Search Publications
Coronal Heating and Photospheric Turbulence Parameters: Observational Aspects
Romano, P.; Abramenko, V. I.; Pevtsov, A. A.
In this study, the soft X-ray luminosity of the solar corona, measured by the Yohkoh spacecraft for 104 well-developed and decaying active regions, is compared to the magnetic field parameters determined from SOHO MDI high-resolution magnetograms. We calculate and compare (1) two area-independent characteristics of the magnetic field: the index (α…
Tracing the Dynamical History of the Globular Cluster 47 Tucanae
Sills, Alison; Beccari, Giacomo; Guhathakurta, Puragra +3 more
We use two stellar populations in the globular cluster 47 Tucanae to trace its dynamical history: blue stragglers and low-mass main-sequence stars. We assumed that the blue stragglers were formed through stellar collisions in all regions of the cluster. We find that in the core of the cluster, models of collisional blue stragglers agree well with …
On the Fraction of X-Ray-obscured Quasars in the Local Universe
Wang, J. X.; Jiang, P.
Recent wide-area hard X-ray and soft gamma-ray surveys have shown that the fraction of X-ray-obscured active galactic nuclei (AGNs) in the local universe significantly decreases with intrinsic luminosity. In this Letter we point out that two corrections have to be made to the samples: (1) radio-loud AGNs have to be excluded, since their X-ray emis…
Multiwavelength Studies of the Optically Dark Gamma-Ray Burst 001025A
Cline, T.; Hurley, K.; Schartel, N. +18 more
We identify the fading X-ray afterglow of GRB 001025A from XMM-Newton observations obtained 1.9-2.3 days, 2 yr, and 2.5 yr after the burst. The nondetection of an optical counterpart to an upper limit of R=25.5, 1.20 days after the burst, makes GRB 001025A a ``dark'' burst. Based on the X-ray afterglow spectral properties of GRB 001025A, we argue …
On the Nature of the Hard X-Ray Source IGR J2018+4043
Kennea, J. A.; Terrier, R.; Rauw, G. +13 more
We found a very likely counterpart to the recently discovered hard X-ray source IGR J2018+4043 in the multiwavelength observations of the source field. The source, originally discovered in the 20-40 keV band, is now confidently detected also in the 40-80 keV band, with a flux of (1.4+/-0.4)×10-11 ergs cm-2 s-1. A 5…
A Blue Straggler Binary with Three Progenitors in the Core of a Globular Cluster?
Knigge, C.; Long, K. S.; Gilliland, R. L. +3 more
We show that the X-ray source W31 in the core of the globular cluster 47 Tucanae is physically associated with the bright blue straggler BSS 7. The two sources are astrometrically matched to 0.061", with a chance coincidence probability of less than 1%. We then analyze optical time-series photometry obtained with the Hubble Space Telescope (HST) a…
Evolution of the X-Ray Jets from 4U 1755-33
Wijnands, R.; Tomsick, J. A.; Kaaret, P. +4 more
We report on new X-ray observations of the large-scale jets recently discovered in X-rays from the black hole candidate 4U 1755-33. Our observations in 2004 show that the jets found in 2001 are still present in X-rays. However, sensitive radio observations in 2004 failed to detect the jets. We suggest that synchrotron radiation is a viable emissio…
Multiwavelength Monitoring of the Dwarf Seyfert 1 Galaxy NGC 4395. II. X-Ray and Ultraviolet Continuum Variability
O'Neill, Paul M.; Kaspi, Shai; Laor, Ari +7 more
We report on two Chandra observations, and a simultaneous Hubble Space Telescope ultraviolet observation, of the dwarf Seyfert 1 galaxy NGC 4395. Each Chandra observation had a duration of ~30 ks, with a separation of ~50 ks. The spectrum was observed to harden between these observations via a scaling down of the soft-band flux. The interobservati…
The Warped Nuclear Disk of Radio Galaxy 3C 449
Baum, Stefi A.; O'Dea, Christopher P.; Sparks, William B. +11 more
Among radio galaxies containing nuclear dust disks, the bipolar jet axis is generally observed to be perpendicular to the disk major axis. The FR I radio source 3C 449 is an outlier to this statistical majority, as it possesses a nearly parallel jet/disk orientation on the sky. We examine the 600 pc dusty disk in this galaxy with images from the H…
X-Ray Number Counts of Normal Galaxies
Zezas, A.; Georgakakis, A.; Georgantopoulos, I. +2 more
We use the number counts of X-ray-selected normal galaxies to explore their evolution by combining the most recent wide-angle shallow and pencil-beam deep samples available. The differential X-ray number counts, dN/dS, for early- and late-type normal galaxies are constructed separately and then compared with the predictions of the local X-ray lumi…