418 research outputs found
CC Bootis: QSO, Not Variable Halo Giant
The poorly-studied, faint (18<m_pg<19.5) variable star CC Bootis has been
noted in the literature as a candidate for a halo red giant. It proves instead
to be a quasi-stellar object of redshift z=0.172, and is detected as an X-ray
source by ROSAT. In addition to its odd heritage, CC Boo exhibits unusually
high amplitude optical variability for an optically-selected QSO.Comment: 6 pages including 1 table and 2 figures; Accepted for publication in
Pub. Astr. Soc. Pacific, Vol 109, June 199
Complex Velocity Fields in the Shell of T Pyxidis
We present spatially-resolved, moderate-resolution spectrophotometry of the
recurrent nova T Pyx and a portion of the surrounding shell. The spectrum
extracted from a strip of width 10'' centered on the star shows well-known,
strong emission lines typical of old novae, plus a prominent, unfamiliar
emission line at 6590 Angstroms. This line, and a weaker companion at 6540
Angstroms which we also detect, have been previously reported by Shahbaz et
al., and attributed to Doppler-shifted H alpha emission from a collimated jet
emerging from T Pyx. We demonstrate that these lines are instead due to [NII]
6548, 6584 from a complex velocity field in the surrounding nebula. The
comments of past workers concerning the great strength of HeII 4686 in T Pyx
itself are also reiterated.Comment: 8 pages including 2 figures; Accepted for publication in The
Astrophysical Journal Letter
An Ultraviolet-Excess Optical Candidate for the Luminous Globular Cluster X-ray Source in NGC1851
The intense, bursting X-ray source in the globular cluster NGC 1851 was one
of the first cluster sources discovered, but has remained optically
unidentified for 25 years. We report here on results from Hubble Space
Telescope WFPC2 multicolor images in NGC 1851. Our high spatial resolution
images resolve ~200 objects in the 3'' radius Einstein X-ray error circle, 40
times as many as in previous ground-based work. A color-magnitude diagram of
the cluster clearly reveals a markedly UV-excess object with B~21, (U-B) ~ -0.9
only 2'' from the X-ray position. The UV-excess candidate is 0.12'' distant
from a second, unremarkable star that is 0.5 mag brighter in B; thus
ground-based studies of this field are probably impractical. Three other
UV-excess objects are also present among the ~16,000 objects in the surveyed
region of the cluster, leaving a ~5% probability that a UV-excess object has
fallen in the X-ray error circle by chance. No variability of the candidate is
seen in these data, although a more complete study is required. If this object
is in fact the counterpart of the X-ray source, previous inferences that some
globular cluster X-ray sources are optically subluminous with respect to
low-mass X-ray binaries in the field are now strengthened.Comment: 13 pages including 1 table and 3 figures in AASTeX 4.0; To appear in
The Astrophysical Journal Letters, volume 472 (1996 December 1). Preprint
with full-resolution figures available at
http://www.astro.washington.edu/deutsch/pubs/pubs.htm
Empirical Uncertainty Estimators for Astrometry from Digital Databases
In order to understand the positional uncertainties of arbitrary objects in
several of the current major databases containing astrometric information, a
sample of extragalactic radio sources with precise positions in the
International Celestial Reference Frame (ICRF) is compared with the available
positions of their optical counterparts. The discrepancies between the radio
and various optical positions are used to derive empirical uncertainty
estimators for the USNO-A2.0, USNO-A1.0, Guide Star Selection System (GSSS)
images, and the first and second Digitized Sky Surveys (DSS-I and DSS-II). In
addition, an estimate of the uncertainty when the USNO-A2.0 catalog is
transferred to different image data is provided. These optical astrometric
frame uncertainties can in some cases be the dominant error term when
cross-identifying sources at different wavelengths.Comment: 12 pages including 2 figures and 1 table. Accepted for publication in
The Astronomical Journal, October 1999. Values in Table 1 for DSS I corrected
99-07-1
Time-Resolved Ultraviolet Observations of the Globular Cluster X-ray Source in NGC 6624: The Shortest Known Period Binary System
Using the Faint Object Spectrograph (FOS) aboard the Hubble Space Telescope,
we have obtained the first time-resolved spectra of the King et al.
ultraviolet-bright counterpart to the 11-minute binary X-ray source in the core
of the globular cluster NGC 6624. This object cannot be readily observed in the
visible, even from HST, due to a much brighter star superposed <0.1'' distant.
Our FOS data show a highly statistically significant UV flux modulation with a
period of 11.46+-0.04 min, very similar to the 685 sec period of the known
X-ray modulation, definitively confirming the association between the King et
al. UV counterpart and the intense X-ray source. The UV amplitude is very large
compared with the observed X-ray oscillations: X-ray variations are generally
reported as 2-3% peak-to-peak, whereas our data show an amplitude of about 16%
in the 126-251 nm range. A model for the system by Arons & King predicts
periodic UV fluctuations in this shortest-known period binary system, due to
the cyclically changing aspect of the X-ray heated face of the secondary star
(perhaps a very low mass helium degenerate). However, prior to our
observations, this predicted modulation has not been detected. Employing the
Arons & King formalism, which invokes a number of different physical
assumptions, we infer a system orbital inclination 35deg<i<50 deg. Amongst the
three best-studied UV/optical counterparts to the intense globular cluster
X-ray sources, two are now thought to consist of exotic double-degenerate
ultrashort period binary systems.Comment: 10 pages including 2 figures in Latex (AASTeX 4.0). Accepted for
publication in vol. 482 (1997 June 10 issue) of The Astrophysical Journal
(Letters
Innate immune receptor NOD2 mediates LGR5+ intestinal stem cell protection against ROS cytotoxicity via mitophagy stimulation
International audienceThe nucleotide-binding oligomerization domain-containing protein 2 (NOD2) agonist muramyl dipeptide (MDP), a peptidoglycan motif common to all bacteria, supports leucine-rich repeat-containing G protein-coupled receptor 5 (LGR5)+ intestinal stem cell (ISC) survival through NOD2 activation upon an otherwise lethal oxidative stress-mediated signal. However, the underlying protective mechanisms remain unknown. Here, using irradiation as stressor and primarily murine-derived intestinal organoids as a model system, we show that MDP induced a significant reduction of total and mitochondrial reactive oxygen species (ROS) within ISCs, which was associated with mitophagy induction. ATG16L1 knockout (KO) and NOD2 KO organoids did not benefit from the MDP-induced cytoprotection. We confirmed the MDP-dependent induction of ISC mitophagy upon stress in vivo. These findings elucidate the NOD2-mediated mechanism of cytoprotection involving the clearance of the lethal excess of ROS molecules through mitophagy, triggered by the coordinated activation of NOD2 and ATG16L1 by a nuclear factor ÎşB (NF-ÎşB)-independent pathway
A Search for the Optical Counterpart of the Luminous X-ray Source in NGC 6652
We examine images of the field of X1832-330, the luminous (Lx ~ 10^36 erg/s)
X-ray burst source near the center of the globular cluster NGC 6652, in order
to identify the optical counterpart for further study. U and B ground-based
images allow us to set a limit M_B > 3.5 for the counterpart at the time of
those observations, provided that the color is (U-B)_0 ~ -1, similar to the
sources known in other clusters. Archival Hubble Space Telescope observations
survey most but not all of the 1 sigma X-ray error circle, and allow us to set
limits M_B > 5.9 and M_B > 5.2 in the WF/PC and WFPC2 regions, respectively. In
the WF/PC images we do weakly detect a faint object with UV-excess, but it is
located 11.7'' from the ROSAT X-ray position. This considerable (2.3 sigma)
discrepancy in position suggests that this candidate be treated with caution,
but it remains the only reasonable one advanced thus far. We measure for this
star m_439 = 20.2 +- 0.2, (m_336 - m_439) = -0.5 +- 0.2, and estimate M_B =
5.5, (U-B)_0 = -0.9, similar to other known optical counterparts. If this
candidate is not the identification, our limits imply that the true
counterpart, not yet identified, is probably the optically-faintest cluster
source yet known, or alternatively that it did not show significant UV excess
at the time of these observations. Finally, we assess the outlook for the
identification of the remaining luminous globular cluster X-ray sources.Comment: 15 pages including 5 figures and no tables. Accepted for publication
in The Astronomical Journal; to appear in Volume 116, September 1998. A
preprint with full resolution figures may be downloaded from
http://www.astro.washington.edu/deutsch/pubs
The Meinunger "Nicht Rote" Objects
Four high-latitude slow variable stars have been noted by Meinunger (1972) as
"nicht rote" ("not red") objects and thus curious. We have previously reported
(Margon & Deutsch 1997) that one of these objects, CC Boo, is in fact a QSO.
Here we present observations demonstrating that the remaining three are also
highly variable active galactic nuclei. The most interesting object of the four
is perhaps S 10765 (= NGP9 F324-0276706), which proves to be a resolved galaxy
at z=0.063. Despite the rapid and large reported variability amplitude (~1.6
mag), the spectrum is that of a perfectly normal galaxy, with no emission lines
or evident nonthermal continuum. We also present new spectroscopic and
photometric observations for AR CVn, suggested by Meinunger to be an RR Lyrae
star despite its very faint magnitude (=19.4). The object is indeed one of
the most distant RR Lyrae stars known, at a galactocentric distance of ~40 kpc.Comment: Accepted for publication in Publications of the Astronomical Society
of the Pacific, Volume 111, January 1999; 14 pages including 4 figures and 1
tabl
Ultracompact X-ray Binaries in Globular Clusters: Variability of the Optical Counterpart of X1832-330 in NGC 6652
Evidence is emerging that the luminous X-ray sources in the cores of globular
clusters may often consist of, or perhaps even as a class be dominated by,
ultracompact (P < 1 hr) binary stars. To the two such systems already known, in
NGC 6624 and NGC 6712, we now add evidence for two more. We detect large
amplitude variability in the candidate optical counterpart for the X-ray source
in the core of NGC 6652. Although the available observations are relatively
brief, the existing Hubble Space Telescope data indicate a strong 43.6 min
periodic modulation of the visible flux of semi-amplitude 30%. Further,
although the orbital period of the source in NGC 1851 is not yet explicitly
measured, we demonstrate that previous correlations of optical luminosity with
X-ray luminosity and accretion disk size, strengthened by recent data, strongly
imply that the period of that system is also less than 1 hr. Thus currently
there is evidence that 4 of the 7 globular cluster X-ray sources with
constrained periods are ultracompact, a fraction far greater than that found in
X-ray binaries the field.Comment: 10 pages including 2 figures and 1 table. Accepted for publication in
The Astrophysical Journal Letter
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