Gigahertz-Peaked Spectrum Radio Sources in Nearby Galaxies

There is now strong evidence that many low-luminosity AGNs (LLAGNs) contain accreting massive black holes and that the nuclear radio emission is dominated by parsec-scale jets launched by these black holes. Here, we present preliminary results on the 1.4 GHz to 667 GHz spectral shape of a well-defined sample of 16 LLAGNs. The LLAGNs have a falling spectrum at high GHz frequencies. Several also show a low-frequency turnover with a peak in the 1-20 GHz range. The results provide further support for jet dominance of the core radio emission. The LLAGNs show intriguing similarities with gigahertz-peaked spectrum (GPS) sources.Comment: 6 pages, to appear in ASP Conference series, 2002, Vol. 25

Radio Cores in Low-Luminosity AGN: ADAFs or Jets?

We have surveyed two large samples of nearby low-luminosity AGN with the VLA to search for flat-spectrum radio cores, similar to Sgr A* in the Galactic Center. Roughly one third of all galaxies are detected (roughly one half if HII transition objects are excluded from the sample), many of which have compact radio cores. Follow-up observations with the VLBA have confirmed that these cores are non-thermal in origin, with lower limits for the brightness temperatures around ~10^8 K. The brightest of these are resolved into linear structures. The radio spectral indices of the cores are quite flat (alpha~0), with no evidence for the highly inverted radio cores predicted in the ADAF model. Spectrum and morphology of the compact radio emission is typical for radio jets seen also in more luminous AGN. The emission-line luminosity seems to be correlated with the radio core flux. Together with the VLBI observations this suggests that optical and radio emission in at least half the low-luminosity Seyferts and LINERs are black hole powered. We find only a weak correlation between bulge luminosity and radio flux and an apparently different efficiency between elliptical and spiral galaxies for producing radio emission at a given optical luminosity.Comment: 5 pages, 2 figures, (ESO) LaTex, to appear in ``Black Holes in Binaries and Galactic Nuclei'', ESO workshop, eds. L. Kaper, E.P.J. van den Heuvel, P.A. Woudt, Springer Verlag; also available at http://www.mpifr-bonn.mpg.de/staff/hfalcke/publications.html#eso9

Binary black hole merger in the extreme-mass-ratio limit: a multipolar analysis

Building up on previous work, we present a new calculation of the gravitational wave (GW) emission generated during the transition from quasi-circular inspiral to plunge, merger and ringdown by a binary system of nonspinning black holes, of masses $m_1$ and $m_2$, in the extreme mass ratio limit, $m_1 m_2\ll(m_1+m_2)^2$. The relative dynamics of the system is computed {\it without making any adiabatic approximation} by using an effective one body (EOB) description, namely by representing the binary by an effective particle of mass $\mu=m_1 m_2/(m_1+m_2)$ moving in a (quasi-)Schwarzschild background of mass $M=m_1+m_2$ and submitted to an \O(\nu) 5PN-resummed analytical radiation reaction force, with $\nu=\mu/M$. The gravitational wave emission is calculated via a multipolar Regge-Wheeler-Zerilli type perturbative approach (valid in the limit $\nu\ll 1$). We consider three mass ratios, $\nu={10^{-2},10^{-3},10^{-4}}$,and we compute the multipolar waveform up to $\ell=8$. We estimate energy and angular momentum losses during the quasi-universal and quasi-geodesic part of the plunge phase and we analyze the structure of the ringdown. We calculate the gravitational recoil, or "kick", imparted to the merger remnant by the gravitational wave emission and we emphasize the importance of higher multipoles to get a final value of the recoil $v/(c\nu^2)=0.0446$. We finally show that there is an {\it excellent fractional agreement} ($\sim 10^{-3}$) (even during the plunge) between the 5PN EOB analytically-resummed radiation reaction flux and the numerically computed gravitational wave angular momentum flux. This is a further confirmation of the aptitude of the EOB formalism to accurately model extreme-mass-ratio inspirals, as needed for the future space-based LISA gravitational wave detector.Comment: 20 pages, 12 figures. Version published in Phys. Rev.

A Chandra X-Ray Survey of Ultraluminous Infrared Galaxies

We present results from Chandra observations of 14 ultraluminous infrared galaxies (ULIRGs; log(L_IR/L_Sun) >= 12) with redshifts between 0.04 and 0.16. The goals of the observations were to investigate any correlation between infrared color or luminosity and the properties of the X-ray emission and to attempt to determine whether these objects are powered by starbursts or active galactic nuclei (AGNs). The sample contains approximately the same number of high and low luminosity objects and ``warm'' and ``cool'' ULIRGs. All 14 galaxies were detected by Chandra. Our analysis shows that the X-ray emission of the two Seyfert 1 galaxies in our sample are dominated by AGN. The remaining 12 sources are too faint for conventional spectral fitting to be applicable. Hardness ratios were used to estimate the spectral properties of these faint sources. The photon indices for our sample plus the Chandra-observed sample from Ptak et al.(2003) peak in the range of 1.0-1.5, consistent with expectations for X-ray binaries in a starburst, an absorbed AGN, or hot bremsstrahlung from a starburst or AGN. The values of photon index for the objects in our sample classified as Seyferts (type 1 or 2) are larger than 2, while those classified as HII regions or LINERs tend to be less than 2. The hard X-ray to far-infrared ratios for the 12 weak sources are similar to those of starbursts, but we cannot rule out the possibility of absorbed, possibly Compton-thick, AGNs in some of these objects. Two of these faint sources were found to have X-ray counterparts to their double optical and infrared nuclei.Comment: 40 pages, 5 tables, 14 figures, accepted by Ap