131 research outputs found
Recent results from COMPTEL observations of Cygnus Xâ1
The COMPTEL experiment on the Compton GammaâRay Observatory (CGRO) has now observed Cyg Xâ1 on four separate occasions during phase 1 and phase 2 of its orbital mission (April, 1991 to August, 1993). Here we report on the results of the latest analysis of these data, which provide a spectrum extending to energies greater than 2 MeV. A spectral analysis of these data, in the context of a classical Comptonization model, indicates an electron temperature much higher than previous hard Xâray measurements would suggest (200 keV vs 60â80 keV). This implies either some limitations in the standard Comptonization model and/or the need to incorporate a reflected component in the hard Xâray spectrum. Although significant variability near 1 MeV has been observed, there is no evidence for any âMeV excess.
Neutron induced background in the COMPTEL detector on the Gamma Ray Observatory
Interactions of neutrons in a prototype of the Compton imaging telescope (COMPTEL) gamma ray detector for the Gamma Ray Observatory were studied to determine COMPTEL's sensitivity as a neutron telescope and to estimate the gamma ray background resulting from neutron interactions. The IUCF provided a pulsed neutron beam at five different energies between 18 and 120 MeV. These measurements showed that the gamma ray background from neutron interactions is greater than previously expected. It was thought that most such events would be due to interactions in the upper detector modules of COMPTEL and could be distinguished by pulse shape discrimination. Rather, the bulk of the gamma ray background appears to be due to interactions in passive material, primarily aluminum, surrounding the D1 modules. In a considerable fraction of these interactions, two or more gamma rays are produced simultaneously, with one interacting in the D1 module and the other interacting in the module of the lower (D2) detector. If the neutron interacts near the D1 module, the D1 D2 time of flight cannot distinguish such an event from a true gamma ray event. In order to assess the significance of this background, the flux of neutrons in orbit has been estimated based on observed events with neutron pulse shape signature in D1. The strength of this neutron induced background is estimated. This is compared with the rate expected from the isotropic cosmic gamma ray flux
FROM MULTIPLE POLYGONS TO SINGLE GEOMETRY: OPTIMIZATION OF POLYGON INTEGRATION FOR CROWDSOURCED DATA
Paid crowdsourcing is a popular approach for creating training data in machine learning, but output quality can suffer from various drawbacks, such as noisy data. One solution is to obtain multiple acquisitions of the same dataset and perform integration steps, which can be challenging for geometries such as polygons. In this paper, we propose a raster-based polygon integration approach for the use of crowdsourced data, providing a solution for integrating multiple geometric shapes into single geometries. We analyze the effects of the choice of the integration threshold parameter for different sample sizes on the quality measures intersection over union (IoU) and Hausdorff distance, and provide a recommendation for its optimal selection based on empirical analysis. Additionally, further possibilities to improve integration results are explored, i.e., methods of filtering data before integration by outlier detection
INTEGRAL observations of the blazar Mrk 421 in outburst (Results of a multi-wavelength campaign)
We report the results of a multi-wavelength campaign on the blazar Mrk 421
during outburst. We observed four strong flares at X-ray energies that were not
seen at other wavelengths (partially because of missing data). From the fastest
rise in the X-rays, an upper limit could be derived on the extension of the
emission region. A time lag between high-energy and low-energy X-rays was
observed, which allowed an estimation of the magnetic-field strength. The
spectral analysis of the X-rays revealed a slight spectral hardening of the
low-energy (3 - 43 keV) spectral index. The hardness-ratio analysis of the
Swift-XRT (0.2 - 10 keV) data indicated a small correlation with the intensity;
i. e., a hard-to-soft evolution was observed. At the energies of IBIS/ISGRI (20
- 150 keV), such correlations are less obvious. A multiwavelength spectrum was
composed and the X-ray and bolometric luminosities are calculated.Comment: 15 pages, 18 figures; accepted by Astronomy & Astrophysic
Multifrequency Observations of the Blazar 3C 279 in January 2006
We report first results of a multifrequency campaign from radio to hard X-ray
energies of the prominent gamma-ray blazar 3C 279, which was organised around
an INTEGRAL ToO observation in January 2006, and triggered on its optical
state. The variable blazar was observed at an intermediate optical state, and a
well-covered multifrequency spectrum from radio to hard X-ray energies could be
derived. The SED shows the typical two-hump shape, the signature of non-thermal
synchrotron and inverse-Compton (IC) emission from a relativistic jet. By the
significant exposure times of INTEGRAL and Chandra, the IC spectrum (0.3 - 100
keV) was most accurately measured, showing - for the first time - a possible
bending. A comparison of this 2006 SED to the one observed in 2003, also
centered on an INTEGRAL observation, during an optical low-state, reveals the
surprising fact that - despite a significant change at the high-energy
synchrotron emission (near-IR/optical/UV) - the rest of the SED remains
unchanged. In particular, the low-energy IC emission (X- and hard X-ray
energies) remains the same as in 2003, proving that the two emission components
do not vary simultaneously, and provides strong constraints on the modelling of
the overall emission of 3C 279.Comment: 4 pages, 6 figures; to be published in the Proc. of the 6th INTEGRAL
workshop "The Obscured Universe" (Moscow, July 2-8, 2006), eds. S. Grebenev,
R. Sunyaev, C. Winkler, ESA SP 622 (2006
Does the Blazar Gamma-Ray Spectrum Harden with Increasing Flux? Analysis of 9 Years of EGRET Data
The Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray
Observatory (CGRO) discovered gamma-ray emission from more than 67 blazars
during its 9 yr lifetime. We conducted an exhaustive search of the EGRET
archives and selected all the blazars that were observed multiple times and
were bright enough to enable a spectral analysis using standard power-law
models. The sample consists of 18 flat-spectrum radio quasars(FSRQs), 6
low-frequency peaked BL Lac objects (LBLs) and 2 high-frequency peaked BL Lac
objects (HBLs). We do not detect any clear pattern in the variation of spectral
index with flux. Some of the blazars do not show any statistical evidence for
spectral variability. The spectrum hardens with increasing flux in a few cases.
There is also evidence for a flux-hardness anticorrelation at low fluxes in
five blazars. The well-observed blazars (3C 279, 3C 273, PKS 0528+134, PKS
1622-297 PKS 0208-512) do not show any overall trend in the long-term spectral
dependence on flux, but the sample shows a mixture of hard and soft states. We
observed a previously unreported spectral hysteresis at weekly timescales in
all three FSRQs for which data from flares lasting for ~(3-4) weeks were
available. All three sources show a counterclockwise rotation, despite the
widely different flux profiles. We analyze the observed spectral behavior in
the context of various inverse Compton mechanisms believed to be responsible
for emission in the EGRET energy range. Our analysis uses the EGRET skymaps
that were regenerated to include the changes in performance during the mission
Challenges in reconciling observations and theory of the brightest high-energy flare ever of 3C 279
INTEGRAL and XMM-Newton observations towards the unidentified MeV source GRO J1411-64
The COMPTEL unidentified source GRO J1411-64 was observed by INTEGRAL, and
its central part, also by XMM-Newton. The data analysis shows no hint for new
detections at hard X-rays. The upper limits in flux herein presented constrain
the energy spectrum of whatever was producing GRO J1411-64, imposing, in the
framework of earlier COMPTEL observations, the existence of a peak in power
output located somewhere between 300-700 keV for the so-called low state. The
Circinus Galaxy is the only source detected within the 4 location error
of GRO J1411-64, but can be safely excluded as the possible counterpart: the
extrapolation of the energy spectrum is well below the one for GRO J1411-64 at
MeV energies. 22 significant sources (likelihood ) were extracted and
analyzed from XMM-Newton data. Only one of these sources, XMMU
J141255.6-635932, is spectrally compatible with GRO J1411-64 although the fact
the soft X-ray observations do not cover the full extent of the COMPTEL source
position uncertainty make an association hard to quantify and thus risky. The
unique peak of the power output at high energies (hard X-rays and gamma-rays)
resembles that found in the SED seen in blazars or microquasars. However, an
analysis using a microquasar model consisting on a magnetized conical jet
filled with relativistic electrons which radiate through synchrotron and
inverse Compton scattering with star, disk, corona and synchrotron photons
shows that it is hard to comply with all observational constrains. This and the
non-detection at hard X-rays introduce an a-posteriori question mark upon the
physical reality of this source, which is discussed in some detail
Discovery of luminous pulsed hard X-ray emission from anomalous X-ray pulsars 1RXS J1708-4009, 4U 0142+61 and 1E 2259+586 by INTEGRAL and RXTE
We report on the discovery of hard spectral tails for energies above 10 keV
in the total and pulsed spectra of anomalous X-ray pulsars 1RXS J1708-4009, 4U
0142+61 and 1E 2259+586 using RXTE PCA (2-60 keV) and HEXTE (15-250 keV) data
and INTEGRAL IBIS ISGRI (20-300 keV) data. Improved spectral information on 1E
1841-045 is presented. The pulsed and total spectra measured above 10 keV have
power-law shapes and there is so far no significant evidence for spectral
breaks or bends up to ~150 keV. The pulsed spectra are exceptionally hard with
indices measured for 4 AXPs approximately in the range -1.0 -- 1.0. We also
reanalyzed archival CGRO COMPTEL (0.75-30 MeV) data to search for signatures
from our set of AXPs. No detections can be claimed, but the obtained
upper-limits in the MeV band indicate that for 1RXS J1708-4009, 4U 0142+61 and
1E 1841-045 strong breaks must occur somewhere between 150 and 750 keV.Comment: Accepted for publication in ApJ; 19 pages; 4 Tables; 15 Figures (6
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Gammaâray burst studies by COMPTEL during its first year of operation
During the first year of Compton GRO operations, more than 20 cosmic gammaâray burstâdetected by the BATSE instrument â occurred inside the 1 sr field of view of the imaging gammaâray telescope COMPTEL. Using COMPTELâs primary mode of operation (the telescope mode) direct images (with âŒ1° GRB location accuracy) and event spectra (0.7 MeV â 30 MeV) with spectral resolution better than 10% FWHM have been obtained. In its secondary mode of burst operations, COMPTEL has recorded time resolved spectra (0.1 MeV â 10 MeV) from its large NaI detectors. This paper summarises the results on cosmic GRB sources obtained by COMPTEL during its first year of operation
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