arXiv subjects

N. Gehrels

Publications and source records attributed to N. Gehrels.

At least 415 records · Page 23Linked to original sources

arXiv2001

Transient Gamma Ray Spectrometer Measurements of Gamma-Ray Lines from Novae. III. The 478 keV Line from 7Be Decay

The "Wind" spacecraft carrying the Transient Gamma Spectrometer (TGRS) moves in an extremely elliptical orbit that largely avoids Earth's trapped radiation belts and albedo gamma-radiation. The TGRS therefore enjoys a relatively low level of background that is also extremely stable. We show how this stability enables modeling of the time variability of background lines, which in turn enables a novel technique of background subtraction to be used in the detection of transient astrophysical lines. We apply a simple version of this method to the line at 478 keV that is expected to arise from nucleosynthesis of 7Be in nearby novae. This search covers the entire southern ecliptic hemisphere during 1995-1997, including five known individual events, and possible undiscovered individual events. The TGRS design also uses "Wind"'s 3 s rotation period to modulate signals from the Galactic Center (GC). We use this feature of the instrument to search for a quasi-constant level of 478 keV emission from the accumulation of 7Be from several novae that are expected to occur in the direction of the GC during that isotope's 53 d half-life. We derive upper limits on the transient (single-nova) emission which improve on previous limits by about an order of magnitude, and limits on the steady (many-nova) emission which represent a factor 2 improvement. Only weak limits can be placed on the key parameters in the nucleosynthesis and ejection of 7Be, however.

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arXiv2000

TGRS Observations of Positron Annihilation in Classical Novae

The TGRS experiment on board the Wind spacecraft has many advantages as a sky monitor --- broad field of view (~2 pi) centered on the south ecliptic pole), long life (1994-present), and stable low background and continuous coverage due to Wind's high altitude high eccentricity orbit. The Ge detector has sufficient energy resolution (3-4 keV at 511 keV) to resolve a cosmic positron annihilation line from the strong background annihilation line from beta-decays induced by cosmic ray impacts on the instrument, if the cosmic line is Doppler-shifted by this amount. Such lines (blueshifted) are predicted from nucleosynthesis in classical novae. We have searched the entire TGRS database for 1995-1997 for this line, with negative results. In principle such a search could yield an unbiased upper limit on the highly-uncertain Galactic nova rate. We carefully examined the times around the known nova events during this period, also with negative results. The upper limit on the nova line flux in a 6-hr interval is typically <3.8 E-3 photon/(cm2 s) at 4.6 sigma. We performed the same analysis for times around the outburst of Nova Vel 1999, obtaining a worse limit due to recent degradation of the detector response caused by cosmic ray induced damage.

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arXiv2000

Transient Gamma Ray Spectrometer Measurements of Gamma-Ray Lines from Novae. I. Limits on the Positron Annihilation Line in Five Individual Novae

The Transient Gamma Ray Spectrometer (TGRS) on board the WIND spacecraft has spent most of the interval 1995-1997 in a high-altitude orbit where gamma-ray backgrounds are low. Its high-resolution Ge spectrometer is thus able to detect weak lines which are slightly offset from stronger background features. One such line is predicted from nucleosynthesis in classical novae, where beta-decays on a time-scale of a few hours in an expanding envelope produce positrons that annihilate to generate a line which is blueshifted by a few keV away from the background annihilation line at 511 keV. The broad TGRS field of view contained five known Galactic novae during 1995 January - 1997 June, and we have searched the spectra taken around the times of these events for the blueshifted nova annihilation line. Although no definite detections were made, the method is shown to be sensitive enough to detect novae occurring on ONeMg-rich white dwarfs out to about 2.5 kpc.

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arXiv2000

Transient Gamma Ray Spectrometer Measurements of Gamma-Ray Lines from Novae. II. Constraining the Galactic Nova Rate from a Survey of the Southern Sky during 1995-1997

The good energy resolution (3--4 keV FWHM) of the Transient Gamma Ray Spectrometer (TGRS) on board the WIND spacecraft makes it sensitive to Doppler-shifted outbursts of 511 keV electron-positron annihilation radiation, the reason being that the Doppler shift causes the cosmic line to be slightly offset from a strong instrumental background 511 keV line at rest, which is ubiquitous in space environments. Such a cosmic line (blueshifted) is predicted to arise in classical novae due to the annihilation of positrons from $β$-decay on a timescale of a few hours in an expanding envelope. A further advantage of TGRS - its broad field of view, containing the entire southern ecliptic hemisphere - has enabled us to make a virtually complete and unbiased 3-year search for classical novae at distances up to ~1 kpc. We present negative results of this search, and estimate its implications for the highly-uncertain Galactic classical nova rate and for future space missions.

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arXiv1999

EXIST: A High Sensitivity Hard X-ray Imaging Sky Survey Mission for ISS

A deep all-sky imaging hard x-ray survey and wide-field monitor is needed to extend soft (ROSAT) and medium (ABRIXAS2) x-ray surveys into the 10-100 keV band (and beyond) at comparable sensitivity (~0.05 mCrab). This would enable discovery and study of >3000 obscured AGN, which probably dominate the hard x-ray background; detailed study of spectra and variability of accreting black holes and a census of BHs in the Galaxy; Gamma-ray bursts and associated massive star formation (PopIII) at very high redshift and Soft Gamma-ray Repeaters throughout the Local Group; and a full galactic survey for obscured supernova remnants. The Energetic X-ray Imaging Survey Telescope (EXIST) is a proposed array of 8 x 1m^2 coded aperture telescopes fixed on the International Space Station (ISS) with 160deg x 40deg field of view which images the full sky each 90 min orbit. EXIST has been included in the most recent NASA Strategic Plan as a candidate mission for the next decade. An overview of the science goals and mission concept is presented.

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arXiv1999

LOTIS Upper Limits and the Prompt OT from GRB 990123

GRB 990123 established the existence of prompt optical emission from gamma-ray bursts (GRBs). The Livermore Optical Transient Imaging System (LOTIS) has been conducting a fully automated search for this kind of simultaneous low energy emission from GRBs since October 1996. Although LOTIS has obtained simultaneous, or near simultaneous, coverage of the error boxes obtained with BATSE, IPN, XTE, and BeppoSAX for several GRBs, image analysis resulted in only upper limits. The unique gamma-ray properties of GRB 990123, such as very large fluence (top 0.4%) and hard spectrum, complicate comparisons with more typical bursts. We scale and compare gamma-ray properties, and in some cases afterglow properties, from the best LOTIS events to those of GRB 990123 in an attempt to determine whether the prompt optical emission of this event is representative of all GRBs. Furthermore, using LOTIS upper limits in conjunction with the relativistic blast wave model, we weakly constrain the GRB and afterglow parameters such as density of the circumburster medium and bulk Lorentz factor of the ejecta.

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arXiv1999

LOTIS Search for Early Time Optical Afterglows: GRB 971227

We report on the very early time search for an optical afterglow from GRB 971227 with the Livermore Optical Transient Imaging System (LOTIS). LOTIS began imaging the `Original' BATSE error box of GRB 971227 approximately 14 s after the onset of gamma-ray emission. Continuous monitoring of the position throughout the evening yielded a total of 499 images (10 s integration). Analysis of these images revealed no steady optical afterglow brighter than R=12.3 +- 0.2 in any single image. Coaddition of different combinations of the LOTIS images also failed to uncover transient optical emission. In particular, assuming a constant early time flux, no optical afterglow brighter than R=14.2 +- 0.2 was present within the first 1200 s and no optical afterglow brighter than R=15.0 +- 0.2 was present in the first 6.0 h. Follow up observations by other groups revealed a likely X-ray afterglow and a possible optical afterglow. Although subsequent deeper observations could not confirm a fading source, we show that these transients are not inconsistent with our present knowledge of the characteristics of GRB afterglows. We also demonstrate that with the upgraded thermoelectrically cooled CCDs, LOTIS is capable of either detecting very early time optical afterglow or placing stringent constraints on the relationship between the gamma-ray emission and the longer wavelength afterglow in relativistic blast wave models.

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arXiv1999

The INTEGRAL Core Observing Programme

The Core Programme of the INTEGRAL mission is defined as the portion of the scientific programme covering the guaranteed time observations for the INTEGRAL Science Working Team. This paper describes the current status of the Core Programme preparations and summarizes the key elements of the observing programme.

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arXiv1998

Gamma-ray limits on Galactic 60Fe nucleosynthesis and implications on the Origin of the 26Al emission

The Gamma Ray Imaging Spectrometer (GRIS) recently observed the gamma-ray emission from the Galactic center region. We have detected the 1809 keV Galactic 26Al emission at a significance level of 6.8-sigma but have found no evidence for emission at 1173 keV and 1332 keV, expected from the decay chain of the nucleosynthetic 60Fe. The isotopic abundances and fluxes are derived for different source distribution models. The resulting abundances are between 2.6+-0.4 and 4.5+-0.7 Solar Masses for 26Al and a 2-sigma upper limit for 60Fe between 1.7 and 3.1 Solar Masses. The measured 26Al emission flux is significantly higher than that derived from the CGRO/COMPTEL 1.8 MeV sky map. This suggests that a fraction of the 26Al emission may come from extended sources with a low surface brightness that are invisible to COMPTEL. We obtain a 60Fe to 26Al flux ratio 2-sigma upper limit of 0.14, which is slightly lower than the 0.16 predicted from current nucleosynthesis models assuming that SNII are the major contributors to the galactic 26Al. Since the uncertainties in the predicted fluxes are large (up to a factor of 2), our measurement is still compatible with the theoretical expectations.

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arXiv1998

TGRS Measurements of the Positron Annihilation Spectrum from the Galactic Center

We have obtained spectra of the Galactic center at energies 400-600 keV from high-resolution data acquired by the TGRS Ge spectrometer on board the WIND mission during 1995-1997. The data were obtained using an on-board occulter, and are relatively free from systematics and backgrounds. Analysis of the spectra reveals a well-resolved electron-positron annihilation line at 511 keV and the associated continuum due to annihilation via positronium formation. Measurements of the line width and the line-to-continuum ratio allow some constraints to be placed on the interstellar sites where annihilation occurs.

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arXiv1998

Spectra of GRB 970228 from the Transient Gamma-Ray Spectrometer

Visible afterglow counterparts have now been detected for two GRBs (970228 and 970508) but are absent, with $L_{opt}/L_γ$ ratios at least two orders of magnitude lower, for other GRBs, e.g., 970828. The causes of this variation are unknown. Any correspondence which could be discovered between the gamma-ray properties of a GRB and its $L_{opt}/L_γ$ would be useful, both in determining the GRB mechanisms, and in allocating resources for counterpart searches and studies. This paper presents the gamma-ray spectra of GRB 970228 as measured by the Transient Gamma-Ray Spectrometer and comments on characteristics of this GRB compared to others that do and do not have observable counterparts.

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arXiv1998

Proposed (to) EXIST: Hard X-ray Imaging All Sky Survey/Monitor

The hard x-ray (10-600 keV) sky is inherently time variable and yet has neither been surveyed nor monitored with a sensitive imaging telescope. The Energetic X-ray Imaging Survey Telescope (EXIST) is a mission concept, proposed for MIDEX, which would conduct the first imaging all-sky hard x-ray survey as well as provide a sensitive all sky monitor. With $\sim 60%$ sky coverage each orbit, and full sky coverage each 50 days, hard x-ray studies of gamma-ray bursts, AGN, galactic transients, x-ray binaries and accretion-powered pulsars can be conducted over a wide range of timescales. We summarize the scientific objectives of EXIST for both the survey and monitoring objectives. We describe the mission concept and the instrumentation approach, which would incorporate a large area array of Cd-Zn-Te (CZT) detectors, as well as some of our ongoing development of CZT array detectors.

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arXiv1997

First Year Results From LOTIS

LOTIS (Livermore Optical Transient Imaging System) is a gamma-ray burst optical counterpart search experiment located near Lawrence Livermore National Laboratory in California. The system is linked to the GCN (GRB Coordinates Network) real-time coordinate distribution network and can respond to a burst trigger in 6-15 seconds. LOTIS has a total field-of-view of 17.4 deg x 17.4 deg with a completeness sensitivity of m_V ~ 11 for a 10 second integration time. Since operations began in October 1996, LOTIS has responded to over 30 GCN/BATSE GRB triggers. Seven of these triggers are considered good events subject to the criteria of clear weather conditions, < 60 s response time, and > 50% coverage of the final BATSE 3sigma error circle. We discuss results from the first year of LOTIS operations with an emphasis on the observations and analysis of GRB 971006 (BATSE trigger 6414).

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arXiv1997

Super-LOTIS: A High-Sensitive Optical Counterpart Search Experiment

We are constructing a 0.6 meter telescope system to search for early time gamma-ray burst(GRB) optical counterparts. Super-LOTIS (Super-Livermore Optical Transient Imaging System) is an automated telescope system that has a 0.8 x 0.8 deg field-of-view, is sensitive to Mv ~ 19 and responds to a burst trigger within 5 min. This telescope will record images of the gamma-ray burst coordinates that is given by the GCN (GRB Coordinate Network). A measurement of GRB light curves at early times will greatly enhance our understanding of GRB physics.

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arXiv1997

New Constraints on Simultaneous Optical Emission From GRBs Measured by the LOTIS Experiment

LOTIS is a gamma-ray burst optical counterpart search experiment located near Lawrence Livermore National Laboratory in California. Since operations began in October 1996, LOTIS has responded to five triggers as of July 30, 1997, which occurred during good weather conditions. GRB970223 (BATSE Trigger #6100) was an exceptionally strong burst lasting $\sim30$ s with a peak at $\sim8$ s. LOTIS began imaging the error box $\sim 11$ s after the burst began, and achieved simultaneous optical coverage of 100% of the region enclosed by the BATSE $3σ$ error circle and the IPN annulus. No optical transients were observed brighter than the m$_V \sim 11$ completeness limit of the resulting images providing a new upper limit on the simultaneous optical to gamma-ray fluence ratio of $R_L < 1.1 \times 10^{-4}$ and on the simultaneous optical (at 700 nm) to gamma-ray (at 100 keV) flux density ratio of $R_F < 305$ for a B type spectrum and $R_F < 475$ for an M type spectrum.

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arXiv1997

OSSE Observations of the Soft Gamma Ray Continuum from the Galactic Plane at Longitude 95 Degrees

We present the results of OSSE observations of the soft gamma ray continuum emission from the Galactic plane at longitude 95 degrees. Emission is detected between 50 and 600 keV where the spectrum is fit well by a power law with photon index -2.6+-0.3 and flux (4.0+-0.5) 10^{-2} photons/s/cm^2/rad/MeV at 100 keV. This spectral shape in this range is similar to that found for the continuum emission from the inner Galaxy but the amplitude is lower by a factor of four. This emission is either due to unresolved and previously unknown point sources or it is of diffuse origin, or a combination of the two. Simultaneous observations with OSSE and smaller field of view instruments operating in the soft gamma ray energy band, such as XTE or SAX, would help resolve this issue. If it is primarily diffuse emission due to nonthermal electron bremsstrahlung, as is the >1 MeV Galactic ridge continuum, then the power in low energy cosmic ray electrons exceeds that of the nuclear component of the cosmic rays by an order of magnitude. This would have profound implications for the origin of cosmic rays and the energetics of the interstellar medium. Alternatively, if the emission is diffuse and thermal, then there must be a component of the interstellar medium at temperatures near 10^9 K.

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arXiv1997

Real-Time Optical Flux Limits From Gamma-Ray Bursts Measured By The GROCSE Experiment

The Gamma-Ray Optical Counterpart Search Experiment (GROCSE) presents new experimental upper limits on the optical flux from gamma-ray bursts (GRBs). Our experiment consisted of a fully-automated very wide-field opto-electronic detection system that imaged locations of GRBs within a few seconds of receiving trigger signals provided by BATSE's real-time burst coordinate distribution network (BACODINE). The experiment acquired ~3800 observing hours, recording 22 gamma-ray burst triggers within $\sim$30 s of the start of the burst event. Some of these bursts were imaged while gamma-ray radiation was being detected by BATSE. We identified no optical counterparts associated with gamma-ray bursts amongst these events at the m$_V$ $\sim$ 7.0 to 8.5 sensitivity level. We find the ratio of the upper limit to the V-band optical flux, F$_ν$, to the gamma-ray fluence, $Φ_γ$, from these data to be $2 \times 10^{-18} < F_ν/Φ_γ< 2 \times 10^{-16}$.

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arXiv1996

Search for TeV Counterparts of Gamma-Ray Bursts with the HEGRA Experiment

The HEGRA experiment is an air shower detector system for the study of neutral and charged cosmic rays in the energy range between 500 GeV to 10 PeV. Here we give an overview of how the HEGRA detector is used to search for TeV gamma-radiation associated with Gamma-Ray Bursts (GRBs) registered with the Burst And Transient Source Experiment (BATSE) on board the Compton Gamma-Ray Observatory. Furthermore, results of an archival search for GRB radiation above 15 TeV carried out with the HEGRA air shower arrays are shown. We conclude with a summary of the search activities planned for the future.

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