The relative growth of Optical and Radio Quasars in SDSS (2009)
Shankar, Francesco, Sivakoff, Gregory R., Vestergaard, Marianne, Dai, Xinyu
We cross-correlate the SDSS DR3 quasar sample with FIRST and the Vestergaard et al. black hole (BH) mass sample to compare the mean accretion histories of optical and radio quasars. We find...
The Evolution of Active Galactic Nuclei in Clusters of Galaxies to Redshift 1.3 (2009)
Martini, Paul, Sivakoff, Gregory R., Mulchaey, John S.
We have measured the luminous AGN population in a large sample of clusters of galaxies and find evidence for a substantial increase in the cluster AGN population from z~0.05 to z~1.3. The present...
Luminosity functions of LMXBs in Centaurus A: globular clusters versus the field (2009)
Voss, Rasmus, Gilfanov, Marat, Sivakoff, Gregory R., Kraft, Ralph P., Jordan, Andres, Raychaudhury, Somak, ...
Sivakoff, Gregory R., Jordán, Andrés, Juett, Adrienne M., Sarazin, Craig L., Irwin, Jimmy A.
Chandra X-ray observations routinely resolve tens to hundreds of low-mass X-ray binaries (LMXBs) per galaxy in nearby massive early-type galaxies. These studies have raised important issues regarding...
Sivakoff, Gregory R., Jord'an, Andrés, Juett, Adrienne M., Sarazin, Craig L., Irwin, Jimmy A.
Multi-epoch Chandra X-ray observations of nearby massive early-type galaxies open up the study of an important regime of low-mass X-ray binary (LMXB) behavior -- long term variability. In a companion...
Wide-Field Chandra X-Ray Observations of AGN in Abell 85 & Abell 754 (2008)
Sivakoff, Gregory R., Martini, Paul, Zabludoff, Ann I., Kelson, Daniel D., Mulchaey, John S.
To better understand the mechanism or mechanisms that lead to AGN activity today, we measure the X-ray AGN fraction in a new sample of nearby clusters and examine how it varies with galaxy...
A Transient Black-Hole Low-Mass X-Ray Binary Candidate in Centaurus A (2008)
Sivakoff, Gregory R., Kraft, Ralph P., Jordán, Andrés, Juett, Adrienne M., Evans, Daniel A., Forman, William R., ...
We report the discovery of a bright transient X-ray source, CXOU J132518.2-430304, towards Centaurus A (Cen A) using six new Chandra X-Ray Observatory observations in 2007 March--May. Between 2003...
2 European Southern Observatory, Karl-Schwarzschild-Str. 2 (2008)
G. Fabbiano, Gregory R. Sivakoff, Andrés Jordán, Adrienne M. Juett, Craig L. Sarazin, ...
Abstract. Chandra observations of early-type galaxies have resolved large populations of lowmass X-ray binaries (LMXBs) in early-type galaxies. The majority of these observations have been snapshots...
Dependence of the BALQSO fraction on Radio Luminosity (2008)
Shankar, Francesco, Dai, Xinyu, Sivakoff, Gregory R.
We find that the fraction of classical Broad Absorption Line quasars (BALQSOs) among the FIRST radio sources in the Sloan Data Release 3, is 20.5^{+7.3}_{-5.9}% at the faintest radio powers detected...
Variable Low-Mass X-ray Binaries in Early-Type Galaxies (2007)
Sivakoff, Gregory R., Jordán, Andrés, Juett, Adrienne M., Sarazin, Craig L., Irwin, Jimmy A.
As the Chandra X-ray Observatory mission matures, increasing numbers of nearby galaxies are being observed multiple times, sampling the variability of extragalactic X-ray binaries on timescales...
Cosmic Structure Traced by Precision (2007)
Measurements Of The, Thomas H. Reiprich, Craig L. Sarazin, Joshua C. Kempner, Michael F, Gregory R. Sivakoff, ...
The current status of our efforts to trace cosmic structure with 10 6 galaxies (2MASS), 10 3 galaxy clusters (NORAS II cluster survey), and precision measurements for 10 galaxy clusters (HIFLUGCS) is...
Low-Mass X-ray Binaries and Globular Clusters in Centaurus A (2007)
Jordan, Andres, Sivakoff, Gregory R., McLaughlin, Dean E., Blakeslee, John P., Evans, Daniel A., Kraft, Ralph P., ...
We present results of Hubble Space Telescope and Chandra X-ray Observatory observations of globular clusters (GCs) and low-mass X-ray binaries (LMXBs) in the central regions of Centaurus A. Out of...
On the nature of the z=0 X-ray absorbers: I. Clues from an external group (2007)
Mathur, Smita, Sivakoff, Gregory R., Williams, Rik J., Nicastro, Fabrizio
Absorption lines of OVII at redshift zero are observed in high quality Chandra spectra of extragalactic sightlines. The location of the absorber producing these lines, whether from the corona of the...
A Photometric Survey for Variables and Transits in the Field of Praesepe with KELT (2007)
Pepper, Joshua, Stanek, K. Z., Pogge, Richard W., Latham, David W., DePoy, D. L., Siverd, Robert, ...
The Kilodegree Extremely Little Telescope (KELT) project is a small aperture, wide-angle search for planetary transits of solar-type stars. In this paper, we present the results of a commissioning...
2MASS Reveals a Large Intrinsic Fraction of BALQSOs (2007)
Dai, Xinyu, Shankar, Francesco, Sivakoff, Gregory R.
The intrinsic fraction of broad absorption line quasars (BALQSOs) is important in constraining geometric and evolutionary models of quasars. We present the fraction of BALQSOs in 2MASS detected...
Andrés Jordán, Gregory R. Sivakoff, Dean E. Mclaughlin, John P. Blakeslee, Daniel A. Evans, Ralph P. Kraft, ...
We present results of Hubble Space Telescope and Chandra X-Ray Observatory observations of globular clusters (GCs) and low-mass X-ray binaries (LMXBs) in the central regions of Centaurus A. Out of...
Sivakoff, Gregory R., Jordán, Andrés, Sarazin, Craig L., Blakeslee, John P., E}, Patrick C{\^ O}t{\', Ferrarese, Laura, ...
(Abridged) Chandra and Hubble ACS observations of 11 early-type galaxies probe the low-mass X-ray binary (LMXB) - globular cluster (GC) connection. We explore the optical properties of 270 GCs with...
Gregory R. Sivakoff, Andrés Jordán, Craig L. Sarazin, John P. Blakeslee, Patrick Côté, Laura Ferrarese, ...
LMXBs form efficiently in GCs. By combining Chandra and HST ACS observations of 11 massive early-type galaxies in the Virgo Cluster, we use the most accurate identification of LMXBs and GCs to date...
Fujita, Yutaka, Sarazin, Craig L., Sivakoff, Gregory R.
We present an analysis of Chandra observations of the galaxy clusters A2670 and A2107. Their cD galaxies have large peculiar velocities (>200km/s) and thus the clusters appear to be undergoing...
Jordan, Andres, Cote, Patrick, Blakeslee, John P., Ferrarese, Laura, McLaughlin, Dean E., Mei, Simona, ...
We have measured half-light radii, r_h, for globular clusters (GCs) belonging to the 100 early-type galaxies observed in the ACS Virgo Cluster Survey and the elliptical galaxy NGC 4697. An analysis...
The Galaxy Distribution Function from the 2MASS Survey (2005)
Sivakoff, Gregory R., Saslaw, William C.
We determine the spatial distribution function of galaxies from a wide range of samples in the 2MASS survey. The results agree very well with the form of the distribution predicted by the theory of...
Luminous X-ray Flares from Low Mass X-ray Binary Candidates in the Early-Type Galaxy NGC 4697 (2005)
Sivakoff, Gregory R., Sarazin, Craig L., Jordan, Andres
We report results of the first search specifically targeting short-timescale X-ray flares from low-mass X-ray binaries in an early-type galaxy. A new method for flare detection is presented. In NGC...
Sivakoff, Gregory R., Sarazin, Craig L., Carlin, Jeffrey L.
(Abridged) We observed the X-ray bright E3 galaxy NGC 1600 and nearby group members with the Chandra X-ray Observatory ACIS S3. Unresolved emission dominates the observation; however, we resolved...
Sivakoff, Gregory R., Sarazin, Craig L., Irwin, Jimmy A.
(Abridged) We used the Chandra X-ray Observatory ACIS S3 to image the X-ray faint elliptical galaxy NGC 4365 and lenticular galaxy NGC 4382. The observations resolve much of the X-ray emission into...
Low Mass X-ray Binaries and Globular Clusters in Early-Type Galaxies (2003)
Sarazin, Craig L., Kundu, Arunav, Irwin, Jimmy A., Sivakoff, Gregory R., Blanton, Elizabeth L., Randall, Scott W.
(Abridged) A high fraction of the Low Mass X-ray Binaries (LMXBs) in early-type galaxies are associated with globular clusters (GCs). Here, we discuss the correlations between LMXBs and GCs in a...
Reiprich, Thomas H., Sarazin, Craigh, Kempner, Joshua C., Skrutskie, Michael F., Sivakoff, Gregory R., Böhringer, Hans, ...
Reiprich, Thomas H., Sarazin, Craig L., Kempner, Joshua C., Skrutskie, Michael F., Sivakoff, Gregory R., Boehringer, Hans, ...
The current status of our efforts to trace cosmic structure with 10^6 galaxies (2MASS), 10^3 galaxy clusters (NORAS II cluster survey), and precision measurements for 10^2 galaxy clusters (HIFLUGCS)...