Now showing items 1-20 of 413

    • Electron-positron jets associated with the quasar 3C279 

      Wardle, J.F.C.; Homan, D.C.; Ojha, R.; Roberts, D.H. (Brandeis University, 1998)
      A long outstanding problem in extragalactic astrophysics is the composition of the relativistic jets of plasma streaming from the nuclei of quasars and active galaxies - whether it is predominantly a "normal" (electron-proton) ...
    • Search for the Identification of 3EG J1835+5918: Evidence for a New Type of High-Energy Gamma-Ray Source 

      Mirabal, N.; Halpern, J. P.; Eracleous, M.; Becker, R. H. (IOP, 2000-05-10)
      The EGRET source 3EG J1835+5918 is the brightest and most accurately positioned of the as-yet unidentified high-energy γ-ray sources at high Galactic latitude (ℓ,b = 89°,25°). We present a multiwavelength study of the ...
    • GRB 991216 Joins the Jet Set: Discovery and Monitoring of its Optical Afterglow 

      Halpern, J. P.; Uglesich, R.; Mirabal, N.; Kassin, S.; Thorstensen, J.; Keel, W. C.; Diercks, A.; Bloom, J. S.; Harrison, F.; Mattox, J.; Eracleous, M. (IOP, 2000-06-15)
      The optical light curve of the energetic γ-ray burst GRB 991216 is consistent with jetlike behavior in which a power-law decay steepens from t⁻¹·²²⁺⁰·⁰⁴ at early times to t⁻¹·⁵³⁺⁰·⁰⁵ in a gradual transition at around 2 ...
    • The Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) 

      Stark, Antony A.; Bally, John; Balm, Simon P.; Bania, T. M.; Bolatto, Alberto D.; Chamberlin, Richard A.; Engargiola, Gregory; Huang, Maohai; Ingalls, James G.; Jacobs, Karl; Jackson, James M.; Kooi, Jacob W.; Lane, Adair P.; Lo, K.-Y.; Marks, Rodney D.; Martin, Christopher L.; Mumma, Dennis; Ojha, Roopesh; Schieder, Rudolf; Staguhn, Johannes; Stutzki, Juergen; Walker, Christopher K.; Wilson, Robert W.; Wright, Gregory A.; Zhang, Xiaolei; Zimmermann, Peter; Zimmermann, Ruediger (IOP, 2000-12-15)
      AST/RO, a 1.7 m diameter telescope for astronomy and aeronomy studies at wavelengths between 200 and 2000 microns, was installed at the South Pole during the 1994-1995 Austral summer. The telescope operates continuously ...
    • A Neutron Star Identification for the High-Energy Gamma-Ray Source 3EG J1835+5918 Detected in the ROSAT All-Sky Survey 

      Mirabal, N.; Halpern, J. P. (IOP, 2001-01-24)
      In the error box of 3EG J1835+5918, the brightest as-yet unidentified EGRET source at intermediate Galactic latitude, we find a weak, ultrasoft X-ray source at energies E < 0.3 keV in the ROSAT All-Sky Survey. Deep optical ...
    • AST/RO Observations of Atomic Carbon near the Galactic Center 

      Ojha, Roopesh; Stark, Antony A.; Hsieh, Henry H.; Lane, Adair P.; Chamberlin, Richard A.; Bania, Thomas M.; Bolatto, Alberto D.; Jackson, James M.; Wright, Gregory A. (IOP, 2001-02-10)
      We present a coarsely sampled map of the region |l| ≤ 2°, |b| ≤ 0°1 in the 492 GHz (³P₁ → ³P₀) fine-structure transition of neutral carbon, observed with the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO). ...
    • Multicolor Observations of the GRB 000926 Afterglow 

      Price, P. A.; Harrison, F. A.; Galama, T. J.; Reichart, D. E.; Axelrod, T. S.; Berger, E.; Bloom, J. S.; Busche, J.; Cline, T.; Diercks, A.; Djorgovski, S. G.; Frail, D. A.; Gal-Yam, A.; Halpern, J.; Holtzman, J. A.; Hunt, M.; Hurley, K.; Jacoby, B.; Kimble, R.; Kulkarni, S. R.; Mirabal, N.; Morrison, G.; Ofek, E.; Pevunova, O.; Sari, R.; Schmidt, B. P.; Turnshek, D.; Yost, S. (IOP, 2001-02-26)
      We present multicolor light curves of the optical afterglow of gamma-ray burst (GRB) 000926. Beginning ~1.5 days after the burst, the light curves of this GRB steepen measurably. The existence of such achromatic breaks is ...
    • Parsec-Scale Blazar Monitoring: Proper Motions 

      Homan, Daniel C.; Ojha, Roopesh; Wardle, John F. C.; Roberts, David H.; Aller, Margo F.; Aller, Hugh D.; Philip A. Hughes (IOP, 2001-03-10)
      We present proper motions obtained from a dual-frequency, six-epoch, VLBA polarization experiment monitoring a sample of 12 blazars. The observations were made at 15 and 22 GHz at bimonthly intervals over 1996. A total of ...
    • GRB 000418: A Hidden Jet Revealed 

      Berger, E.; Diercks, A.; Frail, D. A.; Kulkarni, S. R.; Bloom, J. S.; Sari, R.; Halpern, J.; Mirabal, N.; Taylor, G. B.; Hurley, K.; Pooley, G.; Becker, K. M.; Wagner, R. M.; Terndrup, D. M.; Statler, T.; Wik, D. R.; Mazets, E.; Cline, T. (IOP, 2001-04-05)
      We report on optical, near-infrared, and centimeter radio observations of GRB 000418 that allow us to follow the evolution of the afterglow from 2 to 200 days after the γ-ray burst (GRB). In modeling these broadband data, ...
    • Parsec-Scale Blazar Monitoring: Flux and Polarization Variability 

      Homan, Daniel C.; Ojha, Roopesh; Wardle, John F. C.; Roberts, David H.; Aller, Margo F.; Aller, Hugh D.; Hughes, Philip A. (IOP, 2002-03-20)
      We present analysis of the flux density and polarization variability of parsec-scale radio jets from a dual-frequency, six-epoch, VLBA polarization experiment monitoring 12 blazars. The observations were made at 15 and 22 ...
    • Is the EGRET Source 3EG J1621+8203 the Radio Galaxy NGC 6251? 

      Mukherjee, R.; Halpern, J.; Mirabal, N.; Gotthelf, E. V. (IOP, 2002-04-03)
      We discuss the nature of the unidentified EGRET source 3EG J1621+8203. In an effort to identify the gamma-ray source, we have examined X-ray images of the field from ROSAT PSPC, ROSAT HRI, and ASCA GIS. Of the several faint ...
    • Multiwavelength Searches of Unidentified EGRET Sources 

      Mirabal, N.; Halpern, J.; Mukherjee, R.; Gotthelf, E.; Camilo, F. (2002-05-28)
      3EG J1835+5918 is the brightest of the so-called unidentified EGRET sources at intermediate galactic latitude (l,b)=(89,25). We obtained complete radio, optical, and X-ray coverage of its error box, discovering a faint ...
    • The Next Geminga: Deep Multiwavelength Observations of a Neutron Star Identified with 3EG J1835+5918 

      Halpern, J. P.; Gotthelf, E. V.; Mirabal, N.; Camilo, F. (IOP, 2002-06-02)
      We describe Chandra, Hubble Space Telescope (HST), and radio observations that reveal a radio-quiet but magnetospherically active neutron star in the error circle of the high-energy γ-ray source 3EG J1835+5918, the brightest ...
    • The 2001 Superoutburst of WZ Sagittae 

      Patterson, Joseph; Masi, Gianluca; Richmond, Michael W.; Martin, Brian; Beshore, Edward; Skillman, David R.; Kemp, Jonathan; Vanmunster, Tonny; Rea, Robert; Allen, William; Davis, Stacey; Davis, Tracy; Henden, Arne A.; Starkey, Donn; Foote, Jerry; Oksanen, Arto; Cook, Lewis M.; Fried, Robert E.; Husar, Dieter; Novák, Rudolf; Campbell, Tut; Robertson, Jeff; Krajci, Thomas; Pavlenko, Elena; Mirabal, Nestor; Niarchos, Panos G.; Brettman, Orville; Walker, Stan (IOP, 2002-06-10)
      We report the results of a worldwide campaign to observe WZ Sagittae during its 2001 superoutburst. After a 23 yr slumber at V = 15.5, the star rose within 2 days to a peak brightness of 8.2, and showed a main eruption ...
    • Discovery of a 136 Millisecond Radio and X-Ray Pulsar in Supernova Remnant G54.1+0.3 

      Camilo, F.; Lorimer, D. R.; Bhat, N. D. R.; Gotthelf, E. V.; Halpern, J. P.; Wang, Q. D.; Lu, F. J.; Mirabal, N. (IOP, 2002-06-26)
      We report the discovery of a pulsar with period P = 136 ms and dispersion measure 308 cm⁻³ pc in a deep observation of the supernova remnant (SNR) G54.1+0.3 with the Arecibo radio telescope. Timing measurements of the new ...
    • Time-dependent Optical Spectroscopy of GRB 010222: Clues to the Gamma-Ray Burst Environment 

      Mirabal, N.; Halpern, J. P.; Kulkarni, S. R.; Castro, S.; Bloom, J. S.; Djorgovski, S. G.; Galama, T. J.; Harrison, F. A.; Frail, D. A.; Price, P. A.; Reichart, D. E.; Ebeling, H.; Bunker, A.; Dawson, S.; Dey, A.; Spinrad, H.; Stern, D. (IOP, 2002-07-01)
      We present sequential optical spectra of the afterglow of GRB 010222 obtained 1 day apart using the Low-Resolution Imaging Spectrometer (LRIS) and the Echellette Spectrograph and Imager (ESI) on the Keck Telescopes. Three ...
    • Search for a Point-Source Counterpart of the Unidentified Gamma-Ray Source TeV J2032+4130 in Cygnus 

      Mukherjee, R.; Halpern, J. P.; Gotthelf, E. V.; Eracleous, M.; Mirabal, N. (IOP, 2003-02-05)
      We have made a multiwavelength study of the overlapping error boxes of the unidentified γ-ray sources TeV J2032+4130 and 3EG J2033+4118 in the direction of the Cygnus OB2 association (d = 1.7 kpc) in order to search for a ...
    • GRB 021004: A Possible Shell Nebula around a Wolf-Rayet Star Gamma-Ray Burst Progenitor 

      Mirabal, N.; Halpern, J. P.; Chornock, Ryan; Filippenko, Alexei V.; Terndrup, D. M.; Armstrong, E.; Kemp, J.; Thorstensen, J. R.; Tavarez, M.; Espaillat, C. (IOP, 2003-06-06)
      The rapid localization of GRB 021004 by the HETE-2 satellite allowed nearly continuous monitoring of its early optical afterglow decay, as well as high-quality optical spectra that determined a redshift of z3 = 2.328 for ...
    • Accurate Astrometry of 22 Southern Hemisphere Radio Sources 

      Fey, Alan L.; Ojha, Roopesh; Jauncey, David L.; Johnston, Kenneth J.; Reynolds, John E.; Lovell, James E. J.; Tzioumis, Anastasios K.; Quick, Jonathan F. H.; Nicolson, George D.; Ellingsen, Simon P.; McCulloch, Peter M.; Koyama, Yasuhiro (IOP, 2003-12-03)
      Milliarcsecond-accurate radio positions for 22 southern hemisphere extragalactic sources are reported. These positions are derived from Mark III very long baseline interferometry observations made between 2003 February and ...
    • The Impact of Interstellar Scintillation on Astrometric VLBI 

      Jauncey, David; Lovell, James; Ojha, Roopesh; Fey, Alan L.; Koyama, Yasuhiro; Bignall, Hayley E.; Kedziora-Chudczer, Lucyna; Macquart, Jean-Pierre; Rickett, Barney J.; Titov, Oleg; Tzioumis, Anastasios K. (2004)
      We review the evidence demonstrating the interstellar scintillation (ISS) is the principal mechanism responsible for the intra-day variability (IDV) seen at cm wavelengths in many compact, flat-spectrum extra-galactic radio ...