M. P. Ulmer

Gamma Radiation from PSR B1055–52

The telescopes on the Compton Gamma Ray Observatory (CGRO) have observed PSR B1055-52 a number of times between 1991 and 1998. From these data a more detailed picture of the gamma radiation from this source has been developed, showing several characteristics that distinguish this pulsar: the light curve is complex; there is no detectable unpulsed emission; the energy spectrum is flat, with no evidence of a sharp high-energy cutoff up to greater than 4 GeV. Comparisons of the gamma-ray data with observations at longer wavelengths show that no two of the known gamma-ray pulsars have quite the same characteristics; this diversity makes interpretation in terms of theoretical models difficult.

OSSE Observations of Blazars

Abstract : Results are reported on observations obtained with the Oriented Scintillation Spectrometer Experiment (OSSE) instrument on the Compton Gamma Ray Observatory (CGRO) of 17 active galactic nuclei known to exhibit blazar properties at other wavelengths. These observations span the period from 1991 June through 1994 May. Of the 33 high-confidence EGRET detections of blazars during CGRO observing phases 1 and 2 (1991 May 16 - 1993 Sept. 7), OSSE has observed 8 and detected 5, namely 3C 273, 3C 279, PKS 0528+134, CTA 102, and 3C 454.3. Additionally, OSSE has detected the BL LAC sources H 1517+65.6 and PKS 2155-304, which were not detected with EGRET. Variablility in the energy band 50 -150 keV is observed for all of the detected AGN. The OSSE blazar sources are all well described by simple power law models with photon number indices,,, varying from 1.0 to 2.1 among sources. When combined with available, although not necessarily contrmporaneous, COMPTEL and EGRET observations, 4 out of 5 detected blazars show clear evidence for spectral breaks between the hard X-ray and medium energy gamma-ray bands. The exception is the combined OSSE/EGRET data for 3C 279 during 1991 October, where a simple power-law with, approximately equal 1.9 works equally well. Gamma-ray evidence for beaming in CTA 102, PKS 0528+134, and 3C 454.3 is presented.

Oriented Scintillation Spectrometer Experiment observations of Co-57 in SN 1987A

The Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma Ray Observatory has observed SN 1987A for two 2 week periods during the first 9 months of the mission. Evidence for gamma-ray line and continuum emission from Co-57 is observed with an intensity of about 10 exp -4 gamma/sq cm/s. This photon flux between 50 and 136 keV is demonstrated by Monte Carlo calculations to be independent of the radial distribution of Co-57 for models of low optical depth, viz., models having photoelectric absorption losses of 122 keV photons no greater than several percent. For such models the observed Co-57 flux indicates that the ratio Ni-57/Ni-56 produced in the explosion was about 1.5 times the solar system ratio of Fe-57/Fe-56. When compared with nearly contemporaneous bolometric estimates of the luminosity for SN 1987A, our observations imply that Co-57 radioactivity does not account for most of the current luminosity of the supernova remnant in low optical depth models. We suggest alternatives, including a large optical depth model that is able to provide the SN 1987A luminosity and is consistent with the OSSE flux. It requires a 57/56 production ratio about twice solar.

Large scale diffuse light in the Coma cluster: A multi-scale approach

We have obtained wide field images of the Coma cluster in the B, V, R and I bands with the CFH12K camera at CFHT. To search for large scale diffuse emission, we have applied to these images an iterative multiscale wavelet analysis and reconstruction technique which made it possible to model all the sources (stars and galaxies) and subtract them from the original images. We found various concentrations of diffuse emission present in the central zone around the central galaxies NGC 4874 and NGC 4889. We characterize the positions, sizes and colors of these concentrations. Some sources do not seem to have strong star formation, while one probably exhibits spiral-like colors. One possible origin for the star forming diffuse emission sources is that in the region of the two main galaxies NGC 4874 and NGC 4889 spiral galaxies have recently been disrupted and star formation is still active in the dispersed material. We also use the characteristics of the sources of diffuse emission to trace the cluster dynamics. A scenario in which the group around NGC 4874 is moving north is consistent with our data.

The Bright SHARC Survey: The X-Ray Cluster Luminosity Function*

We present here initial results on the X-ray cluster luminosity function (XCLF) from the Bright Serendipitous High-Redshift Archival Cluster (SHARC) sample of distant X-ray clusters of galaxies. This sample is 97% complete in its optical identifications and contains 12 X-ray-luminous clusters in the redshift range 0.3 ≤ z ≤ 0.83 (median z = 0.42) and 1.1 × 1044 ≤ LX ≤ 8.3 × 1044 ergs s-1 (0.5-2.0 keV). We present a preliminary selection function for the Bright SHARC Survey based on Monte Carlo simulations. Using this selection function, we have computed the Bright SHARC Survey XCLF and find it to be fully consistent with a nonevolving XCLF to LX 5 × 1044 ergs s-1 and z 0.7. At LX > 5 × 1044 ergs s-1, we find evidence for a deficit of clusters compared to that expected from a nonevolving XCLF. We detect only one such cluster in the redshift range 0.3 ≤ z ≤ 0.7 when we would expect 4.9 clusters based on the local XCLF of De Grandi et al. The statistical significance of this deficit is 96%. To increase the statistical significance of this possible deficit, we have combined the Bright SHARC Survey and the 160 deg2 survey of Vikhlinin et al. This joint survey covers 260 deg2 and contains only one confirmed 0.3 ≤ z ≤ 0.7, LX > 5 × 1044 ergs s-1 cluster, while we would expect 7.6 such clusters based on the local XCLF (De Grandi et al.). The statistical significance of the deficit in this joint survey increases to 99.5%. These results remain preliminary because of incompletenesses in the optical follow-up and uncertainties in the local XCLF.

Very deep spectroscopy of the Coma cluster line of sight: exploring new territories

Context. Environmental effects are known to have an important influence on cluster galaxies, but studies at very faint magnitudes (R> 21) are almost exclusively based on imaging. We present here a very deep spectroscopic survey of galaxies on the line of sight to the Coma cluster. Aims. After a series of papers based on deep multi–band imaging of the Coma cluster, we explore spectroscopically part of the central regions of Coma, in order to confirm and generalize previous results, concerning in particular the galaxy luminosity function, red sequence, stellar populations and the most likely formation scenario for the Coma cluster. Methods. We have obtained reliable VIMOS redshifts for 715 galaxies in the direction of the Coma cluster centre in the unprecedented magnitude range 21 ≤R≤ 23, corresponding to the absolute magnitude range −14 ≤ MR ≤ −12. Results. We confirm the substructures previously identified in Coma by Adami et al. (2005a), and identify three new substructures. We detect a large number of groups behind Coma, in particular a large structure at z∼0.5, the SDSS Great Wall, and a large and very young previously unknown structure at z∼0.054, which we named the background massive group (BMG). These structures account for the mass maps derived from a recent weak lensing analysis by Gavazzi et al. (2009). The orbits of dwarf galaxies are probably anisotropic and radial, and could originate from field galaxies radially falling into the cluster along the numerous cosmological filaments surrounding Coma. Spectral characteristics of Coma dwarf galaxies show that red or absorption line galaxies have larger stellar masses and are older than blue or emission line galaxies. R≤22 galaxies show less prominent absorption lines than R≥22 galaxies. This trend is less clear for field galaxies which are similar to R≥22 Coma galaxies. This suggests that part of the faint Coma galaxies could have been recently injected from the field following the NGC 4911 group infall. We present a list of five Ultra Compact Dwarf galaxy candidates which need to be confirmed with high spatial resolution imaging with the HST. We also globally confirm spectroscopically our previous results concerning the galaxy luminosity functions based on imaging down to R=23 (MR = −12) and find that dwarf galaxies follow a red sequence similar to that drawn by bright Coma galaxies. Conclusions. Spectroscopy of faint galaxies in Coma confirms that dwarf galaxies are very abundant in this cluster, and that they are partly field galaxies that have fallen onto the cluster along cosmological filaments.

The pulsed hard X-ray spectrum of PSR B1509-58

Oriented Scintillation Spectrometer Experiment (OSSE) observed the 150 ms X-ray pulsar PSR B1509-58 m the supernova remnant MSH 15-52 for 4 weeks in 1992. The pulsed spectrum from 50 keV to 5 MeV is well represented by a single-power-law photon spectrum of the form (3.14 +/- 0.16) x 10(exp -6) x (E/118.5 keV)(exp -1.68 +/- -0.09) photons cm(exp -2)s(exp -1)keV(exp -1). This is significantly harder than the Crab pulsar spectrum in this energy range. The Ginga soft X-ray spectrum (2-60 keV) reported by Kawai et al. is significantly harder than the observed OSSE spectrum and predicts a flux 2 times higher than we observe in the approximately 55-170 keV energy band. This requires a break to a steeper spectrum somewhere in the intermediate energy range (approximately 20-80 keV). The spectrum must soften again at higher energies or the pulsar would have easily been detected by EGRET, COS B, and SAS 2.

Gamma Ray Observations of Cygnus X-1 With the Oriented Scintillation Spectrometer Experiment

Abstract : We report on ~120 days of observations of Cygnus X-1 with OSSE onboard the Compton Observatory. Emission is detected in the range 50 keV to 1 MeV and we find evidence for a continuum of hard X-ray flux levels rather than the existence of distinct flux states. Comparisons of the source spectra with various theoretical models show that an exponentially truncated power law best describes the average spectrum in the OSSE energy band. Although we have measured a new minimum in the hard X-ray flux from the source, no evidence was found for either a broad 1 MeV feature or a narrow 511 keV line previously reported in association with a low flux state. Upper limits on such emission features are an order of magnitude lower than earlier reported detections. The 5.6-day periodicity of the source measured at optical wavelengths was not detected with a sensitivity to the rms modulation fraction of 5% in the 60-140 keV energy band.

Hard X-rays from SN 1993J

The Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Observatory observed SN 1993J during three intervals centered approximately 12, 30, and 108 days after its outburst. Hard X-ray emission was detected in the first two of these intervals. No emission was seen in the third observation or in two earlier observations in 1991 and 1992. The coincidence of the observed excess with the outburst of SN 1993J and the consistency of the spectra and time evolution with those seen at lower energies by ROSAT and ASCA (Astro-D) argue that the observed emission is indeed from SN 1993J. It is probably due to the interaction of the fast supernova ejecta with circumstellar material. The luminosity, 5 x 10(exp 40) ergs/sec (50-150 keV) in the first interval, is significantly larger than predicted. Extrapolating the spectrum to a few keV accounts for most or all of the observed emission at low energy. The observed high temperature, 10(exp 9) K, is easily obtained in the shocked circumstellar matter, but a surprisingly high density is required there to give the observed luminosity, and little or no additional X-ray emission from denser shocked supernova ejecta is allowed. The hard emission might also be explained in terms of the shocked supernova ejecta itself with unexpectedly high temperature.

The DAFT/FADA survey - I. Photometric redshifts along lines of sight to clusters in the z = [0.4, 0.9] interval

Context. As a contribution to the understanding of the dark energy concept, the Dark energy American French Team (DAFT, in French FADA) has started a large project to characterize statistically high redshift galaxy clusters, infer cosmological constraints from weak lensing tomography, and understand biases relevant for constraining dark energy and cluster physics in future cluster and cosmological experiments. Aims. The purpose of this paper is to establish the basis of reference for the photo-z determination used in all our subsequent papers, including weak lensing tomography studies. Methods. This project is based on a sample of 91 high redshift (z ≥ 0.4), massive (>3 × 10 14 M� ) clusters with existing HST imaging, for which we are presently performing complementary multi-wavelength imaging. This allows us in particular to estimate spectral types and determine accurate photometric redshifts for galaxies along the lines of sight to the first ten clusters for which all the required data are available down to a limit of IAB = 24./24.5 with the LePhare software. The accuracy in redshift is of the order of 0.05 for the range 0.2 ≤ z ≤ 1.5. Results. We verified that the technique applied to obtain photometric redshifts works well by comparing our results to with previous works. In clusters, photo-z accuracy is degraded for bright absolute magnitudes and for the latest and earliest type galaxies. The photo-z accuracy also only slightly varies as a function of the spectral type for field galaxies. As a consequence, we find evidence for an environmental dependence of the photo-z accuracy, interpreted as the standard used spectral energy distributions being not very well suited to cluster galaxies. Finally, we modeled the LCDCS 0504 mass with the strong arcs detected along this line of sight.Context. As a contribution to the understanding of the dark energy con cept, the Dark energy American French Team (DAFT, in French FADA) has started a large project to characterize statistic ally high redshift galaxy clusters, infer cosmological con straints from Weak Lensing Tomography, and understand biases relevant for constraini ng dark energy and cluster physics in future cluster and cosm ological experiments. Aims. The purpose of this paper is to establish the basis of referen c for the photoz determination used in all our subsequent papers, including weak lensing tomography studies. Methods. This project is based on a sample of 91 high redshift ( z ≥0.4), massive ( >∼ 3 × 1014 M⊙) clusters with existing HST imaging, for which we are presently performing complementary multi-wav elength imaging. This allows us in particular to estimate sp ctral types and determine accurate photometric redshifts for galaxies alo ng the lines of sight to the first ten clusters for which all the required data are available down to a limit of IAB = 24./24.5 with the LePhare software. The accuracy in redshift is of th e order of 0.05 for the range 0 .2 ≤ z ≤ 1.5. Results. We verified that the technique applied to obtain photometric redshifts works well by comparing our results to with previo us works. In clusters, photoz accuracy is degraded for bright absolute magnitudes and for the latest and earliest type galaxies. The photoz accuracy also only slightly varies as a function of the spectral type for fie ld galaxies. As a consequence, we find evidence for an environ mental dependence of the photoz accuracy, interpreted as the standard used Spectral Energy Distributions being not very well suited to cluster galaxie s. Finally, we modeled the LCDCS 0504 mass with the strong arcs detected a long this line of sight. ⋆ Based on observations made with the NASA /ESA Hubble Space Telescope, obtained from the data archive at the Space Teles cop Institute and the Space Telescope European Coordinating Fa cility. STScI is operated by the association of Universities for Res earch in Astronomy, Inc. under the NASA contract NAS 5-26555. Also ba sed on observations made with ESO Telescopes at Paranal and La Si lla Observatories under programme ESO LP 166.A-0162. Also base d on visiting astronomer observations, at Cerro Tololo Inter -American Observatory, National Optical Astronomy Observatory, whi ch is op