arXiv subjects

N. Gehrels

Publications and source records attributed to N. Gehrels.

At least 397 records · Page 22Linked to original sources

arXiv2004

SPI observations of positron annihilation radiation from the 4th galactic quadrant: Spectroscopy

The status of the analysis of the electron/positron annihilation radiation performed by INTEGRAL/SPI in the 4th GALACTIC QUADRANT is reported. We use data from the first two Galactic Center Deep Exposures (GCDE) and from the Galactic Plane Scans (GPS). The analysis presented here is focused on the spectroscopic aspects of the electron/positron annihilation radiation. Background substraction and model fitting methods are described, and the parameters of the 511 keV line (flux, energy, and width) are deduced.

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arXiv2004

Swift's Ability to Detect Gamma-Ray Bursts

The Swift satellite will be a self-contained observatory that will bring new capabilities to the observing of the early afterglow emission of Gamma-ray Bursts. Swift is completely autonomous and will do all of the observations without help from the ground. There are three instruments on Swift. A large (5200 sq cm) coded aperture imager will locate the bursts within about 15 seconds. The satellite will be able to slew to point at the location within a minute or two. There are two narrow field of view instruments: an optical telescope and an x-ray telescope. Thus, Swift will provide simultaneous gamma-ray, x-ray, and optical observations of Gamma-ray bursts soon after the burst. A key to the success of Swift will be its ability to detect and locate a large number of gamma-ray bursts quick enough that the narrow field of view instruments can follow up. The results of simulations show that Swift will be able to detect about 300 bursts a year and locate about 150. The number that Swift will be able to slew to depends on constraints built into the satellite bus. Preliminary results indicate that we might be able to slew to 100 bursts per year, but that is heavily dependent on satellite operations.

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arXiv2004

The Trigger Algorithm for the Burst Alert Telescope on Swift

The Swift Burst Alert Telescope (BAT) is a huge (5200 cm2) coded aperture imager that will detect gamma-ray bursts in real time and provide a location that the Swift satellite will use to slew the optical and x-ray telescopes. The huge size of BAT is a challenge for the on-board triggering: a change as small as 1% is equivalent to a 1 sigma statistical variation in 1 second. There will be three types of triggers, two based on rates and one based on images. The first type of trigger is for short time scales (4 msec to 64 msec). These will be traditional triggers (single background) and we check about 25,000 combinations of time-energy-focal plane subregions per second. The second type of trigger will be similar to what is used on HETE: fits to multiple background regions to remove trends for time scales between 64 msec and 64 seconds. About 500 triggers will be checked per second. For these rate triggers, false triggers and variable non-GRB sources will be rejected by requiring a new source to be present in an image. The third type of trigger works on longer time scales (minutes), and will be based on routine images that are made of the field of view.

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arXiv2004

The BAT-Swift Science Software

The BAT instrument tells the Swift satellite where to point to make immediate follow-up observations of GRBs. The science software on board must efficiently process gamma-ray events coming in at up to 34 kHz, identify rate increases that could be due to GRBs while disregarding those from known sources, and produce images to accurately and rapidly locate new Gamma-ray sources.

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arXiv2004

Setting the Triggering Threshold on Swift

Swift has two main types of ``rate'' triggers: short and long. Short trigger time scales range from 4ms to 64ms, while long triggers are 64ms to ~ 16 seconds. While both short and long trigger have criteria with one background sample (traditional ``one-sided'' triggers), the long triggers can also have criteria with two background samples (``bracketed'' triggers) which remove trends in the background. Both long and short triggers can select energy ranges of 15-25, 15-50, 25-100 and 50-350 KeV. There are more than 180 short triggering criteria and approximately 500 long triggering criteria used to detect gamma ray bursts. To fully utilize these criteria, the thresholds must be set correctly. The optimum thresholds are determined by a tradeoff between avoiding false triggers and capturing as many bursts as possible. We use realistic simulated orbital variations, which are the prime cause of false triggers.

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arXiv2004

INTEGRAL and XMM-Newton Spectral Studies of NGC 4388

We present first INTEGRAL and XMM-Newton observations of a Seyfert galaxy, the type 2 AGN NGC 4388. Several INTEGRAL observations performed in 2003 allow us to study the spectrum in the 20 - 300 keV range. In addition two XMM-Newton observations give detailed insight into the 0.2 - 10 keV emission. The measurements presented here and comparison with previous observations by BeppoSAX, SIGMA and CGRO/OSSE show that the overall spectrum from soft X-rays up to the gamma-rays can be described by a highly absorbed (N_H = 2.7e23 1/cm^2) and variable non-thermal component in addition to constant non-absorbed thermal emission (T = 0.8 keV) of low abundance (7% solar), plus a constant Fe K-alpha and K-beta line. The hard X-ray component is well described by a simple power law with a mean photon index of 1.7. During the INTEGRAL observations the 20 - 100 keV flux increased by a factor of 1.4. The analysis of XMM-Newton data implies that the emission below 3 keV is decoupled from the AGN and probably due to extended emission as seen in Chandra observations. The constant iron line emission is apparently also decoupled from the direct emission of the central engine and likely to be generated in the obscuring material, e.g. in the molecular torus.

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arXiv2004

INTEGRAL observation of 3EG J1736-2908

The possible identification by INTEGRAL of the EGRET source 3EG J1736-2908 with the active galactic nucleus GRS 1734-292 is discussed. The latter was discovered in 1990 and later identified with a Seyfert 1 galaxy. At the time of the compilation of the 3rd EGRET Catalog, it was not considered as a possible counterpart of the source 3EG J1736-2908, which remained unidentified. A detailed multiwavelength study of the EGRET error circle is presented, by including archival radio, soft- and hard-X observations, suggesting that GRS 1734-292 could be a likely counterpart of 3EG J1736-2908, even though this poses very interesting questions about the production mechanisms of gamma-rays with energies greater than 100 MeV.

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arXiv2004

SPI observations of positron annihilation radiation from the 4th galactic quadrant: sky distribution

During its first year in orbit the INTEGRAL observatory performed deep exposures of the Galactic Center region and scanning observations of the Galactic plane. We report on the status of our analysis of the positron annihilation radiation from the 4th Galactic quadrant with the spectrometer SPI, focusing on the sky distribution of the 511 keV line emission. The analysis methods are described; current constraints and limits on the Galactic bulge emission and the bulge-to-disk ratio are presented.

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arXiv2004

The Swift Gamma-Ray Burst Mission

The Swift mission, scheduled for launch in early 2004, is a multiwavelength observatory for gamma-ray burst (GRB) astronomy. It is the first-of-its-kind autonomous rapid-slewing satellite for transient astronomy and pioneers the way for future rapid-reaction and multiwavelength missions. It will be far more powerful than any previous GRB mission, observing more than 100 bursts per year and performing detailed X-ray and UV/optical afterglow observations spanning timescales from 1 minute to several days after the burst. The objectives are to determine the origin of GRBs; classify GRBs and search for new types; study the interaction of the ultra-relativistic outflows of GRBs with their surrounding medium; and use GRBs to study the early universe out to z>10. The mission is being developed by a NASA-led international collaboration. It will carry three instruments: a new-generation wide-field gamma-ray (15-150 keV) detector; a narrow-field X-ray telescope; and a narrow-field UV/optical telescope. Redshift determinations will be made for most bursts. In addition to the primary GRB science, the mission will perform a hard X-ray survey to a sensitivity of ~1 mCrab (~2x10^{-11} erg cm^{-2} s^{-1} in the 15-150 keV band), more than an order of magnitude better than HEAO A-4. A flexible data and operations system will allow rapid follow-up observations of all types of high-energy transients, with rapid data downlink and uplink available through the NASA TDRSS system. The mission is currently funded for 2 years of operations and the spacecraft will have a lifetime to orbital decay of ~8 years. [ABRIDGED]

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arXiv2004

Did a gamma-ray burst initiate the late Ordovician mass extinction?

Gamma-ray bursts (hereafter GRB) produce a flux of radiation detectable across the observable Universe, and at least some of them are associated with galaxies. A GRB within our own Ggalaxy could do considerable damage to the Earth's biosphere; rate estimates suggest that a dangerously near GRB should occur on average two or more times per billion years. At least five times in the history of life, the Earth experienced mass extinctions that eliminated a large percentage of the biota. Many possible causes have been documented, and GRB may also have contributed. The late Ordovician mass extinction approximately 440 million years ago may be at least partly the result of a GRB. A special feature of GRB in terms of terrestrial effects is a nearly impulsive energy input of order 10 s. Due to expected severe depletion of the ozone layer, intense solar ultraviolet radiation would result from a nearby GRB, and some of the patterns of extinction and survivorship at this time may be attributable to elevated levels of UV radiation reaching the Earth. In addition a GRB could trigger the global cooling which occurs at the end of the Ordovician period that follows an interval of relatively warm climate. Intense rapid cooling and glaciation at that time, previously identified as the probable cause of this mass extinction, may have resulted from a GRB.

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arXiv2004

INTEGRAL and RXTE observations of broad-line radio galaxy 3C 111

3C 111 is an X-ray bright broad-line radio galaxy which is classified as a Fanaroff-Riley type II source with a double-lobe/single jet morphology, and reported superluminal motion. It is a well-known X-ray source, and was observed by every major X-ray observatory since HEAO-1. In this paper we present the results of the RXTE and INTEGRAL data analysis and compare them with the results of the previous observations.

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arXiv2004

Study of Unidentified EGRET sources with INTEGRAL: first results and future prospects

The primary objective of the study of unidentified EGRET sources with INTEGRAL is to locate with a few arcminute accuracy the hard X-ray / soft gamma-ray counterparts within the EGRET error circle or to determine upper limits on their emission in the INTEGRAL energy range. The INTEGRAL Galactic Plane Scan (GPS) and Galactic Centre Deep Exposure (GCDE) data have been analysed and cross-correlated with the EGRET 3^rd Catalogue to search for counterparts of EGRET sources. The IBIS detection of a source within the EGRET error circle of 3EG J1736-2908 is presented and its possible identification with the active galaxy GRS 1734-292 is discussed. Finally, preliminary results on the cross-correlation between EGRET unidentified sources and the first data from the IBIS survey of the Galactic Centre are presented.

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arXiv2003

A Numerical Gamma-Ray Burst Simulation Using Three-Dimensional Relativistic Hydrodynamics: The Transition from Spherical to Jet-like Expansion

We present the first unrestricted, three-dimensional relativistic hydrodynamical calculations of the blob of gas associated with the jet producing a gamma-ray burst. We investigate the deceleration phase of the blob corresponding to the time when afterglow radiation is produced, concentrating on the transition in which the relativistic beaming 1/gamma goes from being less than theta, where gamma is the bulk Lorentz factor and theta is the angular width of the jet, to 1/gamma > theta. We study the time dependent evolution of the physical parameters associated with the jet, both parallel to the direction of motion and perpendicular to it. We calculate light curves for observers at varying angles with respect to the velocity vector of the blob, assuming optically thin emission that scales with the local pressure. Our main findings are that (i) gas ahead of the advancing blob does not accrete onto and merge with the blob material but rather flows around the blob, (ii) the decay light curve steepens at a time corresponding roughly to 1/gamma equals theta (in accord with earlier studies), and (iii) the rate of decrease of the forward component of momentum in the blob is well-fit by a simple model in which the gas in front of the blob exerts a drag force on the blob, and the cross sectional area of the blob increases quadratically with laboratory time (or distance).

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arXiv2003

Early SPI/INTEGRAL measurements of galactic 511 keV line emission from positron annihilation

We report the first measurements of the 511 keV line emission from the Galactic Centre (GC) region performed with the spectrometer SPI on the space observatory INTEGRAL (International Gamma-Ray Astrophysics Laboratory). Taking into account the range of spatial distribution models which are consistent with the data, we derive a flux of $9.9^{+4.7}_{-2.1} \times 10^{-4}$ ph cm$^{-2}$ s$^{-1}$ and an intrinsic line width of $2.95^{+0.45}_{-0.51}$ keV (FWHM). The results are consistent with other high-spectroscopy measurements, though the width is found to be at the upper bound of previously reported values.

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arXiv2003

First results from the INTEGRAL Galactic plane scans

Scans of the Galactic plane performed at regular intervals constitute a key element of the guaranteed time observations of the INTEGRAL observing programme. These scans are done for two reasons: frequent monitoring of the Galactic plane in order to detect transient sources, and time resolved mapping of the Galactic plane in continuum and diffuse line emission. This paper describes first results obtained from the Galactic plane scans executed so far during the early phase (Dec 2002 - May 2003) of the nominal mission.

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arXiv2002

EXIST: Mission Design Concept and Technology Program

The Energetic X-ray Imaging Survey Telescope (EXIST) is a proposed very large area coded aperture telescope array, incorporating 8m^2 of pixellated Cd-Zn-Te (CZT) detectors, to conduct a full-sky imaging and temporal hard x-ray (10-600 keV) survey each 95min orbit. With a sensitivity (5sigma, 1yr) of ~0.05mCrab (10-150 keV), it will extend the ROSAT soft x-ray (0.5-2.5keV) and proposed ROSITA medium x-ray (2-10 keV) surveys into the hard x-ray band and enable identification and study of sources ~10-20X fainter than with the ~15-100keV survey planned for the upcoming Swift mission. At ~100-600 keV, the ~1mCrab sensitivity is 300X that achieved in the only previous (HEAO-A4, non-imaging) all-sky survey. EXIST will address a broad range of key science objectives: from obscured AGN and surveys for black holes on all scales, which constrain the accretion history of the universe, to the highest sensitivity and resolution studies of gamma-ray bursts it will conduct as the Next Generation Gamma-Ray Burst mission. We summarize the science objectives and mission drivers, and the results of a mission design study for implementation as a free flyer mission, with Delta IV launch. Key issues affecting the telescope and detector design are discussed, and a summary of some of the current design concepts being studied in support of EXIST is presented for the wide-field but high resolution coded aperture imaging and very large area array of imaging CZT detectors. Overall mission design is summarized, and technology development needs and a development program are outlined which would enable the launch of EXIST by the end of the decade, as recommended by the NAS/NRC Decadal Survey.

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arXiv2002

Proposed Next Generation GRB Mission: EXIST

A next generation Gamma Ray Burst (GRB) mission to follow the upcoming Swift mission is described. The proposed Energetic X-ray Imaging Survey Telescope, EXIST, would yield the limiting (practical) GRB trigger sensitivity, broad-band spectral and temporal response, and spatial resolution over a wide field. It would provide high resolution spectra and locations for GRBs detected at GeV energies with GLAST. Together with the next generation missions Constellation-X, NGST and LISA and optical-survey (LSST) telescopes, EXIST would enable GRBs to be used as probes of the early universe and the first generation of stars. EXIST alone would give ~10-50" positions (long or short GRBs), approximate redshifts from lags, and constrain physics of jets, orphan afterglows, neutrinos and SGRs.

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arXiv2001

LOTIS, Super-LOTIS, SDSS and Tautenburg Observations of GRB 010921

We present multi-instrument optical observations of the High Energy Transient Explorer (HETE-2)/Interplanetary Network (IPN) error box of GRB 010921. This event was the first gamma ray burst (GRB) localized by HETE-2 which has resulted in the detection of an optical afterglow. In this paper we report the earliest known observations of the GRB010921 field, taken with the 0.11-m Livermore Optical Transient Imaging System (LOTIS) telescope, and the earliest known detection of the GRB010921 optical afterglow, using the 0.5-m Sloan Digital Sky Survey Photometric Telescope (SDSS PT). Observations with the LOTIS telescope began during a routine sky patrol 52 minutes after the burst. Observations were made with the SDSS PT, the 0.6-m Super-LOTIS telescope, and the 1.34-m Tautenburg Schmidt telescope at 21.3, 21.8, and 37.5 hours after the GRB, respectively. In addition, the host galaxy was observed with the USNOFS 1.0-m telescope 56 days after the burst. We find that at later times (t > 1 day after the burst), the optical afterglow exhibited a power-law decline with a slope of $α= 1.75 \pm 0.28$. However, our earliest observations show that this power-law decline can not have extended to early times (t < 0.035 day).

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