Eric S. Perlman

A public turbulence database cluster and applications to study Lagrangian evolution of velocity increments in turbulence

A public database system archiving a direct numerical simulation (DNS) data set of isotropic, forced turbulence is described in this paper. The data set consists of the DNS output on 10243 spatial points and 1024 time samples spanning about one large-scale turnover time. This complete 10244 spacetime history of turbulence is accessible to users remotely through an interface that is based on the Web-services model. Users may write and execute analysis programs on their host computers, while the programs make subroutine-like calls that request desired parts of the data over the network. The users are thus able to perform numerical experiments by accessing the 27 terabytes (TB) of DNS data using regular platforms such as laptops. The architecture of the database is explained, as are some of the locally defined functions, such as differentiation and interpolation. Test calculations are performed to illustrate the usage of the system and to verify the accuracy of the methods. The database is then used to analyze a dynamical model for small-scale intermittency in turbulence. Specifically, the dynamical effects of pressure and viscous terms on the Lagrangian evolution of velocity increments are evaluated using conditional averages calculated from the DNS data in the database. It is shown that these effects differ considerably among themselves and thus require different modeling strategies in Lagrangian models of velocity increments and intermittency.

The Deep X-Ray Radio Blazar Survey. I. Methods and First Results*

We have undertaken a survey, the Deep X-Ray Radio Blazar Survey (DXRBS), of archived, pointed ROSAT Position Sensitive Proportional Counter data for blazars by correlating the ROSAT WGACAT database with several publicly available radio catalogs, restricting our candidate list to serendipitous flat radio spectrum sources (αr ≤ 0.70, where Sν ∝ ν). We discuss our survey methods, identification procedure, and first results. Our survey is found to be ~95% efficient at finding flat-spectrum radio-loud quasars (FSRQs; 59 of our first 85 identifications) and BL Lacertae objects (22 of our first 85 identifications), a figure that is comparable to or greater than that achieved by other radio and X-ray survey techniques. The identifications presented here show that all previous samples of blazars (even when taken together) did not representatively survey the blazar population, missing critical regions of (LX, LR) parameter space within which large fractions of the blazar population lie. Particularly important is the identification of a large population of FSRQs (25% of DXRBS FSRQs) with ratios of X-ray to radio luminosity 10-6 (αrx 0.78). In addition, as a result of our greater sensitivity, the DXRBS has already more than doubled the number of FSRQs in complete samples with 5 GHz (radio) luminosities between 1031.5 and 1033.5 ergs s-1 Hz-1, and fills in the region of parameter space between X-ray–selected and radio-selected samples of BL Lac objects. The DXRBS is the very first sample to contain statistically significant numbers of blazars at low luminosities, approaching what should be the lower end of the FSRQ luminosity function.

Optical and Radio Polarimetry of the M87 Jet at 02 Resolution

We discuss optical (HST/WFPC2 F555W) and radio (15 GHz VLA) polarimetry observations of the M87 jet taken during 1994-1995. The angular resolution of both of these observations is ∼ 0.2 ′′ , which at the distance of M87 corresponds to 15 pc. Many knot regions are very highly polarized (∼ 40 −50%, approaching the theoretical maximum for optically thin synchrotron radiation), suggesting highly ordered magnetic fields. High degrees of polarization are also observed in interknot regions. The optical and radio polarization maps share many similarities, and in both, the magnetic field is largely parallel to the jet, except in the “shock-like” knot regions (parts of HST-1, A, and C), where it becomes perpendicular to the jet. We do observe significant differences between the radio and optical polarized structures, particularly for bright knots in the inner jet, giving us important insight into the radial structure of the jet. Unlike in the radio, the optical magnetic field position angle becomes perpendicular to the jet at the upstream ends of knots HST-1, D, E and F. Moreover, the optical polarization appears to decrease markedly at the position of the flux maxima in these knots. In contrast, the magnetic field position angle observed in the radio remains parallel to the jet in most of these regions, and the decreases in radio polarization are smaller. More minor differences are seen in other jet regions. Many of the differences between optical and radio polarimetry results can be explained in terms of a model whereby shocks occur in the jet interior, where higher-energy electrons are concentrated and dominate both polarized and unpolarized emissions in the optical, while the radio maps show strong contributions from lower-energy electrons in regions with B parallel, near the jet surface.

A CHANDRA SURVEY OF QUASAR JETS: FIRST RESULTS

We present results from Chandra X-ray imaging and spectroscopy of a flux-limited sample of flat-spectrum radio-emitting quasars with jetlike extended structure. Twelve of 20 quasar jets are detected in 5 ks ACIS-S exposures. The quasars without X-ray jets are not significantly different from those in the sample with detected jets except that the extended radio emission is generally fainter. New radio maps are combined with the X-ray images in order to elucidate the relation between radio and X-ray emission in spatially resolved structures. We find a variety of morphologies, including long straight jets and bends up to 90°. All X-ray jets are one-sided although the radio images used for source selection often show lobes opposite the X-ray jets. The FR II X-ray jets can all be interpreted as inverse Compton scattering of cosmic microwave background photons by electrons in large-scale relativistic jets although deeper observations are required to test this interpretation in detail. Applying this interpretation to the jets as a population, we find that the jets would be aligned to within 30° of the line of sight generally, assuming that the bulk Lorentz factor of the jets is 10.

The WARPS Survey. II. The log N-log S Relation and the X-Ray Evolution of Low-Luminosity Clusters of Galaxies

The strong negative evolution observed in previous X-ray-selected surveys of clusters of galaxies is evidence in favor of hierarchical models of the growth of structure in the universe. A large recent survey has, however, contradicted the low-redshift results, finding no evidence for evolution at z 0.3) the cluster luminosities are in the range 4 × 1043-2 × 1044 h−250 ergs s-1, the luminosities of poor clusters. The number of high-redshift, low-luminosity clusters is consistent with no evolution of the X-ray luminosity function between redshifts of z ≈ 0.4 and z = 0, and it places a limit of a factor of less than 1.7 (at 90% confidence) on the amplitude of any pure negative density evolution of clusters of these luminosities, in contrast with the factor of ≈ 3 [corresponding to number density evolution ∝(1 + z)-2.5] found in the Einstein Extended Medium-Sensitivity Survey at similar redshifts but higher luminosities. Taken together, these results support hierarchical models in which there is mild negative evolution of the most luminous clusters at high redshift, but little or no evolution of the less luminous but more common optically poor clusters. Models involving preheating of the X-ray gas at an early epoch fit the observations, at least for Ω0 = 1.

THE X-RAY EMISSIONS FROM THE M87 JET: DIAGNOSTICS AND PHYSICAL INTERPRETATION

We reanalyze the deep Chandra observations of the M87 jet, first examined by Wilson & Yang in 2002. By employing an analysis chain that also includes image deconvolution, knots HST-1 and I are fully separated from adjacent emission. We derive the spatially resolved X-ray spectrum of the jet using the most recent response functions and find slight but significant variations in the spectral shape, with values of αX(Sν ∝ ν-α) ranging from ~1.2-1.4 (in the nucleus and knots HST-1, D, and C) to ~1.6 (in knots F, A, and B). We make use of VLA radio observations, as well as HST imaging and polarimetry data, to examine the jets broadband spectrum and inquire as to the nature of particle acceleration in the jet. As shown in previous papers, a simple continuous injection model for the synchrotron-emitting knots, in which one holds constant both the filling factor facc of the regions within which particles are accelerated and the energy spectrum of the injected particles, cannot account for the flux or spectrum of the X-ray emission. Instead, we propose that facc is a function of both position and energy and find that in the inner jet, facc ∝ E ∝ E, and in knots A and B, facc ∝ E ∝ E, where Eγ is the energy of the emitted photon and Ee is the energy of the emitting electron. In this model, the index p of the relativistic electron energy spectrum at injection [n(Ee) ∝ E] is p = 2.2 at all energies and all locations along the jet, in excellent agreement with the predictions of models of cosmic-ray acceleration by ultrarelativistic shocks (p = 2.23). There is a strong correlation between the peaks of X-ray emission and minima of optical percentage polarization, i.e., regions where the jet magnetic field is not ordered. We suggest that the X-ray peaks coincide with shock waves that accelerate the X-ray-emitting electrons and cause changes in the direction of the magnetic field; the polarization is thus small because of beam averaging.

The Deep X-Ray Radio Blazar Survey. III. Radio Number Counts, Evolutionary Properties, and Luminosity Function of Blazars

Our knowledge of the blazar surface densities and luminosity functions, which are fundamental parameters, relies still on samples at relatively high flux limits. As a result, our understanding of this rare class of active galactic nuclei is mostly based on relatively bright and intrinsically luminous sources. We present the radio number counts, evolutionary properties, and luminosity functions of the faintest blazar sample with basically complete (~95%) identifications. Based on the Deep X-Ray Radio Blazar Survey (DXRBS), it includes 129 flat-spectrum radio quasars (FSRQs) and 24 BL Lac objects down to a 5 GHz flux and power ~50 mJy and ~1024 W Hz-1, respectively, an order of magnitude improvement as compared to previously published (radio-selected) blazar samples. DXRBS FSRQs are seen to evolve strongly, up to redshift ≈1.5, above which high-power sources show a decline in their comoving space density. DXRBS BL Lac objects, on the other hand, do not evolve. High-energy and low-energy peaked BL Lac objects (HBLs and LBLs, respectively) share the same lack of cosmological evolution, which is at variance with some previous results. The observed luminosity functions are in good agreement with the predictions of unified schemes, with FSRQs getting close to their expected minimum power. Despite the fact that the large majority of our blazars are FSRQs, BL Lac objects are intrinsically ~50 times more numerous. Finally, the relative numbers of HBLs and LBLs in the radio and X-ray bands are different from those predicted by the so-called blazar sequence and support a scenario in which HBLs represent a small minority (≈10%) of all BL Lac objects.

What types of jets does nature make: A New population of radio quasars

We use statistical results from a large sample of about 500 blazars, based on two surveys, the Deep X-Ray Radio Blazar Survey (DXRBS), nearly complete, and the ROSAT All-Sky Survey-Green Bank Survey (RGB), to provide new constraints on the spectral energy distribution of blazars, particularly flat-spectrum radio quasars (FSRQs). This reassessment is prompted by the discovery of a population of FSRQs with spectral energy distribution similar to that of high-energy-peaked BL Lac objects. The fraction of these sources is sample dependent, being ~10% in DXRBS and ~30% in RGB (and reaching ~80% for the Einstein Medium Sensitivity Survey). We show that these X-ray-strong radio quasars, which had gone undetected or unnoticed in previous surveys, indeed are the strong-lined counterparts of high-energy-peaked BL Lac objects and have synchrotron peak frequencies, νpeak, much higher than classical FSRQs, typically in the UV band for DXRBS. Some of these objects may be 100 GeV to TeV emitters, as are several known BL Lac objects with similar broadband spectra. Our large, deep, and homogeneous DXRBS sample does not show anticorrelations between νpeak and radio, broad-line region, or jet power, as expected in the so-called blazar sequence scenario. However, the fact that FSRQs do not reach X-ray-to-radio flux ratios and νpeak values as extreme as BL Lac objects and the elusiveness of high-νpeak high-power blazars suggest that there might be an intrinsic, physical limit to the synchrotron peak frequency that can be reached by strong-lined, powerful blazars. Our findings have important implications for the study of jet formation and physics and its relationship to other properties of active galactic nuclei.

VARIABILITY TIMESCALES IN THE M87 JET: SIGNATURES OF E 2 LOSSES, DISCOVERY OF A QUASI PERIOD IN HST-1, AND THE SITE OF TeV FLARING

We investigate the variability timescales in the jet of M87 with two goals. The first is to use the rise times and decay times in the radio, ultraviolet, and X-ray light curves of HST-1 to constrain the source size and the energy loss mechanisms affecting the relativistic electron distributions. HST-1 is the first jet knot clearly resolved from the nuclear emission by Chandra and is the site of the huge flare of 2005. We find clear evidence for a frequency-dependent decrease in the synchrotron flux being consistent with E 2 energy losses. Assuming that this behavior is predominantly caused by synchrotron cooling, we estimate a value of 0.6 mG for the average magnetic field strength of the HST-1 emission region, a value consistent with previous estimates of the equipartition field. In the process of analyzing the first derivative of the X-ray light curve of HST-1, we discovered a quasi-periodic oscillation which was most obvious in 2003 and 2004 prior to the major flare in 2005. The four cycles observed have a period of order six months. The second goal is to search for evidence of differences between the X-ray variability timescales of HST-1 and the unresolved nuclear region (diameter <06). These features, separated by more than 60 pc, are the two chief contenders for the origin of the TeV variable emissions observed by H.E.S.S. in 2005 and by MAGIC and VERITAS in 2008. The X-ray variability of the nucleus appears to be at least twice as rapid as that of the HST-1 knot. However, the shortest nuclear variability timescale we can measure from the Chandra data (≤20 days) is still significantly longer than the shortest TeV variability of M87 reported by the H.E.S.S. and MAGIC telescopes (1-2 days).

The new BL Lacertae objects discovered by an efficient X-ray/radio/optical technique

The discovery of 13 serendipitous BL Lac objects in the Einstein IPC Slew Survey by means of x ray/radio vs. x ray/optical color-color diagrams and confirmation by optical spectroscopy are reported. These 13 BL Lacs were discovered using a technique which exploits the characteristic broad band spectra of BL Lacs. New VLA detections provide accurate fluxes (f(6 cm) is approximately 0.5 mJy) and 2 in. positions, facilitating the determination of an optical counterpart. All 13 new BL Lacs show essentially featureless optical spectra. Nine of these lie within the range of colors of known x ray selected BL Lacs. Of the remaining four, one is apparently x ray louder (by a factor of 1.5) or optically quieter (by 0.8 mags); and three are optically louder (by 1-1.3 mags) than x ray selected BL Lacs. Approximately 50 new BL Lacs in total are expected from VLA work and upcoming Australia Telescope observations, yielding a complete Slew Survey sample of approximately 90 BL Lacs.