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  • Astronomy  (7)
  • Astrophysics  (6)
  • 1
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    Evidence for Filamentary Jet Structure: The Light Curve of GRB 011211 (2003)
    In:  CASI
    Details
    Publication Date: 2018-06-12
    Description: We report on the discovery of the optical afterglow of the X-ray rich, long-duration gamma-ray burst GRB 011211, and the oscillatory behavior present in its optical and X-ray afterglow light curve. The time scale of the fluctuations, -1 hour, is much smaller than the time of the observations, -12 hours from the onset of the gamma-ray burst. The character and strength of the fluctuations visible in the optical data are unprecedented, and are inconsistent with causally connected variations in the emission of a symmetric, relativistic blast wave. Moreover, the differential time lag between the short-term variations in X-ray and optical energies suggests they do not arise from the same emitting region. Such variability may imply that local spherical symmetry is broken because the energy content across the jet-emitting surface is not uniform, indicating the detection of a small scale substructure within the jet itself.
    Keywords: Astronomy
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  • 2
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    Detection of GRB 060927 at zeta = 5.47: Implications for the Use of Gamma-Ray Bursts as Probes of the End of the Dark Ages (2007)
    In:  CASI
    Details
    Publication Date: 2018-06-12
    Description: We report on follow-up observations of the gamma-ray burst GRB 060927 using the robotic ROTSE-IIIa telescope and a suite of larger aperture groundbased telescopes. An optical afterglow was detected 20 s after the burst, the earliest rest-frame detection of optical emission from any GRB. Spectroscopy performed with the VLT about 13 hours after the trigger shows a continuum break at lambda approx. equals 8070 A, produced by neutral hydrogen absorption at zeta = 5.6. We also detect an absorption line at 8158 A which we interpret as Si II lambda 1260 at zeta = 5.467. Hence, GRB 060927 is the second most distant GRB with a spectroscopically measured redshift. The shape of the red wing of the spectral break can be fitted by a damped Ly(alpha) profile with a column density with log(N(sub HI)/sq cm) = 22.50 +/- 0.15. We discuss the implications of this work for the use of GRBs as probes of the end of the dark ages and draw three main conclusions: i) GRB afterglows originating from zeta greater than or approx. equal to 6 should be relatively easy to detect from the ground, but rapid near-infrared monitoring is necessary to ensure that they are found; ii) The presence of large H I column densities in some GRBs host galaxies at zeta 〉 5 makes the use of GRBs to probe the reionization epoch via spectroscopy of the red damping wing challenging; iii) GRBs appear crucial to locate typical star-forming galaxies at zeta 〉 5 and therefore the type of galaxies responsible for the reionization of the universe.
    Keywords: Astrophysics
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  • 3
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    Constraints on an Optical Afterglow and on Supernova Light Following the Short Burst GRB 050813 (2006)
    In:  CASI
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    Publication Date: 2018-06-12
    Description: We report early follow-up observations of the error box of the short burst 050813 using the telescopes at Calar Alto and at Observatorio Sierra Nevada (OSN), followed by deep VLT/FORS2 I-band observations obtained under very good seeing conditions 5.7 and 11.7 days after the event. No evidence for a GRB afterglow was found in our Calar Alto and OSN data, no rising supernova component was detected in our FORS2 images. A potential host galaxy can be identified in our FORS2 images, even though we cannot state with certainty its association with GRB 050813. IN any case, the optical afterglow of GRB 050813 was very faint, well in agreement with what is known so far about the optical properties of afterglows of short bursts. We conclude that all optical data are not in conflict with the interpretation that GRB 050813 was a short burst.
    Keywords: Astronomy
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  • 4
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    The Optical Afterglow of a Short Gamma-ray Burst (2005)
    In:  Other Sources
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    Publication Date: 2019-07-18
    Description: It has long been known that there are two classes of gamma-ray bursts (GRBs), principally distinguished by their durations. The breakthrough in our understanding of long-duration GRBs (greater than 2 seconds in duration), that ultimately linked them with energetic Type Ic supernovae, came about from the discovery of their long-lived X-ray and optical "afterglow", when precise and rapid localizations of the sources could finally be obtained. Recently, X-ray localizations have become available for short (less than 2 seconds in duration) GRBs, a hitherto elusive GRB population, that has evaded optical detection for more than thirty years. Here we report the discovery of transient optical emission (R approximately 23 mag) associated with a short GRB. This first short GRB afterglow is localized with sub-arcsecond accuracy onto the outskirts of a blue dwarf galaxy. Unless the optical and X-ray afterglow arise from different mechanisms our observations 33 h after the GRB suggest that, analogously to long GRBs, we observe synchrotron emission from ultrarelativistic ejecta (ZZZ CAN WE LIMIT GAMMA?). In contrast, we did not detect a bright supernova, as found in most nearby long GRB afterglows, which suggests a different origidstrongly constrain the nature of the short GRB progenitors.
    Keywords: Astronomy
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  • 5
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    Panchromatic Observations of the Textbook GRB 110205A: Constraining Physical Mechanisms of Prompt Emission and Afterglow (2011)
    In:  CASI
    Details
    Publication Date: 2019-07-12
    Description: We present a comprehensive analysis of a bright, long duration (T(sub 90) approx. 257 s) GRB 110205A at redshift z = 2.22. The optical prompt emission was detected by Swift/UVOT, ROTSE-IIIb and BOOTES telescopes when the GRB was still radiating in the gamma-ray band. Thanks to its long duration, nearly 200 s of observations were obtained simultaneously from optical, X-ray to gamma-ray (1 eV - 5 MeV), which makes it one of the exceptional cases to study the broadband spectral energy distribution across 6 orders of magnitude in energy during the prompt emission phase. In particular, by fitting the time resolved prompt spectra, we clearly identify, for the first time, an interesting two-break energy spectrum, roughly consistent with the standard GRB synchrotron emission model in the fast cooling regime. Although the prompt optical emission is brighter than the extrapolation of the best fit X/ -ray spectra, it traces the -ray light curve shape, suggesting a relation to the prompt high energy emission. The synchrotron + synchrotron self-Compton (SSC) scenario is disfavored by the data, but the models invoking a pair of internal shocks or having two emission regions can interpret the data well. Shortly after prompt emission (approx. 1100 s), a bright (R = 14.0) optical emission hump with very steep rise ( alpha approx. 5.5) was observed which we interpret as the emission from the reverse shock. It is the first time that the rising phase of a reverse shock component has been closely observed.
    Keywords: Astrophysics
    Type: GSFC.JA.5941.2012
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  • 6
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    Identifying the Location in the Host Galaxy of Short GRB 1111l7A with the Chandra Sub- Arcsecond Position (2012)
    In:  CASI
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    Publication Date: 2019-07-12
    Description: We present our successful program using Chandra for identifying the X-ray afterglow with sub-arcsecond accuracy for the short GRB 111117A d iscovered by Swift and Fermi. Thanks to our rapid target of opportuni ty request, Chandra clearly detected the X-ray afterglow, whereas no optical afterglow was found in deep optical observations. Instead, we clearly detect the host galaxy in optica; and also in near-infrared b ands. We found that the best photometric redshift fitofthe host is z = 1.31:(+0.46/-0.23) (90% confidence), making it one of the highest redshift short GRBs. Furthermore, we see an offset of 1.0+/-O.2 arcseco nds, which corresponds to 8.4+/-1.7 kpc aSBuming z= 1.31, between the host and the afterglow position. We discuss the importance of using Chandra for obtaining sub-arcsecond localization of the afterglow in X -rays for short GRBs to study GRB environments in great detail.
    Keywords: Astronomy
    Type: GSFC.JA.7136.2012
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  • 7
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    Strategies for Prompt Searches for GRB Afterglows: The Discovery of GRB 001011 Optical/Near-Infrared Counterpart Using Colour-Colour Selection (2001)
    In:  Other Sources
    Details
    Publication Date: 2019-07-18
    Description: We report the discovery of the optical and near-infrared counterpart to GRB 001011. The GRB 001011 error box determined by Beppo-SAX was simultaneously imaged in the near-infrared by the 3.58-m. New Technology Telescope and in the optical by the 1.54-m Danish Telescope - 8 hr after the gamma-ray event. We implement the colour-colour discrimination technique proposed by Rhoads (2001) and extend it using near-IR data as well. We present the results provided by an automatic colour-colour discrimination pipe-line developed to discern the different populations of objects present in the GRB 001011 error box. Our software revealed three candidates based on single-epoch images. Second-epoch observations carried out approx. 3.2 days after the burst revealed that the most likely candidate had faded thus identifying it with the counterpart to the GRB. In deep R-band images obtained 7 months after the burst a faint (R=25.38 plus or minus 0.25) elongated object, presumably the host galaxy of GRB 001011, was detected at the position of the afterglow. The GRB 001011 afterglow is the first discovered with the assistance of colour-colour diagram techniques. We discuss the advantages of using this method and its application to boxes determined by future missions.
    Keywords: Astronomy
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  • 8
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    Tomography of a Gamma-ray Burst Progenitor and its Host Galaxy (2005)
    In:  Other Sources
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    Publication Date: 2019-07-18
    Description: We have obtained near-infrared and high-resolution optical spectroscopy of the bright afterglow of the very intense gamma-ray burst recorded on 2002, October 4 (GRB 021004). Besides of line emission in the near-IR allowing an independent measurement of the systemic redshift (z = 2.3304 plus or minus 0.0005), we find several absorption line groups spanning a range of about 3,000 kilometers per second in velocity relative to the redshift of the host galaxy. The absorption profiles are very complex with both velocity-broadened components extending over several 100 kilometers per second and narrow lines with velocity widths of only approximately 20 kilometers per second. By analogy with QSO absorption line studies, the relative velocities, widths, and degrees of ionization of the lines ("line-locking", "ionization-velocity correlation") show that the progenitor had both an extremely strong radiation field and several distinct mass loss phases (winds). These results are consistent with GRB progenitors being massive stars, such as Luminous Blue Variables (LBVs) or Wolf-Rayet stars, providing a detailed picture of the spatial and velocity structure of the GRB progenitor star at the time of explosion. The host galaxy is a prolific star-forming galaxy with a SFR of approximately 10 solar mass yr(sup -l).
    Keywords: Astrophysics
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  • 9
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    An Unusual Stellar Death on Christmas Day (2011)
    In:  CASI
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    Publication Date: 2019-07-12
    Description: Long Gamma-Ray Bursts (GRBs) are the most dramatic examples of massive stellar deaths, usually associated with supernovae. They release ultra-relativistic jets producing non-thermal emission through synchrotron radiation as they interact with the surrounding medium. Here we report observations of the peculiar GRB 101225A (the "Christmas burst"). Its gamma-ray emission was exceptionally long and followed by a bright X-ray transient with a hot thermal component and an unusual optical couuterpart. During the first 10 days, the optical emission evolved as an expanding, cooling blackbody after which an additional component, consistent with a faint supernova, emerged. We determine its distance to 1.6 Gpc by fitting the spectral-energy distribution and light curve of the optical emission with a GRB-supernova template. Deep optical observations may have revealed a faint, unresolved host galaxy. Our proposed progenitor is a helium star-neutron star merger that underwent a common envelope phase expelling its hydrogen envelope. The resulting explosion created a GRB-like jet which gets thermalized by interacting with the dense, previously ejected material and thus creating the observed black-body, until finally the emission from the supernova dominated. An alternative explanation is a minor body falling onto a neutron star io the Galaxy
    Keywords: Astronomy
    Type: GSFC.JA.01106.2012
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  • 10
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    A Deep Search with the Hubble Space Telescope for Late-Time Supernova Signatures in the Hosts of XRF 011030 and XRF 020427 (2005)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: X-ray flashes (XRFs) are, like gamma-ray bursts (GRBs), thought to signal the collapse of massive stars in distant galaxies. Many models posit that the isotropic equivalent energies of XRFs are lower than those for GRBs, such that they are visible fiom a reduced range of distances when compared with GRBs. Here we present the results of two-epoch Hubble Space Telescope imaging of two XRFs. These images, taken approximately 45 and 200 days postburst, reveal no evidence of an associated supernova in either case. Supernovae such as SN 1998bw would have been visible out to z approximately 1.5 in each case, while fainter supernovae such as SN 2002ap would have been visible to z approximately 1. If the XRFs lie at such large distances, their energies would not fit the observed correlation between the GRB peak energy and isotropic energy release (E(sub p) proportional to E(sub iso)(sup 1/2), in which soft bursts are less energetic. We conclude that, should these XRFs reside at low redshifts (z less than 0.6), either their line of sight is heavily extinguished, they are associated with extremely faint supernovae, or, unlike GRBs, these XRFs do not have temporally coincident supernovae.
    Keywords: Astrophysics
    Type: The Astrophysical Journal; 622; 977-985
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