arXiv subjects

A. J. Dean

Publications and source records attributed to A. J. Dean.

At least 91 records · Page 5Linked to original sources

arXiv2004

The BATSE 9 year histories of the brightest AGN

The Burst and Transient Source Experiment (BATSE) aboard CGRO monitored the whole sky through 8 NaI(Tl) crystal scintillators sensitive in the 20 keV - 2 MeV energy band continuously from April 1991 until June 2000. Results are presented on the long term variabilty observed in the brightest Active Galactic Nuclei (AGN) present in the BATSE data archive. This was achieved through the application of the Earth Occultation technique to data flat-fielded using the BATSE Mass Model. Removal of the temporal background variations from the data should allow a more sensitive extraction of source parameters. Analysis of the general trends of the 9-year light curves are also presented.

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arXiv2004

Gamma-Ray All Sky Imaging with BATSE

The BATSE mission aboard CGRO observed the whole sky for 9 years in the 20 keV - 2 MeV energy band. Flat-fielding of the temporal variations in the background present in the data set has been accomplished through a GEANT3 Monte-Carlo simulation - the BATSE Mass Model (BAMM). The Earth Occultation technique (EOT) is used together with a maximum-likelihood imaging approach to construct all-sky maps with ~mCrab sensitivity. Additionally, a non-linear CLEAN algorithm is applied to the all-sky maps to remove large artefacts that are systematic of the EOT. The latest survey results produced through the application of this technique to a subset of the BATSE data set are presented.

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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

GRS 1915+105: The first three months with INTEGRAL

GRS 1915+105 is being observed as part of an Open Time monitoring program with INTEGRAL. Three out of six observations from the monitoring program are presented here, in addition to data obtained through an exchange with other observers. We also present simultaneous RXTE observations of GRS 1915+105. During INTEGRAL Revolution 48 (2003 March 6) the source was observed to be in a highly variable state, characterized by 5-minute quasi-periodic oscillations. During these oscillations, the rise is faster than the decline, and is harder. This particular type of variability has never been observed before. During subsequent INTEGRAL revolutions (2003 March-May), the source was in a steady or ''plateau'' state (also known as class $χ$ according to Belloni et al. 2000). Here we discuss both the temporal and spectral characteristics of the source during the first three months of observations. The source was clearly detected with all three gamma-ray and X-ray instruments onboard INTEGRAL.

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arXiv2004

From BeppoSAX to INTEGRAL: PDS observations of hard X-ray sources detected in the IBIS survey

The first IBIS galactic plane survey has provided a list of high energy emitting objects above 20 keV; these sources have been detected mostly in the crowded region of the Galactic Centre and partly along the Galactic Plane. In order to validate the detection procedure, to help in the identification process and to study the nature of these IBIS sources, this list has been cross correlated with the data archive of the PDS instrument on BeppoSAX, which operated in a similar energy band and with a similar sensitivity. We discover a number of associations whose detailed analysis will be particularly useful for the survey work. Also, thanks to the imaging capability of IBIS/ISGRI, objects which could not be studied by the PDS due to contamination from nearby sources can now be associated with a definite source or sources.

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arXiv2004

The sky behind our galaxy as seen by IBIS on INTEGRAL

During the Core Programme, INTEGRAL has surveyed a large portion of the sky (around 9000 square degrees); although INTEGRAL is not optimized for extra-galactic studies its observations have nevertheless given us the opportunity to explore the sky behind our Galaxy, something which is impossible in some wavebands due to the presence of strong absorption. Preliminary results from this exploration are presented and compared with the pre-launch expectations. In particular, we detail all extragalactic detections obtained so far: 10 active galaxies and one cluster of galaxies. Of these sources, many have previously been studied at energies above 10-20 keV while a few are new hard X-ray discoveries. Since the number of detections is smaller than estimated on the basis of the IBIS/ISGRI sensitivity, it is likely that some of the new ISGRI sources found in this survey are extragalactic objects; a few of these are likely to be AGN and are described in detail.

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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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arXiv2004

Gamma-Ray All-Sky Imaging with the Burst and Transient Source Experiment

The BATSE mission aboard CGRO monitored the whole sky in the 20 keV - 1 MeV energy band continuously from April 1991 until June 2000. Although BATSE had very poor intrinsic angular resolution, the data can be used to survey the entire soft gamma-ray sky with < 1 degree angular resolution and ~ 1 mCrab sensitivity by using the Earth occultation method. This method determines flux by measuring the step in the count rate profile in each BATSE detector as a source rises above or sets below the Earth's limb. A maximum likelihood imaging technique can then be used to build up all-sky maps from the images of the Earth's limb produced by occulting sources. However, since the Earth seen from BATSE has a radius of ~ 70 degrees, the limb images that intersect at the positions of bright point sources have a significant effect over the area of the all-sky map. A method for performing image cleaning on likelihood data has also been developed and is used to effectively remove artefacts from the all-sky maps. This paper describes the 'LIMBO' imaging technique and presents preliminary all-sky maps of 25 - 160 keV emission, the first to be made since the HEAO1-A4 mission of 1978-79.

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arXiv2003

First INTEGRAL observations of GRS 1915+105

We present data from the first of six monitoring Open Time observations of GRS 1915+105 undertaken with the orbiting INTEGRAL satellite. The source was clearly detected with all three X-ray and gamma-ray instruments on board. GRS 1915+105 was in a highly variable state, as demonstrated by the JEM X-2 and ISGRI lightcurves. These and simultaneous RXTE/PCA lightcurves point to a novel type of variability pattern in the source. In addition, we fit the combined JEM X-2 and ISGRI spectrum between 3-300 keV with a disk blackbody + powerlaw model leading to typical parameter values found earlier at similar luminosity levels. A new transient, IGR J19140+098, was discovered during the present observation.

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arXiv2003

The INTEGRAL Science Data Centre (ISDC)

The INTEGRAL Science Data Centre (ISDC) provides the INTEGRAL data and means to analyse them to the scientific community. The ISDC runs a gamma ray burst alert system that provides the position of gamma ray bursts on the sky within seconds to the community. It operates a quick-look analysis of the data within few hours that detects new and unexpected sources as well as it monitors the instruments. The ISDC processes the data through a standard analysis the results of which are provided to the observers together with their data.

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arXiv2003

BeppoSAX average spectra of Seyfert galaxies

We have studied the average 3-200 keV spectra of Seyfert galaxies of type 1 and 2, using data obtained with BeppoSAX. The average Seyfert 1 spectrum is well-fitted by a power law continuum with photon spectral index Gamma~1.9, a Compton reflection component R~0.6-1 (depending on the inclination angle between the line of sight and the reflecting material) and a high-energy cutoff at around 200 keV; there is also an iron line at 6.4 keV characterized by an equivalent width of 120 eV. Seyfert 2's on the other hand show stronger neutral absorption (NH=3-4 x 10^{22} atoms cm-2) as expected but are also characterized by an X-ray power law which is substantially harder (Gamma~1.75) and with a cut-off at lower energies (E_c~130 keV); the iron line parameters are instead substantially similar to those measured in type 1 objects. There are only two possible solutions to this problem: to assume more reflection in Seyfert 2 galaxies than observed in Seyfert 1 or more complex absorption than estimated in the first instance. The first possibility is ruled out by the Seyfert 2 to Seyfert 1 ratio while the second provides an average Seyfert 2 intrinsic spectrum very similar to that of the Seyfert 1. The extra absorber is likely an artifact due to summing spectra with different amounts of absorption, althought we cannot exclude its presence in at least some individual sources. Our result argues strongly for a very similar central engine in both type of galaxies as expected under the unified theory.

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arXiv2002

A Mass Model for Estimating the Gamma Ray Background of the Burst and Transient Source Experiment

Orbiting x-ray and gamma ray instruments are subject to large background count rates due to local particle fluxes in the space environment. The ability of an instrument to make calibrated measurements of the flux from a source of interest is highly dependent on accurately determining the background level. We present here a method of calculating the energy dependent background flux for any point in the complete data set recorded by the Burst and Transient Source Experiment (BATSE) in its nine year mission. The BATSE Mass Model (BAMM) uses a Monte Carlo mass modelling approach to produce a data base of the gamma ray background which is then filtered to simulate the background count rate with a 2.048 second time resolution. This method is able to reduce the variations in the background flux by a factor of 8 - 10, effectively `flat-fielding' the detector response. With flat-fielded BATSE data it should be possible to use the Earth occultation technique to produce a hard x-ray all sky survey to the 1-2 mCrab sensitivity limit. BAMM is also capable of estimating the contribution to the spectra measured from gamma ray sources due to the reprocessing of source photons in inactive material surrounding a gamma ray detector. Possible applications of this aspect of the model in the area of Gamma Ray Burst spectral analysis are discussed.

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arXiv2001

Flat-Fielding BATSE Occultation Data for use in a Hard X-Ray All Sky Survey

The BATSE mission aboard CGRO can be used to observe hard X-ray sources by using the Earth occultation method. This method relies on measuring a step in the count rate profile in each BATSE detector as a source rises above or sets below the Earth's limb. A major problem in determining the step sizes (and hence the flux) is in extracting the steps from the varying background. A technique for flat-fielding the response of gamma ray detectors has been developed at Southampton. The technique uses a dynamic Monte-Carlo model to simulate the dominant components of the gamma-ray background encountered by the experiment at any point in its orbit. A maximum likelihood imaging method is also being developed that will be used to make a sky survey with all 9 years of the BATSE Continuous data set in the 20 - 500 keV range. An all sky map of 25 - 35 keV emission has been made using 60 days of data and has a 3 sigma flux sensitivity of 14 mCrab.

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arXiv1999

Hard X-ray detection of the high redshift quasar 4C 71.07

BATSE/OSSE observations of the high redshift quasar 4C 71.07 indicate that this is the brightest and furthest AGN so far detected above 20 keV. BATSE Earth occultation data have been used to search for emission from 4C 71.07 from nearly 3 years of observation. The mean source flux over the whole period in the BATSE energy range 20-100 keV is (13.2 +/- 1.06) x 10^(-11) erg cm^(-2) s^(-1) corresponding to a luminosity of 2 x 10^(48) erg s^(-1). The BATSE light curve over the 3 years of observations shows several flare-like events, one of which (in January 1996) is associated with an optical flare (R=16.1) but with a delay of 55 days. The OSSE/BATSE spectral analysis indicates that the source is characterized by a flat power spectrum (Gamma about 1.1 - 1.3) when in a low state; this spectral form is consistent within errors with the ASCA and ROSAT spectra. This means that the power law observed from 0.1 to 10 keV extends up to at least 1 MeV but steepens soon after to meet EGRET high energy data. BATSE data taken around the January 1996 flare suggests that the spectrum could be steeper when the source is in a bright state. The nuF-nu representation of the source is typical of a low frequency peaked/gamma-ray dominated blazar, with the synchrotron peak in the mm-FIR band and the Compton peak in the MeV band. The BATSE and OSSE spectral data seem to favour a model in which the high energy flux is due to the sum of the synchrotron self-Compton and the external Compton contributions; this is also supported by the variability behaviour of the source.

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arXiv1999

BATSE Observations of the Piccinotti Sample of AGN

BATSE Earth occultation data have been used to search for emission in the 20-100 keV band from all sources in the Piccinotti sample, which represents to date the only complete 2-10 keV survey of the extragalactic sky down to a limiting flux of 3.1 x 10^(-11) erg cm^(-2)$ s^(-1). Nearly four years of observations have been analyzed to reach a 5sigma sensitivity level of about 7.8x 10^(-11) erg cm^(-2) s^(-1) in the band considered. Of the 36 sources in the sample, 14 have been detected above 5sigma confidence level while marginal detection (3<sigma<5) can be claimed for 13 sources; for 9 objects 2sigma upper limits are reported. Comparison of BATSE results with data at higher energies is used to estimate the robustness of our data analysis: while the detection level of each source is reliable, the flux measurement maybe overestimated in some sources by as much as 35%, probably due to incomplete data cleaning. Comparison of BATSE fluxes with X-ray fluxes, obtained in the 2-10 keV range and averaged over years, indicates that a canonical power law of photon index 1.7 gives a good description of the broad band spectra of bright AGNs and that spectral breaks preferentially occur above 100 keV.

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arXiv1999

Diffuse Thermal Emission from Very Hot Gas in Starburst Galaxies: Spatial Results

New BeppoSAX observations of the nearby prototypical starburst galaxies NGC 253 and M82 are presented. A companion paper (Cappi et al. 1998;astro-ph/9809325) shows that the hard (2-10 keV) spectrum of both galaxies, extracted from the source central regions, is best described by a thermal emission model with kT ~ 6-9 keV and abundances ~ 0.1-0.3 solar. The spatial analysis yields clear evidence that this emission is extended in NGC 253, and possibly also in M82. This quite clearly rules out a LLAGN as the main responsible for their hard X-ray emission. Significant contribution from point-sources (i.e. X-ray binaries (XRBs) and Supernovae Remnants (SNRs)) cannot be excluded; neither can we at present reliably estimate the level of Compton emission. However, we argue that such contributions shouldn't affect our main conclusion, i.e., that the BeppoSAX results show, altogether, compelling evidence for the existence of a very hot, metal-poor interstellar plasma in both galaxies.

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arXiv1998

Diffuse Thermal Emission from Very Hot Gas in Starburst Galaxies: Spectral Results

New BeppoSAX observations of the nearby archetypical starburst galaxies (SBGs) NGC253 and M82 are presented. The main observational result is the unambiguous evidence that the hard (2-10 keV) component is (mostly) produced in both galaxies by thermal emission from a metal-poor (~ 0.1-0.3 solar), hot (kT \~ 6- 9 keV) and extended (see companion paper: Cappi et al. 1998) plasma. Possible origins of this newly discovered component are briefly discussed. A remarkable similarity with the (Milky Way) Galactic Ridge's X-ray emission suggests, nevertheless, a common physical mechanism.

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arXiv1998

BeppoSAX detection of the Fe K line in the nearby starburst galaxy NGC 253

We present BeppoSAX results on the nearby starburst galaxy NGC 253. Although extended, a large fraction of the X-ray emission comes from the nuclear region. Preliminary analysis of the LECS/MECS/PDS ~0.2-60 keV data from the central 4' region indicates that the continuum is well fitted by two thermal models: a ``soft'' component with kT ~ 0.9 keV, and a ``hard'' component with kT ~ 6 keV absorbed by a column density of ~ 1.2 x10**22 cm-2. For the first time in this object, the Fe K line at 6.7 keV is detected, with an equivalent width of ~ 300 eV. This detection, together with the shape of the 2--60 keV continuum, implies that most of the hard X-ray emission is thermal in origin, and constrains the iron abundances of this component to be ~0.25 of solar. Other lines clearly detected are Si, S and Fe L/Ne, in agreement with previous ASCA results. We discuss our results in the context of the starburst-driven galactic superwind model.

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