arXiv subjects

J. Ryan

Publications and source records attributed to J. Ryan.

36 records · Page 2Linked to original sources

arXiv2001

The Comptel 1.809 Mev Survey

We present the latest update of the 1.809 MeV sky survey obtained with COMPTEL. Based on all observations taken since the launch of CGRO in spring 1991 to early summer this year we obtain 1.809 MeV all sky maps using different imaging methods. The background is modelled on the basis of an adjacent energy approach. We confirm the previously reported characteristics of the galactic 1.809 MeV emission, specifically excesses in regions away from the inner Galaxy. The observed 1.8 MeV gamma-ray line is ascribed to the radioactive decay of 26Al in the interstellar medium. 26Al has been found to be predominantly synthesised in massive stars and their subsequent core-collapse supernovae, which is confirmed in tracer comparisons. Due to this, one anticipates flux enhancements aligned with regions of recent star formation, such as apparently observed in the Cygnus and Vela regions.

astro-ph↗

arXiv2001

56Co gamma-rays from SN1998bu: COMPTEL upper limits

The type Ia supernova SN1998bu in M96 was observed by COMPTEL for a total of 88 days starting 17 days after the first detection of the supernova. The accumulated effective observation time was 4.14 Msec. The COMPTEL observations were performed in a special instrument mode improving the low-energy sensitivity. We generated images in the 847 keV and 1238 keV lines of 56Co, using improved point spread functions for the low-energy mode. We do not detect SN1998bu. A spectral analysis of our data also confirms the non-detection of the supernova. We discuss the event for which our upper limits constrain the standard supernova models.

astro-ph↗

arXiv2000

The COMPTEL instrumental line background

The instrumental line background of the Compton telescope COMPTEL onboard the Compton Gamma-Ray Observatory is due to the activation and/or decay of many isotopes. The major components of this background can be attributed to eight individual isotopes, namely 2D, 22Na, 24Na, 28Al, 40K, 52Mn, 57Ni, and 208Tl. The identification of instrumental lines with specific isotopes is based on the line energies as well as on the variation of the event rate with time, cosmic-ray intensity, and deposited radiation dose during passages through the South-Atlantic Anomaly. The characteristic variation of the event rate due to a specific isotope depends on its life-time, orbital parameters such as the altitude of the satellite above Earth, and the solar cycle. A detailed understanding of the background contributions from instrumental lines is crucial at MeV energies for measuring the cosmic diffuse gamma-ray background and for observing gamma-ray line emission in the interstellar medium or from supernovae and their remnants. Procedures to determine the event rate from each background isotope are described, and their average activity in spacecraft materials over the first seven years of the mission is estimated.

astro-ph↗

arXiv2000

Multifrequency Observations of the Virgo Blazars 3C 273 and 3C 279 in CGRO Cycle 8

We report first observational results of multifrequency campaigns on the prominent Virgo blazars 3C 273 and 3C 279 which were carried out in January and February 1999. Both blazars are detected from radio to gamma-ray energies. We present the measured X- to gamma-ray spectra of both sources, and for 3C 279 we compare the 1999 broad-band (radio to gamma-ray) spectrum to measured previous ones.

astro-ph↗

arXiv2000

A large high-energy gamma-ray flare from the blazar 3C 273

The Compton Gamma-Ray Observatory (CGRO) experiments EGRET and COMPTEL observed the Virgo sky region continuously for 7 weeks between December 10, 1996 and January 28, 1997. The prominent quasar 3C~273 was found to be the brightest source in gamma-rays and was significantly detected by EGRET and COMPTEL. The EGRET experiment observed a time-variable flux at energies above 100 MeV, which reached in a 2-week flaring period (December 30, 1996 to January 14, 1997) its highest flux level observed during the CGRO-era. COMPTEL, however, does not observe obvious time variability at energies below ~30 MeV contemporaneous to EGRET. In particular, no flare was observed, indicating that this outburst is solely a high-energy (>100 MeV) phenomenon. The energy spectrum between 3 MeV and 10 GeV is well represented by a simple power-law model. Below 3 MeV a spectral turnover is indicated. Performing spectral analysis for different time periods, we found evidence for a spectral hardening during the flaring period, which is consistent with the flare occurring mainly at the higher energies and with its absence at COMPTEL energies of a few MeV. This may be interpreted as an indication that the emission in the EGRET energy range is dominated by a different radiation mechanism than the MeV emission. We argue that the most likely mechanism for the high-energy flare is inverse-Compton scattering of reprocessed accretion-disk radiation.

astro-ph↗

arXiv1999

COMPTEL Time-Averaged All-Sky Point Source Analysis

We use all COMPTEL data from the beginning of the CGRO mission (April '91) up to the end of CGRO Cycle 6 (November '97) to carry out all-sky point source analyses in the four standard COMPTEL energy bands for different time periods. We apply our standard maximum-likelihood method to generate all-sky significance and flux maps for point sources by subtracting off the diffuse emission components via model fitting. In addition, fluxes of known sources have been determined for individual CGRO Phases/Cycles to generate lightcurves with a time resolution of the order of one year. The goal of the analysis is to derive quantitative results -- significances, fluxes, light curves -- of our brightest and most significant sources such as 3C 273, and to search for additional new COMPTEL sources, showing up in time-averaged maps only.

astro-ph↗

arXiv1999

COMPTEL MeV Observations of the TeV Sources Markarian 421 and Markarian 501

The COMPTEL experiment aboard the COMPTON Gamma-Ray Observatory (CGRO) has observed the prominent TeV-blazars Mkn 421 and Mkn 501 many times between the start of its mission in April '91 and December '98. This paper reports first COMPTEL results from mainly time-averaged (CGRO Cycles) data. No evidence for both sources is found up to the end Cycle VI. However, the sum of all 10-30 MeV Cycle VII data shows a weak (3.2 sigma detection) MeV-source being positionally consistent with Mkn 421. During Cycle VII Mkn 421 was rather active at TeV-energies. Due to the lack of other known Gamma-ray sources in this sky region, we consider Mkn 421 as the most likely counterpart for this Gamma-ray emission. However, its connection cannot be proven by COMPTEL.

astro-ph↗

arXiv1999

Image Reconstruction of COMPTEL 1.8 MeV 26Al Line Data

We present a new algorithm, called Multiresolution Regularized Expectation Maximization (MREM), for the reconstruction of gamma-ray intensity maps from COMPTEL data. The algorithm is based on the iterative Richardson-Lucy scheme to which we added a wavelet thresholding step in order to eliminate image-noise in the reconstruction. The wavelet thresholding explicitly accounts for spatial correlations in the data, and adapts the angular resolution locally, depending on the significance of the signal in the data. We compare the performance of MREM to that of the maximum entropy and the Richardson-Lucy algorithms by means of Monte-Carlo simulations of COMPTEL 1.809 MeV gamma-ray line observations. The simulations demonstrate that the maximum entropy and Richardson-Lucy algorithms provide virtually identical reconstructions which are heavily disturbed by image noise. MREM largely suppresses this noise in the reconstructions, showing only the significant structures that are present in the data. Application of MREM to COMPTEL 1.8 MeV gamma-ray line data results in a 1.809 MeV sky map that is much smoother than the maximum entropy or Richardson-Lucy reconstructions presented previously. The essential features of this map are (1) an asymmetric galactic ridge emission reaching from l=45 deg to l=240 deg, (2) a bright localised emission feature in the Cygnus region around (l,b)=(80 deg,0 deg), (3) two emission spots at l=317 deg and l=332 deg situated in the galactic plane, and (4) an extended emission region around (l,b)=(160 deg, 0 deg). Comparison of the MREM map to the simulated reconstructions demonstrates that the 1.809 MeV emission is confined to the galactic plane.

astro-ph↗

arXiv1999

Assessment of Tracers of 1.8 MeV Emission

We examined the question of a possible correlation between 1.8 MeV emission and intensity distributions observed at other wavelengths by comparing the 1.8 MeV data to an extended set of all-sky maps, covering the entire explored wavelength range from the radio band up to high-energy gamma-rays. This analysis revealed that tracers of the old stellar population or the local interstellar medium provide only a poor description of the data. Tracers of the young stellar population considerably improve the fit. Residuals are minimized for the free-free emission map obtained by the DMR at microwave wavelengths and the 158 um fine-structure line map of C+ observed by FIRAS. Within the statistics of the present data, both maps provide an overall satisfactory fit. We therefore conclude that adequate tracers of 26Al sources have now been identified. Implications for 26Al source types are discussed (see also Knodlseder, these proceedings).

astro-ph↗

arXiv1999

A New Look at Image Reconstruction of COMPTEL 1.8 MeV Data

Deconvolving COMPTEL gamma-ray data into images presents a major methodological challenge. We developed a new algorithm called Multiresolution Regularized Expectation Maximization (MREM), which explicitly accounts for spatial correlations in the image by using wavelets (Dixon et al. 1998). We demonstrate that MREM largely suppresses image noise in the reconstruction, showing only significant structures that are present in the data. Application to 1.8 MeV data results in a sky map that is much smoother than the maximum entropy reconstructions presented previously, but it shows the same characteristic emission features which have been established earlier.

astro-ph↗

arXiv1999

COMPTEL Observations of AGN at MeV-Energies

The COMPTEL experiment aboard CGRO, exploring the previously unknown sky at MeV-energies, has so far detected 10 Active Galactic Nuclei (AGN): 9 blazars and the radio galaxy Centaurus A. No Seyfert galaxy has been found yet. With these results COMPTEL has opened the field of extragalactic Gamma-ray astronomy in the MeV-band.

astro-ph↗

arXiv1997

COMPTEL Observations of 3C 279 during the first 4 Years of the CGRO-Mission

The COMPTEL experiment aboard the Compton Gamma-Ray Observatory (CGRO) has observed the gamma-ray blazar 3C 279 several times between April 1991 and September 1995. This paper reports on a consistent analysis of these observations using the most recent COMPTEL data analysis tools. Detections and non-detections of 3C 279 along the CGRO-mission indicate a time-variable MeV-flux. Spectral variability is indicated as well, however can not be significantly proven by spectral fitting. The average MeV-spectrum of 3C 279, as measured by COMPTEL over the four-year period, is consistent with a photon power-law slope of about -1.9. This spectrum smoothly connects to the simultaneous 30 MeV to 10 GeV spectrum obtained from an analysis of the combined EGRET observations. No spectral break is required to fit the time-averaged MeV- to GeV-spectrum of 3C 279.

astro-ph↗

arXiv1997

COMPTEL observations of the quasar PKS 0528+134 during the first 3.5 years of the CGRO mission

The COMPTEL observations of the blazar-type quasar PKS 0528+134 in the energy range 0.75 MeV to 30 MeV carried out between April 1991 and September 1994 have been analyzed. During the first two years PKS 0528+134 was most significantly detected at energies above 3 MeV. During the last year there is only evidence for the quasar at energies below 3 MeV indicating a spectral change. The time-averaged COMPTEL energy spectrum between 0.75 MeV and 30 MeV is well represented by a power-law shape. Spectra collected from different observational periods reveal different power-law shapes: a hard state during flaring observations reported by EGRET, and a soft state otherwise. The combined simultaneous EGRET and COMPTEL spectra indicate these two spectral states as well. During low intensisty gamma-ray phases no spectral break is obvious from the combined COMPTEL and EGRET measurements. For the gamma-ray flaring phases however, the combined COMPTEL and EGRET data require a spectral bending at MeV-energies. By fitting broken power-law functions the best-fit values for the break in photon index range between 0.6 and 1.7, and for the break energy between ~5 MeV and ~20 MeV. Because the flux values measured by COMPTEL below 3 MeV in both states are roughly equal, the observations would be consistent with an additional spectral component showing up during gamma-ray flaring phases of PKS 0528+134. Such a component could be introduced by e.g. a high-energy electron-positron population with a low-energy cutoff in their bulk Lorentz factor distribution. The multiwavelength spectrum of PKS 0528+134 for gamma-ray flaring phases shows that the major energy release across the entire electro-magnetic spectrum is measured at MeV-energies.

astro-ph↗

arXiv1997

COMPTEL gamma-ray study of the Crab nebula

We report on a study of the gamma-ray continuum emission from the Crab supernova nebula and on a search for nuclear de-excitation gamma-ray lines. Crab is the brightest continuum source in the 1-10 MeV gamma-ray sky, and its continuum radiation is most likely of synchrotron origin. It is a likely source of cosmic rays through shock acceleration and thus a potential candidate for gamma-ray line emission from nuclear interactions. Five years of COMPTEL observations enable a fine spectral binning to investigate the behaviour of the 0.75-30 MeV emission in detail and to search for nuclear de-excitation lines on top of the continuum. The nebular spectrum shows a break at the edge of the COMPTEL energy range and connects well to the EGRET spectrum, probably reflecting electron energy losses in the synchrotron emission scenario. Such a smooth continuum model alone may not be sufficient to explain the observations. A weak bump in the spectrum at 1-2 MeV may be present. No significant evidence for distinct line emission is seen, but the presence of a blend of line features or another synchrotron component cannot be excluded.

astro-ph↗

arXiv1997

26Al in the local interstellar medium

We estimate the 1.8 MeV luminosity of the Sco-Cen association due to radioactive decay of 26Al to (4-15) 10e-5 ph cm**-2 s**-1. We propose a low surface brightness, limb brightened bubble for the 1.8 MeV intensity distribution. The detectibility of this distribution with existing gamma-ray telescopes is discussed.

astro-ph↗

arXiv1996

Modelling the 1.8 MeV Sky: Tests for Spiral Structure

COMPTEL imaging analysis revealed a patchy, asymmetric distribution of diffuse 1.8 MeV emission along the Galactic plane, which is attributed to the decay of radioactive 26Al in the ISM. If massive stars were the major source of Galactic 26Al, the 1.8 MeV emission should be asymmetric and trace the spiral arms of the Galaxy, presumed site of massive star formation. Using model fits, we indeed find weak evidence in the COMPTEL data that the observed 1.8 MeV emission is at least partly confined to spiral arms. We derive a total Galactic 26Al mass of 2.5 Msol from which at least 0.7 Msol can be attributed to massive stars.

astro-ph↗

arXiv1996

1.8 MeV Emission from the Carina Region

Significant 1.8 MeV emission from the Carina region has been detected by COMPTEL. The emission is concentrated within 6 degrees or less near the Carina nebula NGC 3372, one of the brightest HII regions known in our Galaxy. This region contains a wealth of extreme young open clusters whose massive stars possibly contributed to an enrichment of 26Al in the ISM within the last few million years. The relation of these clusters and the peculiar object Eta Carinae with the observed emission is discussed. The 26Al yield of the clusters is estimated using current theoretical nucleosynthesis models.

astro-ph↗

arXiv1995

The gamma-ray burst GB 920622

We have analyzed the Ulysses, BATSE, and COMPTEL spectral data from the $γ$-ray burst of June 22, 1992 (GB 920622). COMPTEL data reveal a hard to soft evolution within the first pulse of the burst, while the mean hardness ratios of the three pulses are the same. Unlike the single instrument spectra, the composite spectrum of GB 920622 averaged over the total burst duration ranging from 20 keV up to 10 MeV cannot be fit by a single power law. Instead, the spectrum shows continuous curvature across the full energy range. COMPTEL imaging and BATSE/Ulysses triangulation constrain the source location of GB 920622 to a ring sector 1.1 arcmin wide and 2 degrees long. This area has been searched for quiescent X-ray sources using \ros survey data collected about two years before the burst. After the optical identification of the X-ray sources in and near the GRB location we conclude that no quiescent X-ray counterpart candidate for GB 920622 has been found.

astro-ph↗