

The spectral energy distribution (SED) of TeV blazars has a double-humped shape that is usually interpreted as Synchrotron Self Compton (SSC) model. The one zone SSC model is used broadly but cannot fit the high energy tail of SED very well. It need bulk Lorentz factor which is conflict with the observation. Furthermore one zone SSC model can not explain the entire spectrum. In the paper, we propose a new model that the high energy emission is produced by the accelerated protons in the blob with a small size and high magnetic field, the low energy radiation comes from the electrons in the expanded blob. Because the high and low energy photons are not produced at the same time, the requirement of large Doppler factor from pair production is relaxed. We present the fitting results of the SEDs for Mkn 501 during April 1997 and Mkn 421 during March 2001 respectively.Comment: 5 pages, 1 figures, 1table. accepted for publication in Sciences in China --