Browsing by Author "Eracleous, M."
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Item GRB 991216 Joins the Jet Set: Discovery and Monitoring of its Optical Afterglow(IOP, 2000-06-15) 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.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 days. The derivation of the late-time decay slope takes into account the constant contribution of a host or intervening galaxy, which was measured 110 days after the event at R = 24.56 ± 0.14, although the light curve deviates from a single power law whether or not a constant term is included. The early-time spectral energy distribution of the afterglow can be described as Fν ∝ ν-0.74±0.05 or flatter between optical and X-ray, which, together with the slow initial decay, is characteristic of standard adiabatic evolution in a uniformly dense medium. Assuming that a reported absorption-line redshift of 1.02 is correct, the apparent isotropic energy of 6.7 × 10⁵³ ergs is reduced by a factor of ≈200 in the jet model, and the initial half-opening angle is ≈6°. GRB 991216 is the third good example of a jetlike afterglow (following GRB 990123 and GRB 990510), supporting a trend in which the apparently most energetic γ-ray events have the narrowest collimation and a uniform interstellar medium environment. This, plus the absence of evidence for supernovae associated with jetlike afterglows, suggests that these events may originate from a progenitor in which angular momentum plays an important role but a massive stellar envelope or wind does not, e.g., in the coalescence of a compact binary.Item Search for a Point-Source Counterpart of the Unidentified Gamma-Ray Source TeV J2032+4130 in Cygnus(IOP, 2003-02-05) Mukherjee, R.; Halpern, J. P.; Gotthelf, E. V.; Eracleous, M.; Mirabal, N.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 point-source counterpart of the first unidentified TeV source. Optical identifications and spectroscopic classifications for the brighter X-ray sources in ROSAT PSPC and Chandra ACIS images are obtained, without finding a compelling counterpart. The classified X-ray sources are a mix of early- and late-type stars, with one exception. The brightest source in the Chandra observation is a new, hard absorbed source that is both transient and rapidly variable. It lies 7' from the centroid of the TeV emission, which places it outside of the claimed 2 σ location (r ≈ 4farcm8). A possible eclipse or "dip" transition is seen in its light curve. With a peak 1-10 keV luminosity of ≈7 × 10³²(d/1.7 kpc)2 ergs s⁻¹, this source could be a quiescent low-mass X-ray binary that lies beyond the Cyg OB2 association. A coincident, reddened optical object of R = 20.4, J = 15.4, H = 14.2, and K = 13.4 is observed but not yet classified as a result of the lack of obvious emission or absorption features in its spectrum. Alternatively, this Chandra and optical source might be a considered a candidate for a "proton blazar," a long hypothesized type of radio-weak γ-ray source. More detailed observations will be needed to determine the nature of this variable X-ray source and to assess the possibility of its connection with TeV J2032+4130.Item Search for the Identification of 3EG J1835+5918: Evidence for a New Type of High-Energy Gamma-Ray Source(IOP, 2000-05-10) Mirabal, N.; Halpern, J. P.; Eracleous, M.; Becker, R. H.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 region around it, including X-ray, radio, and optical imaging surveys, as well as optical spectroscopic classification of most of the active objects in this area. Identifications are made of all but one of the ROSAT and ASCA sources in this region to a flux limit of approximately 5 × 10⁻¹⁴ ergs cm⁻² s⁻¹, which is 10⁻⁴ of the γ-ray flux. The identified X-ray sources in or near the EGRET error ellipse are radio-quiet QSOs, a galaxy cluster, and coronal emitting stars. We also find nine quasars using purely optical color selection, and we have monitored the entire field for variable optical objects on short and long timescales without any notable discoveries. The radio sources inside the error ellipse are all fainter than 4 mJy at 1.4 GHz. There are no flat-spectrum radio sources in the vicinity; the brightest neighboring radio sources are steep-spectrum radio galaxies or quasars. Since no blazar-like or pulsar-like candidate has been found as a result of these searches, 3EG J1835+5918 must be lacking one or more of the physically essential attributes of these known classes of γ-ray emitters. If it is an AGN it lacks the beamed emission radio of blazars by at least a factor of 100 relative to identified EGRET blazars. If it is an isolated neutron star, it lacks the steady thermal X-rays from a cooling surface and the magnetospheric nonthermal X-ray emission that is characteristic of all EGRET pulsars. If a pulsar, 3EG J1835+5918 must be either older or more distant than Geminga, and probably an even more efficient or beamed γ-ray engine. One intermittent ROSAT source falls on a blank optical field to a limit of B > 23.4, V > 25.2, and R > 24.5. In view of this conspicuous absence, RX J1836.2+5925 should be examined further as a candidate for identification with 3EG J1835+5918 and possibly the prototype of a new class of high-energy γ-ray source.