arXiv subjects

A. D. Falcone

Publications and source records attributed to A. D. Falcone.

At least 37 records · Page 2Linked to original sources

arXiv2008

The Gamma Ray Burst section of the White Paper on the Status and Future of Very High Energy Gamma Ray Astronomy: A Brief Preliminary Report

This is a short report on the preliminary findings of the gamma ray burst (GRB) working group for the white paper on the status and future of very high energy (VHE; >50 GeV) gamma-ray astronomy. The white paper discusses the status of past and current attempts to observe GRBs at GeV-TeV energies, including a handful of low-significance, possible detections. The white paper concentrates on the potential of future ground-based gamma-ray experiments to observe the highest energy emission ever recorded for GRBs, particularly for those that are nearby and have high Lorentz factors in the GRB jet. It is clear that the detection of VHE emission would have strong implications for GRB models, as well as cosmic ray origin. In particular, the extended emission phase (including both afterglow emission and possible flaring) of nearby long GRBs could provide the best possibility for detection. The difficult-to-obtain observations during the prompt phase of nearby long GRBs and short GRBs could also provide particularly strong constraints on the opacity and bulk Lorentz factors surrounding the acceleration site. The synergy with upcoming and existing observatories will, of course, be critical for both identification of GRBs and for multiwavelength/multimessenger studies.

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arXiv2008

Observations of X-ray Flares from Gamma Ray Bursts

Swift-XRT observations of the X-ray emission from gamma ray bursts (GRBs) and during the GRB afterglow have led to many new results during the past two years. One of these exciting results is that approximately 1/3-1/2 of GRBs contain detectable X-ray flares. The mean fluence of the X-ray flares is ~10 times less than that of the initial prompt emission, but in some cases the flare is as energetic as the prompt emission itself. The flares display fast rises and decays, and they sometimes occur at very late times relative to the prompt emission (sometimes as late as 10^5 s after T_0) with very high peak fluxes relative to the underlying afterglow decay that has clearly begun prior to some flares. The temporal and spectral properties of the flares are found to favor models in which flares arise due to the same GRB internal engine processes that spawned the prompt GRB emission. Therefore, both long and short GRB internal engine models must be capable of producing high fluences in the X-ray band at very late times.

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arXiv2008

VERITAS Observations of the gamma-Ray Binary LS I +61 303

LS I +61 303 is one of only a few high-mass X-ray binaries currently detected at high significance in very high energy gamma-rays. The system was observed over several orbital cycles (between September 2006 and February 2007) with the VERITAS array of imaging air-Cherenkov telescopes. A signal of gamma-rays with energies above 300 GeV is found with a statistical significance of 8.4 standard deviations. The detected flux is measured to be strongly variable; the maximum flux is found during most orbital cycles at apastron. The energy spectrum for the period of maximum emission can be characterized by a power law with a photon index of Gamma=2.40+-0.16_stat+-0.2_sys and a flux above 300 GeV corresponding to 15-20% of the flux from the Crab Nebula.

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arXiv2008

Observation of gamma-ray emission from the galaxy M87 above 250 GeV with VERITAS

The multiwavelength observation of the nearby radio galaxy M87 provides a unique opportunity to study in detail processes occurring in Active Galactic Nuclei from radio waves to TeV gamma-rays. Here we report the detection of gamma-ray emission above 250 GeV from M87 in spring 2007 with the VERITAS atmospheric Cherenkov telescope array and discuss its correlation with the X-ray emission. The gamma-ray emission is measured to be point-like with an intrinsic source radius less than 4.5 arcmin. The differential energy spectrum is fitted well by a power-law function: dPhi/dE=(7.4+-1.3_{stat}+-1.5_{sys})(E/TeV)^{-2.31+-0.17_{stat}+-0.2_{sys}} 10^{-9}m^{-2}s^{-1}TeV^{-1}. We show strong evidence for a year-scale correlation between the gamma-ray flux reported by TeV experiments and the X-ray emission measured by the ASM/RXTE observatory, and discuss the possible short-time-scale variability. These results imply that the gamma-ray emission from M87 is more likely associated with the core of the galaxy than with other bright X-ray features in the jet.

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arXiv2008

A Search for Dark Matter Annihilation with the Whipple 10m Telescope

We present observations of the dwarf galaxies Draco and Ursa Minor, the local group galaxies M32 and M33, and the globular cluster M15 conducted with the Whipple 10m gamma-ray telescope to search for the gamma-ray signature of self-annihilating weakly interacting massive particles (WIMPs) which may constitute astrophysical dark matter (DM). We review the motivations for selecting these sources based on their unique astrophysical environments and report the results of the data analysis which produced upper limits on excess rate of gamma rays for each source. We consider models for the DM distribution in each source based on the available observational constraints and discuss possible scenarios for the enhancement of the gamma-ray luminosity. Limits on the thermally averaged product of the total self-annihilation cross section and velocity of the WIMP, <σv>, are derived using conservative estimates for the magnitude of the astrophysical contribution to the gamma-ray flux. Although these limits do not constrain predictions from the currently favored theoretical models of supersymmetry (SUSY), future observations with VERITAS will probe a larger region of the WIMP parameter phase space, <σv> and WIMP particle mass (m_χ).

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arXiv2007

The First Survey of X-ray Flares from Gamma Ray Bursts Observed by Swift: Spectral Properties and Energetics

Observations of gamma ray bursts (GRBs) with Swift produced the initially surprising result that many bursts have large X-ray flares superimposed on the underlying afterglow. The flares were sometimes intense, had rapid rise and decay phases, and occurred late relative to the ``prompt'' phase. Some remarkable flares are observed with fluence comparable to the prompt GRB fluence. Many GRBs have several flares, which are sometimes overlapping. Short, intense, repetitive, and late flaring can be most easily understood within the context of the standard fireball model with the internal engine that powers the prompt GRB emission in an active state at late times. However, other models for flares have been proposed. Flare origin can be investigated by comparing the flare spectra to that of the afterglow and the initial prompt emission. In this work, we have analyzed all significant X-ray flares from the first 110 GRBs observed by Swift. From this sample 33 GRBs were found to have significant X-ray flares, with 77 flares that were detected above the 3$σ$ level. In addition to temporal analysis presented in a companion paper, a variety of spectral models have been fit to each flare. In some cases, we find that the spectral fits favor a Band function model, which is more akin to the prompt emission than to that of an afterglow. We find that the average fluence of the flares is 2.4e-7 erg/cm^2/s in the 0.2-10 keV energy band, which is approximately a factor of ten below the average prompt GRB fluence. These results, when combined with those presented in the companion paper on temporal properties of flares, supports the hypothesis that most X-ray flares are late-time activity of the internal engine that spawned the initial GRB; not an afterglow related effect.

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arXiv2007

The First Survey of X-ray Flares from Gamma Ray Bursts Observed by Swift: Temporal Properties and Morphology

We present the first systematic investigation of the morphological and timing properties of flares in GRBs observed by Swift/XRT. We consider a large sample drawn from all GRBs detected by Swift, INTEGRAL and HETE-2 prior to 2006 Jan 31, which had an XRT follow-up and which showed significant flaring. Our sample of 33 GRBs includes long and short, at low and high redshift, and a total of 69 flares. The strongest flares occur in the early phases, with a clear anti-correlation between the flare peak intensity and the flare time of occurrence. Fitting each X-ray flare with a Gaussian model, we find that the mean ratio of the width and peak time is = 0.13+/-0.10, albeit with a large scatter. Late flares at times > 2000 seconds have long durations, Delta t>300 s, and can be very energetic compared to the underlying continuum. We further investigated if there is a clear link between the number of pulses detected in the prompt phase by BAT and the number of X-ray flares detected by XRT, finding no correlation. However, we find that the distribution of intensity ratios between successive BAT prompt pulses and that between successive XRT flares is the same, an indication of a common origin for gamma-ray pulses and X-ray flares. All evidence indicates that flares are indeed related to the workings of the central engine and, within the standard fireball scenario, originate from internal shocks rather than external shocks. While all flares can be explained by long-lasting engine activity, 29/69 flares may also be explained by refreshed shocks. However, 10 can only be explained by prolonged activity of the central engine.

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arXiv2007

Attributes of flares in Gamma Ray Bursts: sample I

We discuss some of the preliminary results and findings derived from the analysis of a first sample of flares detected by the XRT on board Swift. The analysis shows that the morphology of flares is the one we expect from the collision of ultra-relativistic shells as it happens during the internal shock model proposed by Rees and Meszaros in 1994. Furthermore the Delta(t)/t ratio and the decay-time to rise-time ratio have mean values that are in good agreement with the values observed in the prompt emission pulses that are believed to originate from internal shocks. The conclusion is that the flare analysis favors the internal shock as due to shells that have been ejected by the central engine after the prompt emission. The central engine seems to remain active and capable of generating large amounts of energy also at later times. More data are needed to establish whether or not some of the flares could be due to shells that have been emitted with small Lorentz factor at the time of the prompt emission and generate later time flares due to the catch up of these shells.

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arXiv2007

Very High Energy Observations of Gamma-Ray Burst Locations with the Whipple Telescope

Gamma-ray burst (GRB) observations at very high energies (VHE, E > 100 GeV) can impose tight constraints on some GRB emission models. Many GRB afterglow models predict a VHE component similar to that seen in blazars and plerions, in which the GRB spectral energy distribution has a double-peaked shape extending into the VHE regime. VHE emission coincident with delayed X-ray flare emission has also been predicted. GRB follow-up observations have had high priority in the observing program at the Whipple 10m Gamma-ray Telescope and GRBs will continue to be high priority targets as the next generation observatory, VERITAS, comes on-line. Upper limits on the VHE emission, at late times (>~4 hours), from seven GRBs observed with the Whipple Telescope are reported here.

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arXiv2006

GRB 050713A: High Energy Observations of the GRB Prompt and Afterglow Emission

Swift discovered GRB 050713A and slewed promptly to begin observing with its narrow field instruments 72.6 seconds after the burst onset, while the prompt gamma-ray emission was still detectable in the BAT. Simultaneous emission from two flares is detected in the BAT and XRT. This burst marks just the second time that the BAT and XRT have simultaneously detected emission from a burst and the first time that both instruments have produced a well sampled, simultaneous dataset covering multiple X-ray flares. The temporal rise and decay parameters of the flares are consistent with the internal shock mechanism. In addition to the Swift coverage of GRB 050713A, we report on the Konus-Wind (K-W) detection of the prompt emission in the energy range 18-1150 keV, an upper limiting GeV measurement of the prompt emission made by the MAGIC imaging atmospheric Cherenkov telescope and XMM-Newton observations of the afterglow. Simultaneous observation between Swift XRT and XMM-Newton produce consistent results, showing a break in the lightcurve at T+~15ks. Together, these four observatories provide unusually broad spectral coverage of the prompt emission and detailed X-ray follow-up of the afterglow for two weeks after the burst trigger. Simultaneous spectral fits of K-W with BAT and BAT with XRT data indicate that an absorbed broken powerlaw is often a better fit to GRB flares than a simple absorbed powerlaw. These spectral results together with the rapid temporal rise and decay of the flares suggest that flares are produced in internal shocks due to late time central engine activity.

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arXiv2006

Observations of the Unidentified TeV Gamma-Ray Source TeV J2032+4130 with the Whipple Observatory 10 m Telescope

We report on observations of the sky region around the unidentified TeV gamma-ray source TeV J2032+4130 carried out with the Whipple Observatory 10 m atmospheric Cherenkov telescope for a total of 65.5 hrs between 2003 and 2005. The standard two-dimensional analysis developed by the Whipple collaboration for a stand-alone telescope reveals an excess in the field of view at a pre-trials significance level of 6.1 standard deviations. The measured position of this excess is alpha(2000) =20 h 32 m 27 s, delta(2000) = 41 deg 39 min 17 s. The estimated integral flux for this gamma-ray source is about 8% of the Crab-Nebula flux. The data are consistent with a point-like source. Here we present a detailed description of the standard two-dimensional analysis technique used for the analysis of data taken with the Whipple Observatory 10 m telescope and the results for the TeV J2032+4130 campaign. We include a short discussion of the physical mechanisms that may be responsible for the observed gamma-ray emission, based on possible association with known astrophysical objects, in particular Cygnus OB2.

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arXiv2006

The First VERITAS Telescope

The first atmospheric Cherenkov telescope of VERITAS (the Very Energetic Radiation Imaging Telescope Array System) has been in operation since February 2005. We present here a technical description of the instrument and a summary of its performance. The calibration methods are described, along with the results of Monte Carlo simulations of the telescope and comparisons between real and simulated data. The analysis of TeV $γ$-ray observations of the Crab Nebula, including the reconstructed energy spectrum, is shown to give results consistent with earlier measurements. The telescope is operating as expected and has met or exceeded all design specifications.

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arXiv2006

TeV Gamma-Ray Observations of the Perseus and Abell 2029 Galaxy Clusters

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.

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arXiv2006

Late-Time X-ray Flares during GRB Afterglows: Extended Internal Engine Activity

Observations of gamma ray bursts (GRBs) with Swift produced the initially surprising result that many bursts have large X-ray flares superimposed on the underlying afterglow. These flares were sometimes intense, rapid, and late relative to the nominal prompt phase. The most intense of these flares was observed by XRT with a flux >500 the afterglow. This burst then surprised observers by flaring again after >10000 s. The intense flare can be most easily understood within the context of the standard fireball model, if the internal engine that powers the prompt GRB emission is still active at late times. Recent observations indicate that X-ray flares are detected in ~1/3 of XRT detected afterglows. By studying the properties of the varieties of flares (such as rise/fall time, onset time, spectral variability, etc.) and relating them to overall burst properties, models of flare production and the GRB internal engine can be constrained.

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arXiv2006

Evidence for a Canonical GRB Afterglow Light Curve in the Swift/XRT Data

We present new observations of the early X-ray afterglows of the first 27 gamma-ray bursts (GRBs) detected with the Swift X-ray Telescope (XRT). The early X-ray afterglows show a canonical behavior, where the light curve broadly consists of three distinct power law segments: (i) an initial very steep decay (t^{-alpha} with 3 4, and augments the already severe requirements for the efficiency of the prompt gamma-ray emission.

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arXiv2006

X-ray flare in XRF 050406: evidence for prolonged engine activity

We present observations of XRF 050406, the first burst detected by Swift showing a flare in its X-ray light curve. During this flare, which peaks at t_peak ~210s after the BAT trigger, a flux variation of (delta F)/F~6 in a very short time (delta t)/t_peak<<1 was observed. Its measured fluence in the 0.2-10 keV band was ~1.4x10^-8 erg cm^-2, which corresponds to 1-15% of the prompt fluence. We present indications of spectral variations during the flare. We argue that the producing mechanism is late internal shocks, which implies that the central engine is still active at 210s, though with a reduced power with respect to the prompt emission. The X-ray light curve flattens to a very shallow slope with decay index of ~0.5 after ~4400s, which also supports continued central engine activity at late times. This burst is classified as an X-ray flash, with a relatively low fluence (~10^-7 erg cm^-2 in the 15-350 keV band, E_iso~10^51 erg), a soft spectrum (photon index 2.65), no significant flux above ~50 keV and a peak energy E_p<15 keV. XRF 050406 is one of the first examples of a well-studied X-ray light curve of an XRF. We show that the main afterglow characteristics are qualitatively similar to those of normal GRBs. In particular, X-ray flares superimposed on a power-law light curve have now been seen in both XRFs and GRBs. This indicates that a similar mechanism may be at work for both kinds of events.

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arXiv2005

The Giant X-ray Flare of GRB 050502B: Evidence for Late-Time Internal Engine Activity

Until recently, X-ray flares during the afterglow of gamma ray bursts (GRBs) were a rarely detected phenomenon, thus their nature is unclear. During the afterglow of GRB 050502B, the largest X-ray flare ever recorded rose rapidly above the afterglow lightcurve detected by the Swift X-ray Telescope. The peak flux of the flare was >500 times that of the underlying afterglow, and it occurred at >12 minutes after the nominal prompt burst emission. The fluence of this X-ray flare, (1.0 +/- 0.05) x 10^{-6} erg cm^{-2} in the 0.2-10.0 keV energy band, exceeded the fluence of the nominal prompt burst. The spectra during the flare were significantly harder than those measured before and after the flare. Later in time, there were additional flux increases detected above the underlying afterglow, as well as a break in the afterglow lightcurve. All evidence presented below, including spectral and particularly timing information during and around the giant flare, suggests that this giant flare was the result of internal dissipation of energy due to late central engine activity, rather than an afterglow-related effect. We also find that the data are consistent with a second central engine activity episode, in which the ejecta is moving slower than that of the initial episode, causing the giant flare and then proceeding to overtake and refresh the afterglow shock, thus causing additional activity at even later times in the lightcurve.

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arXiv2004

A Search for TeV Gamma-Ray Emission from High-Peaked Flat Spectrum Radio Quasars Using the Whipple Air-Cherenkov Telescope

Blazars have traditionally been separated into two broad categories based upon their optical emission characteristics; BL Lacs, with faint or no emission lines, and flat spectrum radio quasars (FSRQs) with prominent, broad emission lines. The spectral energy distribution of FSRQs has generally been thought of as being more akin to the low-peaked BL Lacs, which exhibit a peak in the infrared region of the spectrum, as opposed to high-peaked BL Lacs (HBLs), which exhibit a peak in UV/X-ray region of the spectrum. All blazars currently confirmed as sources of TeV emission are HBLs. Recent surveys have found several FSRQs exhibiting spectral properties similar to HBLs, particularly the synchrotron peak frequency. These objects are potential sources of TeV emission according to several models of blazar jet emission and blazar evolution. Measurements of TeV flux or upper limits could impact existing theories explaining the links between different blazar types and could have a significant impact on our understanding of the nature of objects that are capable of TeV emission. In particular, the presence (or absence) of TeV emission from FSRQs could confirm (or cast doubt upon) recent evolutionary models that expect intermediate objects in a transitionary state between FSRQ and BL Lac. The Whipple 10 meter imaging air-Cherenkov gamma-ray telescope is well suited for TeV gamma-ray observations. Using the Whipple telescope, we have taken data on a small selection of nearby(z<0.1 in most cases), high-peaked FSRQs. Although one of the objects, B2 0321+33, showed marginal evidence of flaring, no significant emission was detected. The implications of this paucity of emission and the derived upper limits are discussed.

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