Search for GRB afterglows in the ROSAT all-sky survey

We report on the status of our search for X-ray afterglows of gamma-ray bursts (GRBs) using the ROSAT all-sky survey data. The number of potential X-ray afterglow candidates with respect to the expected number of beamed GRBs allows to constrain the relative beaming angles of GRB emission and afterglow emission at about 1-5 hrs after the GRB.Comment: 3 pages A&A style, 1 color ps-figure; To appear in A&A Suppl. Series, Proc. of Rome 1998 GRB workshop, also available from http://www.aip.de/~jcg/publis.htm

Search for X-ray Afterglows from Gamma-Ray Bursts in the RASS

We report on a search for X-ray afterglows from gamma-ray bursts using the ROSAT all-sky survey (RASS) data. If the emission in the soft X-ray band is significantly less beamed than in the gamma-ray band, we expect to detect many afterglows in the RASS. Our search procedure generated 23 afterglow candidates, where about 4 detections are predicted. Follow-up spectroscopy of several counterpart candidates strongly suggests a flare star origin of the RASS events in many, if not all, cases. Given the small number of events we conclude that the data are consistent with comparable beaming angles in the X-ray and gamma-ray bands. Models predicting a large amount of energy emerging as a nearly isotropic X-ray component, and a so far undetected class of ``dirty fireballs'' and re-bursts are constrained.Comment: 5 pages, LATEX with aipproc.sty, incl. 1 ps-Fig., Proc. of the 5th Huntsville Gamma Ray Burst Symposium, Oct. 1999, ed. R.M. Kippen, AIP; also available at http://www.aip.de/~jcg/publis.htm

Using Virtual Observatory techniques to search for Adaptive Optics suitable AGN

Until recently, it has been possible only for nearby galaxies to study the scaling relations between central black hole and host galaxy in detail. Because of the small number densities at low redshift, (luminous) AGN are underrepresented in such detailed studies. The advent of adaptive optics (AO) at large telescopes helps overcoming this hurdle, allowing to reach small linear scales over a wide range in redshift. Finding AO-suitable targets, i.e., AGN having a nearby reference star, and carrying out an initial multiwavelength classification is an excellent use case for the Virtual Observatory. We present our Virtual-Observatory approach to select an AO-suitable catalog of X-ray-emitting AGN at redshifts 0.1<z<1.Comment: 4 pages, 5 figures, submitted to "EURO-VO AIDA workshop: Multiwavelength astronomy and Virtual Observatory", ESAC, Spain, 1-3 Dec. 200

Cluster Evolution in the ROSAT North Ecliptic Pole Survey

The deepest region of the ROSAT All-Sky Survey, at the North Ecliptic Pole, has been studied to produce a complete and unbiased X-ray selected sample of clusters of galaxies. This sample is used to investigate the nature of cluster evolution and explore potential implications for large-scale structure models. The survey is 99.6% optically identified. Spectroscopic redshifts have been measured for all the extragalactic identifications. In this Letter, first results on cluster evolution are presented based on a comparison between the number of the observed clusters in the North Ecliptic Pole survey and the number of expected clusters assuming no-evolution models. At z>0.3 there is a deficit of clusters with respect to the local universe which is significant at > 4.7sigma. The evolution appears to commence at L_{0.5-2.0} > 1.8x10^{44} erg s^{-1} in our data. The negative evolution goes in the same direction as the original EMSS result, the results from the 160 deg^{2} survey by Vikhlinin et al. (1998) and the recent results from the RDCS (Rosati et al. 2000). At lower redshifts there is no evidence for evolution, a result in agreement with these and other cluster surveys.Comment: 17 pages, 3 figures. Accepted for publication in ApJ Letter

The size distribution of galaxies in the Sloan Digital Sky Survey

abridged: We use a complete sample of about 140,000 galaxies from the Sloan Digital Sky Survey (SDSS) to study the size distribution of galaxies and its dependence on their luminosity, stellar mass, and morphological type. The large SDSS database provides statistics of unprecedented accuracy. For each type of galaxy, the size distribution at given luminosity (or stellar mass) is well described by a log-normal function, characterized by its median $\bar{R}$ and dispersion $\sigma_{\ln R}$. For late-type galaxies, there is a characteristic luminosity at $M_{r,0}\sim -20.5$ (assuming $h=0.7$) corresponding to a stellar mass M_0\sim 10^{10.6}\Msun. Galaxies more massive than $M_0$ have $\bar{R}\propto M^{0.4}$ and $\sigma_{\ln R}\sim 0.3$, while less massive galaxies have $\bar{R}\propto M^{0.15}$ and $\sigma_{\ln R}\sim 0.5$. For early-type galaxies, the $\bar{R}$ - $M$ relation is significantly steeper, $\bar{R}\propto M^{0.55}$, but the $\sigma_{\ln R}$ - $M$ relation is similar to that of late-type galaxies. Faint red galaxies have sizes quite independent of their luminosities.Comment: 42 pages, 18 figures, 2 tables; replaced with the version accepted by MNRA

Follow-Up Chandra Observations of Three Candidate Tidal Disruption Events

Large-amplitude, high-luminosity soft X-ray flares were detected by the ROSAT All-Sky Survey in several galaxies with no evidence of Seyfert activity in their ground-based optical spectra. These flares had the properties predicted for a tidal disruption of a star by a central supermassive black hole. We report Chandra observations of three of these galaxies taken a decade after their flares that reveal weak nuclear X-ray sources that are from 240 to 6000 times fainter than their luminosities at peak, supporting the theory that these were special events and not ongoing active galactic nucleus (AGN) variability. The decline of RX J1624.9+7554 by a factor of 6000 is consistent with the (t-t_D)^(-5/3) decay predicted for the fall-back phase of a tidal disruption event, but only if ROSAT was lucky enough to catch the event exactly at its peak in 1990 October. RX J1242.6-1119A has declined by a factor of 240, also consistent with (t-t_D)^(-5/3). In the H II galaxy NGC 5905 we find only resolved, soft X-ray emission that is undoubtedly associated with starburst activity. When accounting for the starburst component, the ROSAT observations of NGC 5905, as well as the Chandra upper limit on its nuclear flux, are consistent with a (t-t_D)^(-5/3) decay by at least a factor of 1000. Although we found weak Seyfert~2 emission lines in Hubble Space Telescope spectra of NGC 5905, indicating that a low-luminosity AGN was present prior to the X-ray flare, we favor a tidal disruption explanation for the flare itself.Comment: 17 pages, 4 figures, to appear in ApJ April 1 200

On the Evidence for Axion-like Particles from Active Galactic Nuclei

Burrage, Davis, and Shaw recently suggested exploiting the correlations between high and low energy luminosities of astrophysical objects to probe possible mixing between photons and axion-like particles (ALP) in magnetic field regions. They also presented evidence for the existence of ALP's by analyzing the optical/UV and X-ray monochromatic luminosities of AGNs. We extend their work by using the monochromatic luminosities of 320 unobscured Active Galactic Nuclei from the Sloan Digital Sky Survey/Xmm-Newton Quasar Survey (Young et al., 2009), which allows the exploration of 18 different combinations of optical/UV and X-ray monochromatic luminosities. However, we do not find compelling evidence for the existence of ALPs. Moreover, it appears that the signal reported by Burrage et al. is more likely due to X-ray absorption rather than to photon-ALP oscillation.Comment: 16 pages, 12 figures. Updated to reflect the minor changes introduced in the published versio