Showing papers by "Roger Blandford published in 2008"

LSST: from Science Drivers to Reference Design and Anticipated Data Products

Abstract: (Abridged) We describe here the most ambitious survey currently planned in the optical, the Large Synoptic Survey Telescope (LSST). A vast array of science will be enabled by a single wide-deep-fast sky survey, and LSST will have unique survey capability in the faint time domain. The LSST design is driven by four main science themes: probing dark energy and dark matter, taking an inventory of the Solar System, exploring the transient optical sky, and mapping the Milky Way. LSST will be a wide-field ground-based system sited at Cerro Pachon in northern Chile. The telescope will have an 8.4 m (6.5 m effective) primary mirror, a 9.6 deg$^2$ field of view, and a 3.2 Gigapixel camera. The standard observing sequence will consist of pairs of 15-second exposures in a given field, with two such visits in each pointing in a given night. With these repeats, the LSST system is capable of imaging about 10,000 square degrees of sky in a single filter in three nights. The typical 5$\sigma$ point-source depth in a single visit in $r$ will be $\sim 24.5$ (AB). The project is in the construction phase and will begin regular survey operations by 2022. The survey area will be contained within 30,000 deg$^2$ with $\delta<+34.5^\circ$, and will be imaged multiple times in six bands, $ugrizy$, covering the wavelength range 320--1050 nm. About 90\% of the observing time will be devoted to a deep-wide-fast survey mode which will uniformly observe a 18,000 deg$^2$ region about 800 times (summed over all six bands) during the anticipated 10 years of operations, and yield a coadded map to $r\sim27.5$. The remaining 10\% of the observing time will be allocated to projects such as a Very Deep and Fast time domain survey. The goal is to make LSST data products, including a relational database of about 32 trillion observations of 40 billion objects, available to the public and scientists around the world.

The sloan digital sky survey-II supernova survey: technical summary

Abstract: The Sloan Digital Sky Survey-II (SDSS-II) has embarked on a multi-year project to identify and measure light curves for intermediate-redshift (0.05 < z < 0.35) Type Ia supernovae (SNe Ia) using repeated five-band (ugriz) imaging over an area of 300 sq. deg. The survey region is a stripe 2.5° wide centered on the celestial equator in the Southern Galactic Cap that has been imaged numerous times in earlier years, enabling construction of a deep reference image for the discovery of new objects. Supernova imaging observations are being acquired between September 1 and November 30 of 2005-7. During the first two seasons, each region was imaged on average every five nights. Spectroscopic follow-up observations to determine supernova type and redshift are carried out on a large number of telescopes. In its first two three-month seasons, the survey has discovered and measured light curves for 327 spectroscopically confirmed SNe Ia, 30 probable SNe Ia, 14 confirmed SNe Ib/c, 32 confirmed SNe II, plus a large number of photometrically identified SNe Ia, 94 of which have host-galaxy spectra taken so far. This paper provides an overview of the project and briefly describes the observations completed during the first two seasons of operation.

Cosmic Evolution of Black Holes and Spheroids. III. The MBH-σ* Relation in the Last Six Billion Years

Abstract: We measure the evolution of the correlation between black hole mass and host spheroid velocity dispersion (MBH-σ*) over the last 6 billion years, by studying three carefully selected samples of active galaxies at z = 0.57, z = 0.36 and z < 0.1. For all three samples, virial black hole masses are consistently estimated using the line dispersion of Hβ and the continuum luminosity at 5100 A or Hα line luminosity, based on our cross calibration of the broad-line region size-luminosity relation. For the z = 0.57 sample, new stellar velocity dispersions are measured from high signal-to-noise ratio spectra obtained at the Keck Telescope, while for the two lower redshift samples they are compiled from previous works. Extending our previous result at z = 0.36, we find an offset from the local relation, suggesting that for fixed MBH, distant spheroids have on average smaller velocity dispersions than local ones. The measured offset at z = 0.57 is Δ log σ* = 0.12 ± 0.05 ± 0.06 (or Δ log MBH = 0.50 ± 0.22 ± 0.25), i.e., Δ log M BH = (3.1 ± 1.5) log (1 + z) + 0.05 ± 0.21. This is inconsistent with a tight and nonevolving universal M BH-σ* relation at the 95% CL. © 2008. The American Astronomical Society. All rights reserved.

Cosmic Evolution of Black Holes and Spheroids. III. The M-sigma relation in the last six billion years

Abstract: We measure the evolution of the correlation between black hole mass and host spheroid velocity dispersion over the last 6 billion years, by studying three carefully selected samples of active galaxies at z=0.57, z=0.36 and z<0.1. For all three samples, virial black hole masses are consistently estimated using the line dispersion of H$\beta$ and the continuum luminosity at 5100A or Halpha line luminosity, based on our cross calibration of the broad line region size-luminosity relation. For the z=0.57 sample, new stellar velocity dispersions are measured from high signal-to-noise ratio spectra obtained at the Keck Telescope, while for the two lower redshift samples they are compiled from previous works. Extending our previous result at z=0.36, we find an offset from the local relation, suggesting that for fixed M_{BH}, distant spheroids have on average smaller velocity dispersions than local ones. The measured offset at z=0.57 is d log sigma_{*}=0.12 \pm 0.05\pm 0.06 (or d log M_{BH}=0.50 \pm 0.22\pm 0.25), i.e. d log M_{BH} = (3.1\pm1.5)\log (1+z) + 0.05\pm0.21. This is inconsistent with a tight and non-evolving universal M-sigma relation at the 95%CL

Dark matter and baryons in the most x-ray luminous and merging galaxy cluster rx j1347.5−1145 *

Abstract: The galaxy cluster RX J1347.5−1145 is one of the most X-ray luminous and most massive clusters known. Its extreme mass makes it a prime target for studying issues addressing cluster formation and cosmology. Despite the naive expectation that mass estimation for this cluster should be straightforward (high mass and favorable redshift make it an efficient lens, and in addition it is bright in X-rays and appears to be in a fairly relaxed state), some studies have reported very discrepant mass estimates from X-ray, dynamical and gravitational lensing. In this paper we present new high-resolution HST/ACS and Chandra X-ray data. The high resolution and sensitivity of ACS enabled us to detect and quantify several new multiply imaged sources, we now use a total of eight for the strong lensing analysis. Combining this information with shape measurements of weak lensing sources in the central regions of the cluster, we derive a high-resolution, absolutely-calibrated mass map. This map provides the best available quantification of the total mass of the central part of the cluster to date. We compare the reconstructed mass with that inferred from the new Chandra X-ray data, and conclude that both mass estimates agree extremely well in the observed region, namely within 400h −1 70 kpc of the cluster center. In addition we study the major baryonic components (gas and stars) and hence derive the dark matter distribution in the center of the cluster. We find that the dark matter and baryons are both centered on the BCG within the uncertainties (alignment is better than < 10 kpc). We measure the corresponding 1-D profiles and find that dark matter distribution is consistent with both NFW and cored profiles, indicating that a more extended radial analysis is needed to pinpoint the concentration parameter, and hence the inner slope of the dark matter profile. Subject headings: cosmology: dark matter – gravitational lensing – galaxies:clusters:individual:RX J1347.5-1145

The Fermi Gamma Ray Space Telescope discovers the Pulsar in the Young Galactic Supernova-Remnant CTA 1

Abstract: Energetic young pulsars and expanding blast waves (supernova remnants, SNRs) are the most visible remains after massive stars, ending their lives, explode in core-collapse supernovae. The Fermi Gamma-Ray Space Telescope has unveiled a radio quiet pulsar located near the center of the compact synchrotron nebula inside the supernova remnant CTA 1. The pulsar, discovered through its gamma-ray pulsations, has a period of 316.86 ms, a period derivative of 3.614 x 10{sup -13} s s{sup -1}. Its characteristic age of 10{sup 4} years is comparable to that estimated for the SNR. It is conjectured that most unidentified Galactic gamma ray sources associated with star-forming regions and SNRs are such young pulsars.

Dark Matter and Baryons in the X-Ray Luminous Merging Galaxy Cluster RX J1347.5–1145*

Abstract: The galaxy cluster RX J1347.5–1145 is one of the most X-ray luminous and most massive clusters known. This makes it a prime target for studying issues addressing cluster formation and cosmology. Despite the naive expectation that mass estimation for this cluster should be straightforward (high mass and favorable redshift make it an efficient lens; it is bright in X-rays and appears to be fairly relaxed), some studies have reported very discrepant mass estimates. In this paper we present new high-resolution HST ACS and Chandra X-ray data. The high resolution and sensitivity of ACS enabled us to detect and quantify several new multiple images; we use a total of eight for the strong-lensing analysis. Combining it with shape measurements of weak-lensing sources in the central regions of the cluster, we derive a high-resolution, absolutely calibrated mass map. This map provides the best available quantification of the total mass of the central part of the cluster to date. We compare the reconstructed mass with that inferred from the new Chandra X-ray data. Both mass estimates agree extremely well in the observed region (within 400 -->h−170 kpc of the cluster center). In addition we study the major baryonic components (gas and stars) and hence derive the dark matter distribution in the center of the cluster. We find that the dark matter and baryons are both centered on the BCG within the uncertainties (alignment is better <10 kpc). We measure the corresponding dark matter profile and find it consistent with both NFW and cored profiles, indicating that a more extended radial analysis is needed to pinpoint the concentration parameter, and hence the inner slope of the dark matter profile.

First-year spectroscopy for the Sloan Digital Sky Survey-II supernova survey

Abstract: This paper presents spectroscopy of supernovae (SNe) discovered in the first season of the Sloan Digital Sky Survey-II SN Survey. This program searches for and measures multi-band light curves of SNe in the redshift range z = 0.05-0.4, complementing existing surveys at lower and higher redshifts. Our goal is to better characterize the SN population, with a particular focus on SNe Ia, improving their utility as cosmological distance indicators and as probes of dark energy. Our SN spectroscopy program features rapid-response observations using telescopes of a range of apertures, and provides confirmation of the SN and host-galaxy types as well as precise redshifts. We describe here the target identification and prioritization, data reduction, redshift measurement, and classification of 129 SNe Ia, 16 spectroscopically probable SNe Ia, 7 SNe Ib/c, and 11 SNe II from the first season. We also describe our efforts to measure and remove the substantial host-galaxy contamination existing in the majority of our SN spectra.

Hot Self-Similar Relativistic Magnetohydrodynamic Flows

Abstract: We consider axisymmetric relativistic jets with a toroidal magnetic field and an ultrarelativistic equation of state, to study the lateral structure of jets whose pressure is matched to the pressure of the surrounding medium. We find all self-similar steady state solutions of the relativistic MHD equations for this setup. One of the solutions is a parabolic jet being accelerated by the pressure gradient as it propagates through a medium with pressure declining as p(z) ∝ z−2. As the jet material expands due to internal pressure gradients, it runs into the ambient medium resulting in a pileup of material along the jet boundary, while the magnetic field acts to produce a magnetic pinch along the axis of the jet. Such jets can be in a lateral pressure equilibrium only if their opening angle θj at distance z is smaller than about 1/γ , where γ is the characteristic bulk Lorentz factor at this distance; otherwise, different parts of the jet cannot maintain causal contact. We construct maps of optically thin synchrotron emission from our self-similar models. We suggest that the boundary pileup may be the reason for the limb-brightening of the subparsec jet of M87. We find that if the synchrotron emissivity falls with the distance from the jet axis, the polarization fraction rises toward the edge, as seen in 3C273 and Mrk501. Projection effects and the emissivity pattern of the jet have a strong effect on the observed polarization signal, so the interpretation of the polarization data in terms of the geometry of magnetic fields is rather uncertain. For example, jets with toroidal magnetic fields display the "spine-sheath" polarization angle pattern seen in some BL Lac objects.

Hot self-similar relativistic MHD flows

Abstract: We consider axisymmetric relativistic jets with a toroidal magnetic field and an ultrarelativistic equation of state, with the goal of studying the lateral structure of jets whose pressure is matched to the pressure of the medium through which they propagate. We find all self-similar steady-state solutions of the relativistic MHD equations for this setup. One of the solutions is the case of a parabolic jet being accelerated by the pressure gradient as it propagates through a medium with pressure declining as p(z)\propto z^{-2}. As the jet material expands due to internal pressure gradients, it runs into the ambient medium resulting in a pile-up of material along the jet boundary, while the magnetic field acts to produce a magnetic pinch along the axis of the jet. Such jets can be in a lateral pressure equilibrium only if their opening angle \theta_j at distance z is smaller than about 1/\gamma, where \gamma is the characteristic bulk Lorentz-factor at this distance; otherwise, different parts of the jet cannot maintain causal contact. We construct maps of optically thin synchrotron emission from our self-similar models. We suggest that the boundary pile-up may be the reason for the limb-brightening of the sub-parsec jet of M87. We find that if the synchrotron emissivity falls with the distance from the jet axis, the polarization fraction rises toward the edge, as seen in 3C273 and Mkn501. Projection effects and the emissivity pattern of the jet have a strong effect on the observed polarization signal, so the interpretation of the polarization data in terms of the geometry of magnetic fields is rather uncertain. For example, jets with toroidal magnetic fields display the `spine-sheath' polarization angle pattern seen in some BL Lac objects.

First-Year Spectroscopy for the SDSS-II Supernova Survey

Abstract: This paper presents spectroscopy of supernovae discovered in the first season of the Sloan Digital Sky Survey-II Supernova Survey. This program searches for and measures multi-band light curves of supernovae in the redshift range z = 0.05 - 0.4, complementing existing surveys at lower and higher redshifts. Our goal is to better characterize the supernova population, with a particular focus on SNe Ia, improving their utility as cosmological distance indicators and as probes of dark energy. Our supernova spectroscopy program features rapid-response observations using telescopes of a range of apertures, and provides confirmation of the supernova and host-galaxy types as well as precise redshifts. We describe here the target identification and prioritization, data reduction, redshift measurement, and classification of 129 SNe Ia, 16 spectroscopically probable SNe Ia, 7 SNe Ib/c, and 11 SNe II from the first season. We also describe our efforts to measure and remove the substantial host galaxy contamination existing in the majority of our SN spectra.

A subhorizon framework for probing the relationship between the cosmological matter distribution and metric perturbations

Abstract: The relationship between the metric and non-relativistic matter distribution depends on the theory of gravity and additional fields, hence providing a possible way of distinguishing competing theories. With the assumption that the geometry and kinematics of the homogeneous Universe have been measured to sufficient accuracy, we present a procedure for understanding and testing the relationship between the cosmological matter distribution and metric perturbations (along with their respective evolution) using the ratio of the physical size of the perturbation to the size of the horizon as our small expansion parameter. We expand around Newtonian gravity on linear, subhorizon scales with coefficient functions in front of the expansion parameter. Our framework relies on an ansatz which ensures that (i) the Poisson equation is recovered on small scales and (ii) the metric variables (and any additional fields) are generated and supported by the non-relativistic matter overdensity. The scales for which our framework is intended are small enough so that cosmic variance does not significantly limit the accuracy of the measurements and large enough to avoid complications due to non-linear effects and baryon cooling. From a theoretical perspective, the coefficient functions provide a general framework for contrasting the consequences of ACDM (cosmological constant + cold dark matter) and its alternatives. We calculate the coefficient functions for general relativity (GR) with a cosmological constant and dark matter, GR with dark matter and quintessence, scalar-tensor theories (STT), f(R) gravity and braneworld models. We identify a possibly unique signature of braneworld models. For observers, constraining the coefficient functions provides a streamlined approach for testing gravity in a scale-dependent manner. We briefly discuss the observations best suited for an application of our framework.

Stability of Relativistic Jets from Rotating, Accreting Black Holes via Fully Three-Dimensional Magnetohydrodynamic Simulations

Abstract: Rotating magnetized compact objects and their accretion discs can generate strong toroidal magnetic fields driving highly magnetized plasmas into relativistic jets. Of significant concern, however, has been that a strong toroidal field in the jet should be highly unstable to the non-axisymmetric helical kink (screw) $m=1$ mode leading to rapid disruption. In addition, a recent concern has been that the jet formation process itself may be unstable due to the accretion of non-dipolar magnetic fields. We describe large-scale fully three-dimensional global general relativistic magnetohydrodynamic simulations of rapidly rotating, accreting black holes producing jets. We study both the stability of the jet as it propagates and the stability of the jet formation process during accretion of dipolar and quadrupolar fields. For our dipolar model, despite strong non-axisymmetric disc turbulence, the jet reaches Lorentz factors of $\Gamma\sim 10$ with opening half-angle $\theta_j\sim 5^\circ$ at $10^3$ gravitational radii without significant disruption or dissipation with only mild substructure dominated by the $m=1$ mode. On the contrary, our quadrupolar model does not produce a steady relativistic ($\Gamma\gtrsim 3$) jet due to mass-loading of the polar regions caused by unstable polar fields. Thus, if produced, relativistic jets are roughly stable structures and may reach up to an external shock with strong magnetic fields. We discuss the astrophysical implications of the accreted magnetic geometry playing such a significant role in relativistic jet formation, and we outline avenues for future work.

Dissecting the Gravitational Lens B1608+656. I. Lens Potential Reconstruction

Abstract: Strong gravitational lensing is a powerful technique for probing galaxy mass distributions and for measuring cosmological parameters. We present a pixelated approach to modeling simultaneously the lens potential and source intensity of strong gravitational lens systems with extended source-intensity distributions. For systems with sources of sufficient extent such that the separate lensed images are connected by intensity measurements, the accuracy in the reconstructed potential is solely limited by the quality of the data. We apply this potential reconstruction technique to deep HST observations of B1608+656, a four-image gravitational lens system formed by a pair of interacting lens galaxies. We present a comprehensive Bayesian analysis of the system that takes into account the extended source-intensity distribution, dust extinction, and the interacting lens galaxies. Our approach allows us to compare various models of the components of the lens system, which include the point-spread function (PSF), dust, lens galaxy light, source-intensity distribution, and lens potential. Using optimal combinations of the PSF, dust, and lens galaxy light models, we successfully reconstruct both the lens potential and the extended source-intensity distribution of B1608+656. The resulting reconstruction can be used as the basis of a measurement of the Hubble constant. We use our reconstruction of the gravitational potential to study the relative distribution of mass and light in the lensing galaxies. We find that the mass-to-light ratio for the primary lens galaxy is (2.0+/-0.2)h M_{\sun} L_{B,\sun}^{-1} within the Einstein radius 3.9 h^{-1} kpc, in agreement with what is found for noninteracting lens galaxies at the same scales. (Abridged)

Gamma Ray Burst Section of the White Paper on the Status and Future of Ground-based TeV Gamma-ray Astronomy

Abstract: This is a report on the findings of the gamma ray burst working group for the white paper on the status and future of TeV gamma-ray astronomy. The white paper is an APS commissioned document, and the overall version has also been released and can be found on astro-ph. This detailed section of the white paper discusses the status of past and current attempts to observe gamma ray bursts at GeV-TeV energies. We concentrate on the potential of future ground-based gamma-ray experiments to observe the highest energy emission ever recorded for GRBs, particularly for those that are nearby and have high Lorentz factors in the GRB jet. It is clear that major advances are possible and that the detection of very high energy emission would have strong implications for GRB models, as well as cosmic ray origin.

What is the largest Einstein radius in the universe

Abstract: The Einstein radius plays a central role in lens studies as it characterises the strength of gravitational lensing. The distribution of Einstein radii near the upper cutoff should probe the largest mass concentrations in the universe. Adopting a triaxial halo model, we compute expected distributions of large Einstein radii. To assess the cosmic variance, we generate a number of all-sky Monte-Carlo realisations. We find that the expected largest Einstein radius in the universe is sensitive to the cosmological model: for a source redshift z=1, they are 42^{+9}_{-7}, 35^{+8}_{-6}, and 54^{+12}_{-7} arcseconds, assuming best-fit parameters of the WMAP5, WMAP3 and WMAP1 data, respectively. These values are broadly consistent with current observations given their incompleteness. For the same source redshift, we expect in all-sky 35 (WMAP5), 15 (WMAP3), and 150 (WMAP1) clusters that have Einstein radii larger than 20". Whilst the values of the largest Einstein radii are almost unaffected by the primordial non-Gaussianity currently of interest, the abundance of large lens clusters should probe non-Gaussianity competitively with CMB, but only if other cosmological parameters are well-measured. We also find that these "superlens" clusters constitute a highly biased population. For instance, a substantial fraction of these superlens clusters have major axes preferentially aligned with the line-of-sight. As a consequence, the projected mass distributions of the clusters are rounder by an ellipticity of 0.2 and have 40%-60% larger concentrations compared with typical clusters with similar redshifts and masses. We argue that the large concentration measured in A1689 is consistent with our model prediction at the 1.2\sigma level. (Abridged)

A Measurement of The Faint Source Correlation Function in the GOODS and UDF Survey

Abstract: We present a stable procedure for defining and measuring the two point angular autocorrelation function, w, of faint (25 < V < 29), barely resolved and unresolved sources in the HST GOODS and UDF datasets. We construct catalogs that include close pairs and faint detections. We show for the first time that on subarcsecond scales, the correlation function exceeds unity. This correlation function is well fit by a power law with index of 2.5 and a characteristic angular scale that decrease slowly with magnitude. This is very different from the purely gravitationalcorrelation function of brighter galaxies which has a index of 0.7 and a characteristic angular scale which decreases quickly with magnitude. This observed clustering probably reflects the presence of giant star-forming regions within galactic-scale potential wells. Its measurement enables a new approach to measuring the redshift distribution of the faintest sources in the sky.

The standard model and some new directions

Abstract: A 'standard' model of Active Galactic Nuclei (AGN), based upon a massive black hole surrounded by a thin accretion disk, is defined It is argued that, although there is good evidence for the presence of black holes and orbiting gas, most of the details of this model are either inadequate or controversial Magnetic field may be responsible for the confinement of continuum and line-emitting gas, for the dynamical evolution of accretion disks and for the formation of jets It is further argued that gaseous fuel is supplied in molecular form and that this is responsible for thermal re-radiation, equatorial obscuration and, perhaps, the broad line gas clouds Stars may also supply gas close to the black hole, especially in low power AGN and they may be observable in discrete orbits as probes of the gravitational field Recent observations suggest that magnetic field, stars, dusty molecular gas and orientation effects must be essential components of a complete description of AGN The discovery of quasars with redshifts approaching 5 is an important clue to the mechanism of galaxy formation

Astrophysical particle acceleration

Abstract: The observed properties of high energy particles in different astrophysical environments are briefly summarized. It appears that cosmic rays are freely produced with high (up to 10 per cent) efficiency by supersonic flows. At relativistic energies, the integral distribution function of the accelerated particles (i.e., the number of particles with energy in excess of some value E) generally has a power law form with slope lying in the range ∼1.2 to 1.8. Various particle acceleration schemes are examined. Fermi acceleration at strong shock fronts seems a particularly promising process. A simple test particle analysis suggests that this mechanism can fulfill the necessary energetic and spectral requirements of galactic cosmic rays. More detailed studies of shock acceleration including the mediating influence of the accelerated particles and the generation of scattering Alfven modes together with models of the gas subshock give inconclusive results. Future progress depends upon combining the results of in sit...

A Statistical Description of AGN Jet Evolution from the VLBA Imaging and Polarimetry Survey (VIPS)

Abstract: A detailed analysis of the evolution of the properties of core-jet systems within the VLBA Imaging and Polarimetry Survey (VIPS) is presented. We find a power-law relationship between jet intensity and width that suggests that for the typical jet, little if any energy is lost as it moves away from its core. Using VLA images at 1.5 GHz, we have found evidence that parsec-scale jets tend to be aligned with the direction of emission on kiloparsec scales. We also found that this alignment improves as the jets move farther from their cores on projected scales as small as ~50-100 pc. This suggests that realignment of jets on these projected scales is relatively common. We typically find a modest amount of bending (a change in jet position angle of ~5°) on these scales, suggesting that this realignment may typically occur relatively gradually.

Section on Supernova remnants and cosmic rays of the White Paper on the Status and Future of Ground-based Gamma-ray Astronomy

Abstract: This is a report on the findings of the SNR/cosmic-ray working group for the white paper on the status and future of ground-based gamma-ray astronomy The white paper is an APS commissioned document, and the overall version has also been released and can be found on astro-ph This detailed section of the white paper discusses the status of past and current attempts to observe shell-type supernova remnants and diffuse emission from cosmic rays at GeV-TeV energies We concentrate on the potential of future ground-based gamma-ray experiments to study the acceleration of relativistic charged particles which is one of the main unsolved, yet fundamental, problems in modern astrophysics The acceleration of particles relies on interactions between energetic particles and magnetic turbulence In the case of SNRs we can perform spatially resolved studies in systems with known geometry, and the plasma physics deduced from these observations will help us to understand other systems where rapid particle acceleration is believed to occur and where observations as detailed as those of SNRs are not possible

Automated detection of galaxy-scale gravitational lenses in high resolution imaging data

Abstract: Lens modeling is the key to successful and meaningful automated strong galaxy-scale gravitational lens detection. We have implemented a lens-modeling "robot" that treats every bright red galaxy (BRG) in a large imaging survey as a potential gravitational lens system. Using a simple model optimized for "typical" galaxy-scale lenses, we generate four assessments of model quality that are used in an automated classification. The robot infers the lens classification parameter H that a human would have assigned; the inference is performed using a probability distribution generated from a human-classified training set, including realistic simulated lenses and known false positives drawn from the HST/EGS survey. We compute the expected purity, completeness and rejection rate, and find that these can be optimized for a particular application by changing the prior probability distribution for H, equivalent to defining the robot's "character." Adopting a realistic prior based on the known abundance of lenses, we find that a lens sample may be generated that is ~100% pure, but only ~20% complete. This shortfall is due primarily to the over-simplicity of the lens model. With a more optimistic robot, ~90% completeness can be achieved while rejecting ~90% of the candidate objects. The remaining candidates must be classified by human inspectors. We are able to classify lens candidates by eye at a rate of a few seconds per system, suggesting that a future 1000 square degree imaging survey containing 10^7 BRGs, and some 10^4 lenses, could be successfully, and reproducibly, searched in a modest amount of time. [Abridged]

The Gamma Ray Burst section of the White Paper on the Status and Future of Very High Energy Gamma Ray Astronomy: A Brief Preliminary Report

Abstract: This is a short report on the preliminary findings of the gamma ray burst (GRB) working group for the white paper on the status and future of very high energy (VHE; >50 GeV) gamma-ray astronomy. The white paper discusses the status of past and current attempts to observe GRBs at GeV-TeV energies, including a handful of low-significance, possible detections. The white paper concentrates on the potential of future ground-based gamma-ray experiments to observe the highest energy emission ever recorded for GRBs, particularly for those that are nearby and have high Lorentz factors in the GRB jet. It is clear that the detection of VHE emission would have strong implications for GRB models, as well as cosmic ray origin. In particular, the extended emission phase (including both afterglow emission and possible flaring) of nearby long GRBs could provide the best possibility for detection. The difficult-to-obtain observations during the prompt phase of nearby long GRBs and short GRBs could also provide particularly strong constraints on the opacity and bulk Lorentz factors surrounding the acceleration site. The synergy with upcoming and existing observatories will, of course, be critical for both identification of GRBs and for multiwavelength/multimessenger studies.

Section on Extragalactic Science Topics of the White Paper on the Status and Future of Ground-Based TeV Gamma-Ray Astronomy

Abstract: This is a report on the findings of the extragalactic science working group for the white paper on the status and future of TeV gamma-ray astronomy. The white paper was commissioned by the American Physical Society, and the full white paper can be found on astro-ph (arXiv:0810.0444). This detailed section discusses extragalactic science topics including active galactic nuclei, cosmic ray acceleration in galaxies, galaxy clusters and large scale structure formation shocks, and the study of the extragalactic infrared and optical background radiation. The scientific potential of ground based gamma-ray observations of Gamma-Ray Bursts and dark matter annihilation radiation is covered in other sections of the white paper.