Gamma-ray Instruments Research Articles

Space-based gamma-ray detectors

Instruments for nuclear astrophysics collect photons using geometric optics, quantum optics or wave optics. Despite their disparate aperture systems, hard X- and soft gamma-ray telescopes have in common the way photons are detected, i.e. the non-dispersive spectroscopy performed in their detectors – gaz-filled counters, scintillators, and semiconductor detectors. The objective of the present short review is to focus on the status of these detectors in space-based gamma-ray instruments.

Global characteristics of GRBs observed with INTEGRAL and the inferred large population of low-luminosity GRBs

INTEGRAL has two sensitive gamma-ray instruments that have detected 46 gamma-ray bursts (GRBs) up to July 2007. We present the spectral, spatial, and temporal properties of the bursts in the INTEGRAL GRB catalogue using data from the imager, IBIS, and spectrometer, SPI. Spectral properties of the GRBs are determined using power-law, Band model and quasithermal model fits to the prompt emission. Spectral lags, i.e. the time delay in the arrival of low-energy gamma-rays with respect to high-energy gamma-rays, are measured for 31 of the GRBs. The photon index distribution of power-law fits to the prompt emission spectra is consistent with that obtained by Swift. The peak flux distribution shows that INTEGRAL detects proportionally more weak GRBs than Swift because of its higher sensitivity in a smaller field of view. The all-sky rate of GRBs above ~0.15 ph cm^-2 s^-1 is ~1400 yr^-1 in the fully coded field of view of IBIS. Two groups are identified in the spectral lag distribution, one with short lags <0.75 s (between 25-50 keV and 50-300 keV) and one with long lags >0.75 s. Most of the long-lag GRBs are inferred to have low redshifts because of their long spectral lags, their tendency to have low peak energies and their faint optical and X-ray afterglows. They are mainly observed in the direction of the supergalactic plane with a quadrupole moment of Q=-0.225+/-0.090 and hence reflect the local large-scale structure of the Universe. The rate of long-lag GRBs with inferred low luminosity is ~25% of Type Ib/c supernovae. Some of these bursts could be produced by the collapse of a massive star without a supernova or by a different progenitor, such as the merger of two white dwarfs or a white dwarf with a neutron star or black hole, possibly in the cluster environment without a host galaxy.

Airborne Geophysical survey of the PNG Highlands and the Papuan Peninsula

The Airborne Geophysical survey of the PNG Highlands and the Papuan Peninsula involves the collection of geophysical data using helicopter-borne magnetic and gamma-ray spectrometric instruments over 2 areas of Papua New Guinea (PNG), where little or no prior geophysical coverage is available. The availability of these data in both digital and paper-map formats is expected to encourage and facilitate exploration in PNG by international mining companies. All of Area 1, comprising the PNG Highlands portion of the survey, is anticipated to be completed by the end of the 2007 field season in late October. Flight operations will recommence in March 2008 in the Papuan Peninsula. Preliminary interpretation of the magnetic and radiometric data collected to date is providing a much higher degree of resolution and accuracy than has been available to date; with more detail in some areas, and new features being identified in others.

The MESSENGER Gamma-Ray and Neutron Spectrometer

A Gamma-Ray and Neutron Spectrometer (GRNS) instrument has been developed as part of the science payload for NASA’s Discovery Program mission to the planet Mercury. Mercury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) launched successfully in 2004 and will journey more than six years before entering Mercury orbit to begin a one-year investigation. The GRNS instrument forms part of the geochemistry investigation and will yield maps of the elemental composition of the planet surface. Major elements include H, O, Na, Mg, Si, Ca, Ti, Fe, K, and Th. The Gamma-Ray Spectrometer (GRS) portion detects gamma-ray emissions in the 0.1- to 10-MeV energy range and achieves an energy resolution of 3.5 keV full-width at half-maximum for 60Co (1332 keV). It is the first interplanetary use of a mechanically cooled Ge detector. Special construction techniques provide the necessary thermal isolation to maintain the sensor’s encapsulated detector at cryogenic temperatures (90 K) despite the intense thermal environment. Given the mission constraints, the GRS sensor is necessarily body-mounted to the spacecraft, but the outer housing is equipped with an anticoincidence shield to reduce the background from charged particles. The Neutron Spectrometer (NS) sensor consists of a sandwich of three scintillation detectors working in concert to measure the flux of ejected neutrons in three energy ranges from thermal to ∼7 MeV. The NS is particularly sensitive to H content and will help resolve the composition of Mercury’s polar deposits. This paper provides an overview of the Gamma-Ray and Neutron Spectrometer and describes its science and measurement objectives, the design and operation of the instrument, the ground calibration effort, and a look at some early in-flight data.

The Giant Flare of 2004 December 27 from SGR 1806−20

The giant flare of December 27, 2004 from SGR 1806-20 represents one of the most extraordinary events captured in over three decades of monitoring the gamma-ray sky. One measure of the intensity of the main peak is its effect on X- and gamma-ray instruments. RHESSI, an instrument designed to study the brightest solar flares, was completely saturated for ~0.5 s following the start of the main peak. A fortuitous alignment of SGR 1806-20 near the Sun at the time of the giant flare, however, allowed RHESSI a unique view of the giant flare event, including the precursor, the main peak decay, and the pulsed tail. Since RHESSI was saturated during the main peak, we augment these observations with Wind and RHESSI particle detector data in order to reconstruct the main peak as well. Here we present detailed spectral analysis and evolution of the giant flare. We report the novel detection of a relatively soft fast peak just milliseconds before the main peak, whose timescale and sizescale indicate a magnetospheric origin. We present the novel detection of emission extending up to 17 MeV immediately following the main peak, perhaps revealing a highly-extended corona driven by the hyper-Eddington luminosities. The spectral evolution and pulse evolution during the tail are presented, demonstrating significant magnetospheric twist and evolution during this phase. Blackbody radii are derived for every stage of the flare, which show remarkable agreement despite the range of luminosities and temperatures covered. Finally, we place significant upper limits on afterglow emission in the hundreds of seconds following the giant flare.

Gamma rays from colliding winds of massive stars

Colliding winds of massive binaries have long been considered as potential sites of non-thermal high-energy photon production. This is motivated by the detection of non-thermal spectra in the radio band, as well as by correlation studies of yet unidentified EGRET γ-ray sources with source populations appearing in star formation regions. This work re-considers the basic radiative processes and its properties that lead to high energy photon production in long-period massive star systems. We show that Klein–Nishina effects as well as the anisotropic nature of the inverse Compton scattering, the dominating leptonic emission process, likely yield spectral and variability signatures in the γ-ray domain at or above the sensitivity of current or upcoming gamma ray instruments like GLAST-LAT. In addition to all relevant radiative losses, we include propagation (such as convection in the stellar wind) as well as photon absorption effects, which a priori can not be neglected. The calculations are applied to WR 140 and WR 147, and predictions for their detectability in the γ-ray regime are provided. Physically similar specimen of their kind like WR 146, WR 137, WR 138, WR 112 and WR 125 may be regarded as candidate sources at GeV energies for near-future γ-ray experiments. Finally, we discuss several aspects relevant for eventually identifying this source class as a γ-ray emitting population. Thereby we utilize our findings on the expected radiative behavior of typical colliding wind binaries in the γ-ray regime as well as its expected spatial distribution on the γ-ray sky.

Demystifying an unidentified EGRET source by VHE gamma-ray observations

In a novel approach in observational high-energy gamma-ray astronomy, observations carried out by imaging atmospheric Cherenkov telescopes provide necessary templates to pinpoint the nature of intriguing, yet unidentified EGRET gamma-ray sources. Using GeV-photons detected by EGRET and taking advantage of high spatial resolution images from H.E.S.S. observations, we were able to shed new light on the EGRET observed gamma-ray emission in the Kookaburra complex, whose previous coverage in the literature is somewhat contradictory. 3EG J1420–6038 very likely accounts for two GeV gamma-ray sources (E>1 GeV), both in positional coincidence with the recently reported pulsar wind nebulae (PWN) by HESS in the Kookaburra/Rabbit complex. PWN associations at VHE energies, supported by accumulating evidence from observations in the radio and X-ray band, are indicative for the PSR/plerionic origin of spatially coincident, but still unidentified Galactic gamma-ray sources from EGRET. This not only supports the already suggested connection between variable, but unidentified low-latitude gamma-ray sources with pulsar wind nebulae (3EG J1420–6038 has been suggested as PWN candidate previously), it also documents the ability of resolving apparently confused EGRET sources by connecting the GeV emission as measured from a large-aperture space-based gamma-ray instrument with narrow field-of-view but superior spatial resolution observations by ground-based atmospheric Cherenkov telescopes, a very promising identification technique for achieving convincing individual source identifications in the era of GLAST-LAT.

Search for pulsed VHE gamma-ray emission from young pulsars with HESS

We present the results of a search for pulsed very-high-energy (VHE) gamma-ray emission from young pulsars using data taken with the H.E.S.S. imaging atmospheric Cherenkov telescope system. Data on eleven pulsars, selected according to their spin-down luminosity relative to distance, are searched for gamma-ray signals with periodicity at the respective pulsar spin period. Special analysis efforts were made to improve the sensitivity in the 100 GeV gamma-ray energy domain in an attempt to reduce the gap between satellite and ground-based gamma-ray instruments. No significant evidence for pulsed emission is found in any data set. Differential upper limits on pulsed energy flux are determined for all selected pulsars in the approximate gamma-ray energy range between 100 GeV and 50 TeV, using different limit determination methods, testing a wide range of possible pulsar light curves and energy spectra. The limits derived here imply that the magnetospheric VHE gamma-ray production efficiency in young pulsars is less than 10^-4 of the pulsar spin-down luminosity, requiring spectral turnovers for the high-energy emission of four established gamma-ray pulsars, and constrain the inverse Compton radiation component predicted by several outer gap models.

The Observed Galactic Annihilation Line: Possible Signature of Accreting Small‐Mass Black Holes in the Galactic Center

Various balloon and satellite observatories have revealed what appears to be an extended source of 0.511 MeV annihilation radiation with flux of approx. 10(exp -3) photons/sq cm/s centered on the Galactic Center. Positrons from radioactive products of stellar explosions can account for a significant fraction of the emission. We discuss an additional source for this emission: namely e(+)e(-) pairs produced when X-rays generated from the approx. 2.6 x 10(exp 6) solar mass Galactic Center Black Hole interact with approx. 10 MeV temperature blackbody emission from 10(exp 17) g black holes within 10(exp 14-l5) cm of the center. The number of such Small Mass Black Holes (SMMBHs) can account for the production of the 10(exp 42) e(+)/s that produces the observed annihilation in the inner Galaxy when transport effects are taken into account. We consider the possibility for confirming the presence of these SMMBHs in the Galactic Center region with future generations of gamma-ray instruments if a blackbody like emission of approx. 10 MeV temperature would be detected by them. Small Mass Black Hole can be a potential candidate for dark (invisible) matter hal

A microquasar model applied to unidentified gamma-ray sources

the date of receipt and acceptance should be inserted later Abstract. Among unidentified gamma-ray sources in the galactic plane, there are some that present significant variability and have been proposed to be high-mass microquasars. To deepen the study of the possible associa- tion between variable low galactic latitude gamma-ray sources and microquasars, we have applied a leptonic jet model based on the microquasar scenario that reproduces the gamma-ray spectrum of three unidentified gamma- ray sources, 3EG J1735 1500, 3EG J1828+0142 and GRO J1411 64, and is consistent with the observational constraints at lower energies. We conclude that if these sources were generated by microquasars, the particle acceleration processes could not be as efficient as in other objects of this type that present harder gamma-ray spectra. Moreover, the dominant mechanism of high-energy emission should be synchrotron self-Compton (SSC) scattering, and the radio jets may only be observed at low frequencies. For each particular case, further predictions of jet physical conditions and variability generation mechanisms have been made in the context of the model. Although there might be other candidates able to explain the emission coming from these sources, microquasars cannot be excluded as counterparts. Observations performed by the next generation of gamma-ray instruments, like GLAST, are required to test the proposed model.

The INTEGRAL Gamma-Ray Sky after 1000 days in orbit

This paper will review the main astrophysical results obtained in the field of high energy Galactic sources with the INTEGRAL/IBIS Gamma-ray Imager (Ubertini et al. 2003) on-board INTEGRAL (Winkler et al. 2003), the ESA space Observatory successfully launched the 17th October 2002 from Baikonur with a Proton vehicle. In view of the high sensitivity of the two gamma ray instruments IBIS and SPI and their capability to provide at the same time image, spectra and time profiles of all the sources in their wide field of view, a key project was approved as “Core Programme” to deeply observe the Galactic Centre (GCDE) and to exploit regular scans of the whole Galaxy Plane. The major results obtained in terms of classes of high energy emitters are shortly outlined.

High‐Energy Emission and Cosmic Rays from Gamma‐Ray Bursts

This paper is devoted to the analysis of particle acceleration in gamma-ray bursts (GRBs) and its radiative consequences. Therefore, we get on one hand constraints on the physics and on the other hand possible signatures of particle acceleration that could be recorded by new gamma-ray instruments. In a recent paper we have shown that UHECRs can be generated in GRBs even with conservative assumptions about the magnetic field and the scattering capability of its perturbations, provided that a suitable relativistic Fermi process is at work during the so-called internal shock phase. In this paper we extend the analysis of the consequences of these assumptions to the whole prompt emission of both electrons and protons. Indeed, assuming that the magnetic field decays as 1/r2 and that the scattering time of particles is longer than the Bohm assumption, in particular, with a rule derived from Kolmogorov scaling, we show that the four following events naturally happen with no other parameter adaptation than the intensity of the magnetic field, which turns out to be subequipartition: (1) UHECRs can be generated with a sufficient flux (≃1 km-2 yr-1) within the GZK sphere to account for the CR spectrum at the ankle. (In the previous paper, we showed that the associated pγ-neutrino emission is tiny.) (2) A thermal component below the so-called Epeak is often unavoidable and even amplified when the shocks start before the photosphere. (3) The CRs could radiate gamma rays around 67 MeV (in the comoving frame, which implies ≃20 GeV for the observer) due to π0 decay and a low-energy neutrino emission (around 0.2 GeV) associated with neutron decay and also neutrinos of energy between 5 and 150 GeV due to muon decay (as predicted in the previous paper). (4) The UHECRs radiate high-energy gamma rays between a few hundred MeV and 10 GeV (taking the pair creation process into account) due to their synchrotron emission, with a sufficient flux to be observable.

26Al production in [formula omitted] Velorum

The closest Wolf-Rayet star, WR 11 in the binary system γ 2 Velorum, is the only star for which the spectral signature of the 26Al produced in its core is expected to be detectable with current gamma-ray instruments, through the 1.8 MeV gamma-ray line produced by the radioactive decay of the 26Al nuclei. We present here the current status of both model predictions, from calculations of massive star evolution including rotation of stellar interior, and from data on γ 2 Velorum obtained by the ESA's gamma-ray satellite INTEGRAL over the first year of its mission.

Constraints on Extra-Dimensions and Variable Constants from Cosmological Gamma-Ray Bursts

The observation of the time delay between the soft emission and the high-energy radiation from cosmological gamma ray bursts can be used as an important observational test of multi-dimensional physical theories. The main source of the time delay is the variation of the electromagnetic coupling, due to dimensional reduction, which induces an energy dependence of the speed of light. For photons with energies around 1 TeV, the time delay could range from a few seconds in the case of Kaluza–Klein models to a few days for models with large extra-dimensions. Based on these results we suggest that the detection of the 18-GeV photon ∼4500 s after the keV/MeV burst in GRB 940217 provides a strong evidence for the existence of extra-dimensions. The time delay of photons, if observed by the next generation of high energy detectors, like, for example, the SWIFT and GLAST satellite based detectors, or the VERITAS ground-based TeV gamma-ray instrument, could differentiate between the different models with extra-dimensions.

TeV γ-radiation from Dark Matter annihilation in the Galactic center

Recent observations of the Galactic center with ground based gamma-ray instruments (CANGAROO-II, VERITAS, H.E.S.S.) have revealed a TeV ( 10 12 eV ) gamma-ray signal consistent with the position of Sgr A * . The observed energy spectrum and angular distribution of the excess could be explained by a massive Dark Matter candidate with a minimum mass of 12 TeV (90% c.l.) and an upper limit on the WIMP density for r < 10 pc of ( 1261 M ⊙ pc −3 ) ( 〈 σ v 〉 / ( 3 × 10 −26 cm 3 s −1 ) ) − 1 / 2 . The angular distribution of the excess events is consistent with a cuspy profile with ρ ( r ) ∝ r − α with α > 1.0 at a confidence level of 90%. Even though the mass and the cross section of the Dark Matter constituents are unexpectedly high in the framework of most models of non-baryonic Dark Matter, it cannot be ruled out.

High-mass microquasars and low-latitude gamma-ray sources

Population studies of unidentified EGRET sources suggest that there exist at least three different populations of galactic gamma-ray sources. One of these populations is formed by young objects distributed along the galactic plane with a strong concentration toward the inner spiral arms of the Galaxy. Variability, spectral and correlation analysis indicate that this population is not homogeneous. In particular, there is a subgroup of sources that display clear variability in their gamma-ray fluxes on timescales from days to months. Following the proposal by Kaufman Bernadó et al. (2002), we suggest that this group of sources might be high-mass microquasars, i.e. accreting black holes or neutron stars with relativistic jets and early-type stellar companions. We present detailed inhomogeneous models for the gamma-ray emission of these systems that include both external and synchrotron self-Compton interactions. We have included effects of interactions between the jet and all external photon fields to which it is exposed: companion star, accretion disk, and hot corona. We make broadband calculations to predict the spectral energy distribution of the emission produced in the inner jet of these objects up to GeV energies. The results and predictions can be tested by present and future gamma-ray instruments like INTEGRAL, AGILE, and GLAST.

Time resolved spectroscopy of GRB030501 using INTEGRAL

Abstract The Gamma-ray instruments on-board INTEGRAL offer a unique opportunity to perform time resolved analysis on GRBs. The imager IBIS allows accurate positioning of GRBs and broad band spectral analysis, while SPI provides high resolution spectroscopy. GRB 030501 was discovered by the INTEGRAL Burst Alert System in the ISGRI field of view. Although the burst was fairly weak (fluence F 20–200 keV ≃3.5×10 −6 erg cm−2) it was possible to perform time resolved spectroscopy with a resolution of a few seconds. The GRB shows a spectrum in the 20 - 400 keV range which is consistent with a spectral photon index Γ=−1.8. No emission line or spectral break was detectable in the spectrum. Although the flux seems to be correlated with the hardness of the GRB spectrum, there is no clear soft to hard evolution seen over the duration of the burst.

Changes in the X‐Ray Emission from the Magnetar Candidate 1E 2259+586 during Its 2002 Outburst

An outburst of more than 80 individual bursts, similar to those seen from Soft Gamma Repeaters (SGRs), was detected from the anomalous X-ray pulsar (AXP) 1E 2259+586 in 2002 June. Coincident with this burst activity were gross changes in the pulsed flux, persistent flux, energy spectrum, pulse profile, and spin-down of the underlying X-ray source. We present Rossi X-Ray Timing Explorer and X-Ray Multi-Mirror Mission observations of 1E 2259+586 that show the evolution of the aforementioned source parameters during and following this episode and identify recovery timescales for each. Specifically, we observe an X-ray flux increase (pulsed and phase-averaged) by more than an order of magnitude having two distinct components. The first component is linked to the burst activity and decays within ~2 days, during which the energy spectrum is considerably harder than during the quiescent state of the source. The second component decays over the year following the glitch according to a power law in time with an exponent -0.22 ? 0.01. The pulsed fraction decreased initially to ~15% rms but recovered rapidly to the preoutburst level of ~23% within the first 3 days. The pulse profile changed significantly during the outburst and recovered almost fully within 2 months of the outburst. A glitch of size ??max/? = (4.24 ? 0.11) ? 10-6 was observed in 1E 2259+586, which preceded the observed burst activity. The glitch could not be well fitted with a simple partial exponential recovery. An exponential rise of ~20% of the frequency jump with a timescale of ~14 days results in a significantly better fit to the data; however, contamination from a systematic drift in the phase of the pulse profile cannot be excluded. A fraction of the glitch (~19%) was recovered in a quasi-exponential manner having a recovery timescale of ~16 days. The long-term postglitch spin-down rate decreased in magnitude relative to the preglitch value. The changes in the source properties of 1E 2259+586 during its 2002 outburst are shown to be qualitatively similar to changes seen during or following burst activity in two SGRs, thus further solidifying the common nature of SGRs and AXPs as magnetars. The changes in persistent emission properties of 1E 2259+586 suggest that the star underwent a plastic deformation of the crust that simultaneously impacted the superfluid interior (crustal and possibly core superfluid) and the magnetosphere. Finally, the changes in persistent emission properties coincident with burst activity in 1E 2259+586 enabled us to infer previous burst-active episodes from this and other AXPs. The nondetection of these outbursts by all-sky gamma-ray instruments suggests that the number of active magnetar candidates in our Galaxy is larger than previously thought.

Status of the H.E.S.S. experiment and first results

The High Energy Stereoscopic System (H.E.S.S.) consists of four 12m diameter imaging Cherenkov telescopes, and is presently under construction in the Khomas Highland region of Namibia, 1.8 km above sea level (23∘16′18′′ S, 16∘30′00′′ E). The H.E.S.S. telescopes provide very good angular resolution and background rejection capability resulting in a sensitivity ~ 10 mCrab with an energy threshold of around 100 GeV. The first two telescopes are currently operating, and the array will be complete in early 2004. This paper provides a description of the H.E.S.S. telescopes and reports preliminary results obtained with the first telescope. PACS: 95.55.Ka X- and gamma ray telescopes and instrumentation – 95.55.Pw Astronomical observations: gamma ray

Detection of Hard X-Ray Emission from the Galactic Nuclear Region with INTEGRAL

This Letter presents the first results of an observational campaign to study the Galactic center with INTEGRAL, the International Gamma-Ray Astrophysics Laboratory. Mosaicked images were constructed using data obtained with ISGRI, the soft gamma-ray instrument of the coded aperture IBIS imager, in the energy ranges 20-40 and 40-100 keV. These give a yet unseen view of the high-energy sources of this region in hard X-rays and gamma rays with an angular resolution of 12' (FWHM). We report on the discovery of a source, IGR J1745.6-2901, coincident with the Galactic nucleus Sgr A* to within 09. Located at R.A. = 17h45m385 (J2000.0), decl. = -29°01'15'' (J2000.0), the source is visible up to about 100 keV with a 20-100 keV luminosity at 8 kpc of (2.89 ± 0.41) × 1035 ergs s-1. Although the new INTEGRAL source cannot unequivocally be associated to the Galactic nucleus, this is the first report of significant hard X-ray emission from within the inner 10' of the Galaxy and a contribution from the Galactic supermassive black hole itself cannot be excluded.