31 research outputs found
Identification of Faint Chandra X-ray Sources in the Core-Collapsed Globular Cluster NGC 6752
We have searched for optical identifications for 39 Chandra X-ray sources
that lie within the 1.9 arcmin half-mass radius of the nearby (d = 4.0 kpc),
core-collapsed globular cluster, NGC 6752, using deep Hubble Space Telescope
ACS/WFC imaging in B435, R625, and H alpha. Photometry of these images allows
us to classify candidate counterparts based primarily on color-magnitude and
color-color diagram location. The color-color diagram is particularly useful
for quantifying the H alpha line equivalent width. In addition to recovering 11
previously detected optical counterparts, we propose 20 new optical IDs. In
total, there are 16 likely or less certain cataclysmic variables (CVs), nine
likely or less certain chromospherically active binaries, three galaxies, and
three active galactic nuclei (AGNs). The latter three sources, which had been
identified as likely CVs by previous investigations, now appear to be
extragalactic objects based on their proper motions. As we previously found for
NGC 6397, the CV candidates in NGC 6752 fall into a bright group that is
centrally concentrated relative to the turnoff-mass stars and a faint group
that has a spatial distribution that is more similar to that of the
turnoff-mass stars. This is consistent with an evolutionary scenario in which
CVs are produced by dynamical interactions near the cluster center and diffuse
to larger radius orbits as they age.Comment: 26 pages, 18 figure
Initial Optical Results for the ChaMPlane Survey
We provide a brief description of the optical survey being conducted under
the NOAO Long Term Surveys program in support of the Chandra Multiwavelength
Plane (ChaMPlane) Survey (see paper by Grindlay et al. in this Volume). A
representative photometry result is shown, along with spectroscopic followup.Comment: 1 page, 2 figures (in 3 files). Astronomische Nachrichten, in press
(Feb 2003). Proceedings of "X-ray Surveys, in the Light of New
Observatories", 4-6 September, Santander, Spai
Chandra X-ray Sources in the Collapsed-Core Globular Cluster M30 (NGC 7099)
We report the detection of six discrete, low-luminosity (Lx < 10^33 erg/s)
X-ray sources, located within 12 arcsec of the center of the collapsed-core
globular cluster M30 (NGC 7099), and a total of 13 sources within the half-mass
radius, from a 50 ksec Chandra ACIS-S exposure. Three sources lie within the
very small upper limit of 1.9 arcsec on the core radius. The brightest of the
three core sources has a luminosity of Lx (0.5-6 keV) = 6x10^32 erg/s and a
blackbody-like soft X-ray spectrum, which are both consistent with it being a
quiescent low-mass X-ray binary (qLMXB). We have identified optical
counterparts to four of the six central sources and a number of the outlying
sources, using deep Hubble Space Telescope and ground-based imaging. While the
two proposed counterparts that lie within the core may represent chance
superpositions, the two identified central sources that lie outside of the core
have X-ray and optical properties consistent with being CVs. Two additional
sources outside of the core have possible active binary counterparts. We
discuss the X-ray source population of M30 in light of its collapsed-core
status.Comment: 18 pages, 13 figures (8 color), resubmitted to ApJ after
incorporating referee comment
Chandra Multi-wavelength Plane (ChaMPlane) Survey: Design and Initial Results
The Chandra Multiwavength Plane (ChaMPlane) Survey of the galactic plane
incorporates serendipitous sources from selected Chandra pointings in or near
the galactic plane (b 20 ksec; lack of bright diffuse or point
sources) to measure or constrain the luminosity function of low-luminosity
accretion sources in the Galaxy. The primary goal is to detect and identify
accreting white dwarfs (cataclysmic variables, with space density still
uncertain by a factor of >10-100), neutron stars and black holes (quiescent low
mass X-ray binaries) to constrain their space densities and thus origin and
evolution. Secondary objectives are to identify Be stars in high mass X-ray
binaries and constrain their space densities, and to survey the H-R diagram for
stellar coronal sources. A parallel optical imaging under the NOAO Long Term
Survey program provides deep optical images using the Mosaic imager on the CTIO
and KPNO 4-m telescopes. The 36arcmin X 36arcmin optical images (Halpha, R, V
and I) cover ~5X the area of each enclosed Chandra ACIS FOV, providing an
extended survey of emission line objects for comparison with Chandra.
Spectroscopic followup of optical counterparts is then conducted, thus far with
WIYN and Magellan. The X-ray preliminary results from both the Chandra and
optical surveys will be presented, including logN-logS vs. galactic position
(l,b) and optical idenifications.Comment: 4 pages, 7 figures (in 8 files), Astronomishe Nachrichten, in press
(Feb 2003). Proceedings of "X-ray Surveys, in the Light of New
Observatories", 4-6 September, Santander, Spain. Higher resolution figures
available at:
http://hea-www.harvard.edu/ChaMPlane/papers/champlane-santander.pd
Stellar Variability in the Globular Cluster Terzan 5
We present the results of a search for variability in and near the core of
the metal-rich, obscured globular cluster Terzan 5, using NICMOS on HST. This
extreme cluster has approximately solar metallicity and a central density that
places it in the upper few percent of all clusters. It is estimated to have the
highest interaction rate of any galactic globular cluster. The large extinction
towards Terzan 5 and the severe stellar crowding near the cluster center
present a substantial observational challenge. Using time series analysis we
discovered two variable stars in this cluster. The first is a RRab Lyrae
variable with a period of ~0.61 days, a longer period than that of field stars
with similar high metallicities. This period is, however, shorter than the
average periods of RR Lyraes found in the metal-rich globular clusters NGC
6441, NGC 6388 and 47 Tuc. The second variable is a blue star with a 7-hour
period sinusoidal variation and a likely orbital period of 14 hours. This star
is probably an eclipsing blue straggler, or (less likely) the infrared
counterpart to the low mass X-ray binary known in Terzan 5. Due to the extreme
crowding and overlapping Airy profile of the IR PSF, we fall short of our
original goal of detecting CVs via Palpha emission and detecting variable
infrared emission from the location of the binary MSP in Terzan 5.Comment: 17 pages including 12 figures. Accepted for publication in Ap
Exotica in the Globular Cluster M4, Studied with Chandra, HST, and the VLA
Using the Hubble Ultraviolet Globular Cluster Survey (HUGS) and additional
HST archival data, we have carried out a search for optical counterparts to the
low-luminosity Chandra X-ray sources in the globular cluster M4 (NGC 6121). We
have also searched for optical or X-ray counterparts to radio sources detected
by the VLA. We find 24 new confident optical counterparts to Chandra sources
for a total of 40, including the 16 previously identified. Of the 24 new
identifications, 18 are stellar coronal X-ray sources (active binaries, ABs),
the majority located along the binary sequence in a V-I colour-magnitude
diagram and generally showing an H-alpha excess. In addition to confirming the
previously detected cataclysmic variable (CV, CX4), we identify one confident
new CV (CX76), and two candidates (CX81 and CX101). One MSP is known in M4
(CX12), and another strong candidate has been suggested (CX1); we identify some
possible MSP candidates among optical and radio sources, such as VLA20, which
appears to have a white dwarf counterpart. One X-ray source with a sub-subgiant
optical counterpart and a flat radio spectrum (CX8, VLA31) is particularly
mysterious. The radial distribution of X-ray sources suggests a relaxed
population of average mass ~ 1.2 - 1.5 Msun. Comparing the numbers of ABs,
MSPs, and CVs in M4 with other clusters indicates that AB numbers are
proportional to cluster mass (primordial population), MSPs to stellar encounter
rate (dynamically formed population), while CVs seem to be produced both
primordially and dynamically.Comment: 30 pages, 12 figures, 2 pages of supplementary material containing
finding chart
Cataclysmic Variables and a New Class of Faint UV Stars in the Globular Cluster NGC 6397
We present evidence that the globular cluster NGC 6397 contains two distinct
classes of centrally-concentrated UV-bright stars. Color-magnitude diagrams
constructed from U, B, V, and I data obtained with the HST/WFPC2 reveal seven
UV-bright stars fainter than the main-sequence turnoff, three of which had
previously been identified as cataclysmic variables (CVs). Lightcurves of these
stars show the characteristic ``flicker'' of CVs, as well as longer-term
variability. A fourth star is identified as a CV candidate on the basis of its
variability and UV excess. Three additional UV-bright stars show no photometric
variability and have broad-band colors characteristic of B stars. These
non-flickering UV stars are too faint to be extended horizontal branch stars.
We suggest that they could be low-mass helium white dwarfs, formed when the
evolution of a red giant is interrupted, due either to Roche-lobe overflow onto
a binary companion, or to envelope ejection following a common-envelope phase
in a tidal-capture binary. Alternatively, they could be very-low-mass
core-He-burning stars. Both the CVs and the new class of faint UV stars are
strongly concentrated toward the cluster center, to the extent that mass
segregation from 2-body relaxation alone may be unable to explain their
distribution.Comment: 11 pages plus 3 eps figures; LaTeX using aaspp4.sty; to appear in The
Astrophysical Journal Letter