The VLA Low-frequency Sky Survey

The Very Large Array (VLA) Low-frequency Sky Survey (VLSS) has imaged 95% of the 3*pi sr of sky north of declination = -30 degrees at a frequency of 74 MHz (4 meter wavelength). The resolution is 80" (FWHM) throughout, and the typical RMS noise level is ~0.1 Jy/beam. The typical point-source detection limit is 0.7 Jy/beam and so far nearly 70,000 sources have been catalogued. This survey used the 74 MHz system added to the VLA in 1998. It required new imaging algorithms to remove the large ionospheric distortions at this very low frequency throughout the entire ~11.9 degree field of view. This paper describes the observation and data reduction methods used for the VLSS and presents the survey images and source catalog. All of the calibrated images and the source catalog are available online (http://lwa.nrl.navy.mil/VLSS) for use by the astronomical community.Comment: 53 pages, including 3 tables and 15 figures. Has been accepted for publication in the Astronomical Journa

GraphSE2^2: An Encrypted Graph Database for Privacy-Preserving Social Search

In this paper, we propose GraphSE$^2$, an encrypted graph database for online social network services to address massive data breaches. GraphSE$^2$ preserves the functionality of social search, a key enabler for quality social network services, where social search queries are conducted on a large-scale social graph and meanwhile perform set and computational operations on user-generated contents. To enable efficient privacy-preserving social search, GraphSE$^2$ provides an encrypted structural data model to facilitate parallel and encrypted graph data access. It is also designed to decompose complex social search queries into atomic operations and realise them via interchangeable protocols in a fast and scalable manner. We build GraphSE$^2$ with various queries supported in the Facebook graph search engine and implement a full-fledged prototype. Extensive evaluations on Azure Cloud demonstrate that GraphSE$^2$ is practical for querying a social graph with a million of users.Comment: This is the full version of our AsiaCCS paper "GraphSE$^2$: An Encrypted Graph Database for Privacy-Preserving Social Search". It includes the security proof of the proposed scheme. If you want to cite our work, please cite the conference version of i

The Origin of Radio Emission in Low-Luminosity Active Galactic Nuclei: Jets, Accretion Flows, or Both?

The low-luminosity active galactic nuclei in NGC 3147, NGC 4203, and NGC 4579 have been imaged at four frequencies with the Very Long Baseline Array. The galaxies are unresolved at all frequencies, with size upper limits of $10^3-10^4$ times the Schwarzschild radii of their central massive black holes. The spectral indices between 1.7 and 5.0 GHz range from 0.2 to 0.4; one and possibly two of the galaxies show spectral turnovers between 5.0 and 8.4 GHz. The high brightness temperatures ($> 10^9$ K) and relatively straight spectra imply that free-free emission and/or absorption cannot account for the slightly inverted spectra. Although the radio properties of the cores superficially resemble predictions for advection-dominated accretion flows, the radio luminosities are too high compared to the X-ray luminosities. We suggest that the bulk of the radio emission is generated by a compact radio jet, which may coexist with a low radiative efficiency accretion flow.Comment: To appear in ApJ (Letters). 4 page

Scaling Laws for Advection Dominated Flows: Applications to Low Luminosity Galactic Nuclei

We present analytical scaling laws for self-similar advection dominated flows. The spectra from these systems range from 10$^{8}$ - 10$^{20}$ Hz, and are determined by considering cooling of electrons through synchrotron, bremsstrahlung, and Compton processes. We show that the spectra can be quite accurately reproduced without detailed numerical calculations, and that there is a strong testable correlation between the radio and X-ray fluxes from these systems. We describe how different regions of the spectrum scale with the mass of the accreting black hole, $M$, the accretion rate of the gas, $\dot{M}$, and the equilibrium temperature of the electrons, $T_e$. We show that the universal radio spectral index of 1/3 observed in most elliptical galaxies (Slee et al. 1994) is a natural consequence of self-absorbed synchrotron radiation from these flows. We also give expressions for the total luminosity of these flows, and the critical accretion rate, $\dot{M}_{crit}$, above which the advection solutions cease to exist. We find that for most cases of interest the equilibrium electron temperature is fairly insensitive to $M$, $\dot{M}$, and parameters in the model. We apply these results to low luminosity black holes in galactic nuclei. We show that the problem posed by Fabian & Canizares (1988) of whether bright elliptical galaxies host dead quasars is resolved, as pointed out recently by Fabian & Rees (1995), by considering advection-dominated flows.Comment: 30 pages, 5 postscript files. Accepted to ApJ. Also available http://cfa-www.harvard.edu/~rohan/publications.htm

Chandra Observations of the Disruption of the Cool Core in Abell 133

We present the analysis of a Chandra observation of the galaxy cluster Abell 133, which has a cooling flow core, a central radio source, and a diffuse, filamentary radio source which has been classified as a radio relic. The X-ray image shows that the core has a complex structure. The most prominent feature is a "tongue" of emission which extends from the central cD galaxy to the northwest and partly overlaps the radio relic. One possibility is that this tongue is produced by Kelvin-Helmholtz (KH) instabilities through the interaction between the cold gas around the cD galaxy and hot intracluster medium. We estimate the critical velocity and time scale for the KH instability to be effective for the cold core around the cD galaxy. We find that the KH instability can disrupt the cold core if the relative velocity is >~400 km s^-1. We compare the results with those of clusters in which sharp, undisrupted cold fronts have been observed; in these clusters, the low temperature gas in their central regions has a more regular distribution. In contrast to Abell 133, these cluster cores have longer timescales for the disruption of the core by the KH instability when they are normalized to the timescale of the cD galaxy motion. Thus, the other cores are less vulnerable to KH instability. Another possible origin of the tongue is that it is gas which has been uplifted by a buoyant bubble of nonthermal plasma that we identify with the observed radio relic. From the position of the bubble and the radio estimate of the age of the relic source, we estimate avelocity of ~700 km s^-1 for the bubble. The structure of the bubble and this velocity are consistent with numerical models for such buoyant bubbles. (abridged)Comment: 38 pages, 15 figures, accepted for publication in Ap

Particle Energies and Filling Fractions of Radio Bubbles in Cluster Cores

Using Chandra images of cluster cores with clear radio bubbles, we have determined k, which is the ratio of the total particle energy to that of the electrons radiating between 10 MHz and 10 GHz. Radiative and dynamical constraints on the bubbles indicate that the ratio of the energy factor, k, to the volume filling factor, f, lies within the range 1 < k/f < 1000. Assuming pressure equilibrium between the radio-emitting plasma and the surrounding X-ray gas, none of the lobes have equipartition between relativistic particles and magnetic field. There is no evidence for any dependence of the upper limit of the k/f ratio on any physical parameter of the cluster or the radio source. The distribution of the upper limit on k/f appears to be bimodal, the value for some clusters being ~3 and for the others ~300. We show that this is may due to the composition of the jet which forms the bubbles, the variation in the volume filling fraction or variation in the amount of re-acceleration occurring in the bubble.Comment: 12 pages, 9 figures; accepted for publication in MNRA

A Chandra Observation of Abell 13: Investigating the Origin of the Radio Relic

We present results from the Chandra X-ray observation of Abell 13, a galaxy cluster that contains an unusual noncentral radio source, also known as a radio relic. This is the first pointed X-ray observation of Abell 13, providing a more sensitive study of the properties of the X-ray gas. The X-ray emission from Abell 13 is extended to the northwest of the X-ray peak and shows substructure indicative of a recent merger event. The cluster X-ray emission is centered on the bright galaxy H of Slee et al. 2001. We find no evidence for a cooling flow in the cluster. A knot of excess X-ray emission is coincident with the other bright elliptical galaxy F. This knot of emission has properties similar to the enhanced emission associated with the large galaxies in the Coma cluster. With these Chandra data we are able to compare the properties of the hot X-ray gas with those of the radio relic from VLA data, to study the interaction of the X-ray gas with the radio emitting electrons. Our results suggest that the radio relic is associated with cooler gas in the cluster. We suggest two explanations for the coincidence of the cooler gas and radio source. First, the gas may have been uplifted by the radio relic from the cluster core. Alternatively, the relic and cool gas may have been displaced from the central galaxy during the cluster merger event.Comment: 11 pages, 9 figures, Accepted for Publication in the Astrophysical Journal, higher-resolution figures can be found at http://www.astro.virginia.edu/~amj3r/Abell13

Quality of Publicly Available Physical Activity Apps: Review and Content Analysis

BACKGROUND: Within the new digital health landscape, the rise of health apps creates novel prospects for health promotion. The market is saturated with apps that aim to increase physical activity (PA). Despite the wide distribution and popularity of PA apps, there are limited data on their effectiveness, user experience, and safety of personal data. OBJECTIVE: The purpose of this review and content analysis was to evaluate the quality of the most popular PA apps on the market using health care quality indicators. METHODS: The top-ranked 400 free and paid apps from iTunes and Google Play stores were screened. Apps were included if the primary behavior targeted was PA, targeted users were adults, and the apps had stand-alone functionality. The apps were downloaded on mobile phones and assessed by 2 reviewers against the following quality assessment criteria: (1) users' data privacy and security, (2) presence of behavior change techniques (BCTs) and quality of the development and evaluation processes, and (3) user ratings and usability. RESULTS: Out of 400 apps, 156 met the inclusion criteria, of which 65 apps were randomly selected to be downloaded and assessed. Almost 30% apps (19/65) did not have privacy policy. Every app contained at least one BCT, with an average number of 7 and a maximum of 13 BCTs. All but one app had commercial affiliation, 12 consulted an expert, and none reported involving users in the app development. Only 12 of 65 apps had a peer-reviewed study connected to the app. User ratings were high, with only a quarter of the ratings falling below 4 stars. The median usability score was excellent-86.3 out of 100. CONCLUSIONS: Despite the popularity of PA apps available on the commercial market, there were substantial shortcomings in the areas of data safety and likelihood of effectiveness of the apps assessed. The limited quality of the apps may represent a missed opportunity for PA promotion

High energy protons from PKS 1333-33

In this letter we give an account of the possible acceleration of protons in the outer radio lobes of the active galaxy PKS 1333-33. We also make estimates of the arrival energy spectrum.Comment: 3 pages revtex, two figures, to appear in Mod. Phys. Lett.