High Energy Astrophysics

J. S. Perkins et al. (with F. Krennrich and M. Pohl)

Galaxy clusters might be sources of TeV gamma rays emitted by high-energy protons and electrons accelerated by large scale structure formation shocks, galactic winds, or active galactic nuclei. Furthermore, gamma rays may be produced in dark matter particle annihilation processes at the cluster cores. We report on observations of the galaxy clusters Perseus and Abell 2029 using the 10 m Whipple Cherenkov telescope during the 2003-2004 and 2004-2005 observing seasons. We apply a two-dimensional analysis technique to scrutinize the clusters for TeV emission. In this paper we first determine flux upper limits on TeV gamma-ray emission from point sources within the clusters. Second, we derive upper limits on the extended cluster emission. We subsequently compare the flux upper limits with EGRET upper limits at 100 MeV and theoretical models. Assuming that the gamma-ray surface brightness profile mimics that of the thermal X-ray emission and that the spectrum of cluster cosmic rays extends all the way from thermal energies to multi-TeV energies with a differential spectral index of -2.1, our results imply that the cosmic ray proton energy density is less than 7.9% of the thermal energy density for the Perseus cluster.

J. S. Perkins, H. M. Badran, G. Blaylock, S. M. Bradbury, P. Cogan, Y. C. K. Chow, W. Cui, M. K. Daniel, A. D. Falcone, S. J. Fegan, J. P. Finley, P. Fortin, L. F. Fortson, G. H. Gillanders, K. J. Gutierrez, J. Grube, J. Hall, D. Hanna, J. Holder, D. Horan, S. B. Hughes, G. E. Kenny, M. Kertzman, D. B. Kieda, J. Kildea, K. Kosack, H. Krawczynski, F. Krennrich, M. J. Lang, S. LeBohec, G. Maier, P. Moriarty, R. A. Ong, M. Pohl, K. Ragan, P. F. Rebillot, G. H. Sembroski, D. Steele, S. P. Swordy, L. Valcarcel, V. V. Vassiliev, S. P. Wakely, T. C. Weekes, D. A. Williams

K. Gutierrez et al. (with F. Krennrich and M. Pohl)

In April-May 2003, the blazar 1ES 1959+650 showed an increased level of X-ray activity. This prompted a multiwavelength observation campaign with the Whipple 10 m gamma-ray telescope, the Rossi X-ray Timing Explorer, the Bordeaux Optical Observatory, and the University of Michigan Radio Astrophysical Observatory. We present the multiwavelength data taken from May 2, 2003 to June 7, 2003 and compare the source characteristics with those measured during observations taken during the years 2000 and 2002. The X-ray observations gave a data set with high signal-to-noise light curves and energy spectra; however, the gamma-ray observations did not reveal a major TeV gamma-ray flare. Furthermore, we find that the radio and optical fluxes do not show statistically significant deviations from those measured during the 2002 flaring periods. While the X-ray flux and X-ray photon index appear correlated during subsequent observations, the apparent correlation evolved significantly between the years 2000, 2002, and 2003. We discuss the implications of this finding for the mechanism that causes the flaring activity.

K. Gutierrez, H. M. Badran, S. M. Bradbury, J. H. Buckley, O. Celik, Y. C. Chow, P. Cogan, W. Cui, M. Daniel, A. Falcone, S. J. Fegan, J. P. Finley, G. H. Gillanders, J. Grube, J. Holder, D. Horan, S. B. Hughes, I. Jung, D. Kieda, K. Kosack, H. Krawczynski, F. Krennrich, M. J. Lang, S. Le Bohec, G. Maier, P. Moriarty, J. Perkins, M. Pohl, J. Quinn, P. F. Rebillot, H. J. Rose, M. Schroedter, G. H. Sembroski, S. P. Wakely, T. C. Weekes, R. J. White

E. Dwek, G. Arendt and F. Krennrich, Astrophysical Journal, in press, (astro-ph/0508262), 2005

The intensity of the diffuse ~ 1 - 4 micron sky emission from which solar system and Galactic foregrounds have been subtracted is in excess of that expected from energy released by galaxies and stars that formed during the z < 5 redshift interval (Arendt & Dwek 2003, Matsumoto et al. 2005). The spectral signature of this excess near-infrared background light (NIRBL) component is almost identical to that of reflected sunlight from the interplanetary dust cloud, and could therefore be the result of the incomplete subtraction of this foreground emission component from the diffuse sky maps. Alternatively, this emission component could be extragalactic. Its spectral signature is consistent with that of redshifted continuum and recombination line emission from HII regions formed by the first generation of very massive stars. In this paper we analyze the implications of this spectral component for the formation rate of these Population III stars, the redshift interval during which they formed, the reionization of the universe and evolution of collapsed halo masses. We find that to reproduce the intensity and spectral shape of the NIRBL requires a peak star formation rate that is higher by about a factor of 4 to 10 compared to those derived from hierarchical models. Furthermore, an extragalactic origin for the NIRBL leads to physically unrealistic absorption-corrected spectra of distant TeV blazars. All these results suggest that Pop III stars contribute only a fraction of the NIRBL intensity with zodiacal light, star forming galaxies, and/or non-nuclear sources giving rise to the remaining fraction.

Eli Dwek, Richard G. Arendt, Frank Krennrich

Dwek, F. Krennrich and G. Arendt, Astrophysical Journal, in press, (see astro-ph0508133), 2005

The 1 - 5 micron diffuse sky emission from which local foreground emission from the solar system and the Galaxy have been subtracted exceeds the brightness that can be attributed to normal star forming galaxies. The nature of this excess near-infrared background light (NIRBL) is still controversial. On one hand, it has been interpreted as a distinct spectral feature created by the redshifted emission from primordial (Population III) stars that have formed at redshifts > 8. On the other hand, the NIRBL spectrum is almost identical to that of the zodiacal cloud, raising the possibility that it is of local origin. Blazars can, in principle, offer a simple test for the nature and origin of the NIRBL. Very high energy gamma-ray photons emitted by these objects are attenuated on route to earth by photon-photon interactions with the extragalactic background light (EBL). This paper examines whether the extragalactic nature of the NIRBL can be determined from the analysis of the TeV spectra of blazars.

Eli Dwek, Frank Krennrich, Richard G. Arendt

M. Blazejowski et al. (with F. Krennrich, D.A. Lewis, T. Nagai)

We report results from a multi-wavelength monitoring campaign on Mrk 421 over the period of 2003-2004. The source was observed simultaneously at TeV and X-ray energies, with supporting observations frequently carried out at optical and radio wavelengths. The large amount of simultaneous data has allowed us to examine the variability of Mrk 421 in detail. The variabilities are generally correlated between the X-ray and gamma-ray bands, although the correlation appears to be fairly loose. The light curves show the presence of flares with varying amplitudes on a wide range of timescales both at X-ray and TeV energies. Of particular interest is the presence of TeV flares that have no coincident counterparts at longer wavelengths, because the phenomenon seems difficult to understand in the context of the proposed emission models for TeV blazars. We have also found that the TeV flux reached its peak days before the X-ray flux during a giant flare in 2004. Such a difference in the development of the flare presents a further challenge to the emission models. Mrk 421 varied much less at optical and radio wavelengths. Surprisingly, the normalized variability amplitude in optical seems to be comparable to that in radio, perhaps suggesting the presence of different populations of emitting electrons in the jet. The spectral energy distribution (SED) of Mrk 421 is seen to vary with flux, with the two characteristic peaks moving toward higher energies at higher fluxes. We have failed to fit the measured SEDs with a one-zone SSC model; introducing additional zones greatly improves the fits. We have derived constraints on the physical properties of the X-ray/gamma-ray flaring regions from the observed variability (and SED) of the source. The implications of the results are discussed.

M. Blazejowski, I. C. De La Calle Perez, I. H. Bond, P. J. Boyle, S. M. Bradbury, J. H. Buckley, D. A. Carter-Lewis, W. Cui, , M. Daniel , C. Dowdall, C. Duke, I. de la Calle Perez, A. Falcone, D. J. Fegan, S. J. Fegan, J. P. Finley, L. Fortson, J. A. Gaidos, K. Gibbs, S. Gammell, J. Hall, T. A. Hall, A. M. Hillas, J. Holder, D. Horan, M. Jordan, M. Kertzman, D. Kieda, J. Kildea, J. Knapp, H. Krawczynski, F. Krennrich, S. LeBohec, E. T. Linton, J. Lloyd-Evans, P. Moriarty, D. Müller, T. N. Nagai, R. Ong, M. Page, R. Pallassini, D. Petry, B. Power-Mooney, J. Quinn, P. Rebillot, P. T. Reynolds, H. J. Rose, G. H. Sembroski, S. P. Swordy, V. V. Vassiliev, S. P. Wakely, G. Walker, and T. C. Weekes

M. Schroedter et al. Astrophysical Journal, in press, 2005

The BL Lacertae (BL Lac) object 1ES 2344+514 (1ES 2344), at a redshift of 0.044, was discovered as a source of very high energy (VHE) gamma rays by the Whipple Collaboration in 1995 \citep{2344Catanese98}. This detection was recently confirmed by the HEGRA Collaboration \citep{2344Hegra03}. As is typical for high-frequency peaked blazars, the VHE gamma-ray emission is highly variable. On the night of 20 December, 1995, a gamma-ray flare of 5.3-sigma significance was detected, the brightest outburst from this object to-date. The emission region is compatible with a point source. The spectrum between 0.8 TeV and 12.6 TeV can be described by a power law $\frac{\ud^3 N}{\ud E \ud A \ud t}=(5.1\pm1.0_{st}\pm1.2_{sy})\times10^{-7} (E/ \mathrm{TeV})^{-2.54 \pm0.17_{st}\pm0.07_{sy}} \mathrm{\frac{1}{TeV m^2 s}}$. Comparing the spectral index with that of the other five confirmed TeV blazars, the spectrum of 1ES 2344 is similar to 1ES 1959+650, located at almost the same distance. The spectrum of 1ES 2344 is steeper than the brightest flare spectra of Markarian 421 (Mrk~421) and Markarian 501 (Mrk~501), both located at a distance about 2/3 that of 1ES 2344, and harder than the spectra of PKS 2155-304 and H~1426+428, which are located almost three times as far. This trend is consistent with attenuation caused by the infrared extragalactic background radiation.

M. Schroedter, H. M. Badran, J. H. Buckley, J. Bussons Gordo, D. A. Carter-Lewis, C. Duke, D. J. Fegan, S. F. Fegan, J. P. Finley, G. H. Gillanders, J. Grube, D. Horan, G. E. Kenny, M. Kertzman, K. Kosack, F. Krennrich, D. B. Kieda, J. Kildea, M. J. Lang, Kuen Lee, P. Moriarty, J. Quinn, M. Quinn, B. Power-Mooney, G. H. Sembroski, S. P. Wakely, V. V. Vassiliev, T. C. Weekes, and J. Zweerink

M. Daniel et al. (with F. Krennrich, D. A. Lewis, T. Nagai, M. Schroedter, A. Imran, M. Pohl) Astrophysical Journal, 621, 181, 2005

The blazar 1ES 1959+650 was observed in a flaring state with the Whipple 10 m Imaging Atmospheric Cherenkov Telescope during May of 2002. A spectral analysis has been carried out on the data from that time period and the resulting very high energy gamma-ray spectrum ($E \geq 316$ GeV) can be well fit by a power-law of differential spectral index \alpha = 2.78 +/- 0.12_{stat.} +/- 0.21_{sys.}. On June 4th 2002, the source flared dramatically in the gamma-ray range without any coincident increase in the X-ray emission, providing the first unambiguous example of an `orphan' gamma-ray flare from a blazar. The gamma-ray spectrum for these data can also be described by a simple power-law fit with \alpha = 2.82 +/- 0.15_{stat.} +/- 0.30_{sys.}. There is no compelling evidence for spectral variability, or for any cut-off to the spectrum.

J. Holder, R.W. Atkins, H.M. Badran, G. Blaylock, S.M. Bradbury, J.H. Buckley, K.L. Byrum, D.A. Carter-Lewis, O. Celik, Y.C.K. Chow, P. Cogan, W. Cui, M.K. Daniel, I. de la Calle Perez, C. Dowdall, P. Dowkontt, C. Duke, A.D. Falcone, S.J. Fegan, J.P. Finley, P. Fortin, L.F. Fortson, K. Gibbs, G. Gillanders, O.J. Glidewell, J. Grube, K.J. Gutierrez, G. Gyuk, J. Hall, D. Hanna, E. Hays, D. Horan, S.B. Hughes, T.B. Humensky, A. Imran, I. Jung, P. Kaaret, G.E. Kenny, D. Kieda, J. Kildea, J. Knapp, H. Krawczynski, F. Krennrich, M.J. Lang, S. LeBohec, E. Linton, E.K. Little, G. Maier, H. Manseri, A. Milovanovic, P. Moriarty, R. Mukherjee, P.A. Ogden, R.A. Ong, J.S. Perkins, F. Pizlo, M. Pohl, J. Quinn, K. Ragan, P.T. Reynolds, E.T. Roache, H.J. Rose, M. Schroedter, G.H. Sembroski, G. Sleege, D. Steele

S. LeBohec, F. Krennrich & G. Sleege, Astroparticle Physics, 23, 238-248, 2005

The Short GAmma Ray Front Air Cherenkov Experiment (SGARFACE) is operated at the Whipple Observatory utilizing the Whipple 10m gamma-ray telescope. SGARFACE is sensitive to gamma-ray bursts of more than 100MeV with durations from 100ns to 35us and provides a fluence sensitivity as low as 0.8 gamma-rays per m^2 above 200MeV (0.05 gamma-rays per m^2 above 2GeV) and allows to record the burst time structure.

S. LeBohec, F. Krennrich & G. Sleege

E. Dwek and F. Krennrich, Astrophysical Journal, 618, 657, 2005.

Very high energy (~ TeV) $\gamma$-rays from blazars are attenuated by photons from the extragalactic background light (EBL). Observations of blazars can therefore provide an ideal opportunity for determining the EBL intensity if their intrinsic spectrum is known. Conversely, knowledge of the EBL intensity can be used to determine the intrinsic blazar spectrum. Unfortunately, neither the EBL intensity nor the intrinsic blazar spectrum is known with high enough precision to accurately derive one quantity from the other. In this paper we use the most recent data on the EBL to construct twelve different realizations representing all possible permutations between EBL limits and the detections in the different wavelength regions. We use these realizations to explore the effects of the EBL on the inferred spectra of blazars. In particular, we show that the frequently cited "IR background-TeV gamma-ray crisis" does not exist, and derive the intrinsic spectra and peak energies of the blazars Mrk 421, 501 and H1426+428 for EBL realizations that give rise to physically viable intrinsic blazar spectra. We also show that the intrinsic spectrum of Mrk~421 during a period of intense flaring activity has a peak energy that seems to shift to higher energies at higher flux states. Finally, we also explore the effect of the uncertainties in the absolute calibration of the gamma-ray energies on derived TeV opacities and the intrinsic blazar spectra.

E. Dwek and F. Krennrich

S. J. Fegan et al. (with F. Krennrich, D. A. Carter-Lewis,T. Nagai, M. Schroedter) Astrophysical Journal, 624, 638, 2005

Observations of unidentified EGRET sources were made with the Whipple 10m imaging atmospheric Cherenkov telescope between Fall 1999 and Spring 2001. During this period, a high resolution 490 pixel camera with 4 degree field of view was present on the telescope. Characterization of the off-axis response of this instrument was done using observations of the Crab Nebula. No significant emission was detected from the eight unidentified EGRET sources observed and upper limits are presented as a function of position.

S. J. Fegan, The VERITAS collaboration

M. Blazejowski, I. C. De La Calle Perez, I. H. Bond, P. J. Boyle, S. M. Bradbury, J. H. Buckley, D. A. Carter-Lewis, W. Cui, , M. Daniel , C. Dowdall, C. Duke, I. de la Calle Perez

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