128 research outputs found
RJK Observations of the Optical Afterglow of GRB 991216
We present near-infrared and optical observations of the afterglow to the
Gamma-Ray Burst (GRB) 991216 obtained with the F. L. Whipple Observatory 1.2-m
telescope and the University of Hawaii 2.2-m telescope. The observations range
from 15 hours to 3.8 days after the burst. The temporal behavior of the data is
well described by a single power-law decay with index -1.36 +/-0.04,
independent of wavelength. The optical spectral energy distribution, corrected
for significant Galactic reddening of E(B-V)=0.626, is well fitted by a single
power-law with index -0.58 +/- 0.08. Combining the IR/optical observations with
a Chandra X-ray measurement gives a spectral index of -0.8 +/- 0.1 in the
synchrotron cooling regime. A comparison between the spectral and temporal
power-law indices suggest that a jet is a better match to the observations than
a simple spherical shock.Comment: Accepted to the Astrophysical Journal, 12 pages, 4 postscript figure
Understanding the mechanisms underlying cognitive control in psychosis
Background
Cognitive control (CC) involves a topâdown mechanism to flexibly respond to complex stimuli and is impaired in schizophrenia.
Methods
This study investigated the impact of increasing complexity of CC processing in 140 subjects with psychosis and 39 healthy adults, with assessments of behavioral performance, neural regions of interest and symptom severity.
Results
The lowest level of CC (Stroop task) was impaired in all patients; the intermediate level of CC (Faces task) with explicit emotional information was most impaired in patients with first episode psychosis. Patients showed activation of distinct neural CC and reward networks, but iterative learning based on the higher-order of CC during the trust game, was most impaired in chronic schizophrenia. Subjects with first episode psychosis, and patients with lower symptom load, demonstrate flexibility of the CC network to facilitate learning, which appeared compromised in the more chronic stages of schizophrenia.
Conclusion
These data suggest optimal windows for opportunities to introduce therapeutic interventions to improve CC
Ground-based Transit Spectroscopy of the Hot-Jupiter WASP-19b in the Near-infrared
We present ground-based measurements of the transmission and emission spectra of the hot-Jupiter WASP-19b in nine spectroscopic channels from 1.25 to 2.35 ÎŒm. The measurements are based on the combined analysis of time-series spectroscopy obtained during two complete transits and two complete secondary eclipses of the planet. The observations were performed with the MMIRS instrument on the Magellan II telescope using the technique of multi-object spectroscopy with wide slits. We compare the transmission and emission data to theoretical models to constrain the composition and thermal structure of the planet's atmosphere. Our measured transmission spectrum exhibits a scatter that corresponds to 1.3 scale heights of the planet's atmosphere, which is consistent with the size of spectral features predicted by theoretical models for a clear atmosphere. We detect the secondary eclipses of the planet at significances ranging from 2.2Ï to 14.4Ï. The secondary eclipse depths, and the significances of the detections increase toward longer wavelengths. Our measured emission spectrum is consistent with a 2250 K effectively isothermal one-dimensional model for the planet's dayside atmosphere. This model also matches previously published photometric measurements from the Spitzer Space Telescope and ground-based telescopes. These results demonstrate the important role that ground-based observations using multi-object spectroscopy can play in constraining the properties of exoplanet atmospheres, and they also emphasize the need for high-precision measurements based on observations of multiple transits and eclipses
Two 'b's in the Beehive: The Discovery of the First Hot Jupiters in an Open Cluster
We present the discovery of two giant planets orbiting stars in Praesepe
(also known as the Beehive Cluster). These are the first known hot Jupiters in
an open cluster and the only planets known to orbit Sun-like, main-sequence
stars in a cluster. The planets are detected from Doppler shifted radial
velocities; line bisector spans and activity indices show no correlation with
orbital phase, confirming the variations are caused by planetary companions.
Pr0201b orbits a V=10.52 late F dwarf with a period of 4.4264 +/- 0.0070 days
and has a minimum mass of 0.540 +/- 0.039 Mjup, and Pr0211b orbits a V=12.06
late G dwarf with a period of 2.1451 +/- 0.0012 days and has a minimum mass of
1.844 +/- 0.064 Mjup. The detection of 2 planets among 53 single members
surveyed establishes a lower limit on the hot Jupiter frequency of 3.8
(+5.0)(-2.4) % in this metal-rich open cluster. Given the precisely known age
of the cluster, this discovery also demonstrates that, in at least 2 cases,
giant planet migration occurred within 600 Myr after formation. As we endeavor
to learn more about the frequency and formation history of planets,
environments with well-determined properties -- such as open clusters like
Praesepe -- may provide essential clues to this end.Comment: 5 pages, 3 tables, 2 figures. Published in ApJ Letter
KELT-1b: A Strongly Irradiated, Highly Inflated, Short Period, 27 Jupiter-mass Companion Transiting a mid-F Star
We present the discovery of KELT-1b, the first transiting low-mass companion
from the wide-field Kilodegree Extremely Little Telescope-North (KELT-North)
survey. The V=10.7 primary is a mildly evolved, solar-metallicity, mid-F star.
The companion is a low-mass brown dwarf or super-massive planet with mass of
27.23+/-0.50 MJ and radius of 1.110+0.037-0.024 RJ, on a very short period
(P=1.21750007) circular orbit. KELT-1b receives a large amount of stellar
insolation, with an equilibrium temperature assuming zero albedo and perfect
redistribution of 2422 K. Upper limits on the secondary eclipse depth indicate
that either the companion must have a non-zero albedo, or it must experience
some energy redistribution. Comparison with standard evolutionary models for
brown dwarfs suggests that the radius of KELT-1b is significantly inflated.
Adaptive optics imaging reveals a candidate stellar companion to KELT-1, which
is consistent with an M dwarf if bound. The projected spin-orbit alignment
angle is consistent with zero stellar obliquity, and the vsini of the primary
is consistent with tidal synchronization. Given the extreme parameters of the
KELT-1 system, we expect it to provide an important testbed for theories of the
emplacement and evolution of short-period companions, and theories of tidal
dissipation and irradiated brown dwarf atmospheres.Comment: 30 pages, 19 figures. Submitted to Ap
A Putative Early-Type Host Galaxy for GRB 060502B: Implications for the Progenitors of Short-Duration Hard-Spectrum Bursts
Starting with the first detection of an afterglow from a short-duration
hard-spectrum gamma-ray burst (SHB) by Swift last year, a growing body of
evidence has suggested that SHBs are associated with an older and
lower-redshift galactic population than long-soft GRBs and, in a few cases,
with large (>~ 10 kpc) projected offsets from the centers of their putative
host galaxies. Here we present observations of the field of GRB 060502B, a SHB
detected by Swift and localized by the X-ray Telescope (XRT). We find a massive
red galaxy at a redshift of z=0.287 at an angular distance of 17.1 arcsec from
our revised XRT position. Using associative and probabilistic arguments we
suggest that this galaxy hosted the progenitor of GRB 060502B. If true, this
offset would correspond to a physical displacement of 73 +- 19 kpc in
projection, about twice the largest offset inferred for any SHB to date and
almost an order of magnitude larger than a typical long-soft burst offset.
Spectra and modeling of the star-formation history of this possible host show
it to have undergone a large ancient starburst. If the progenitor of GRB
060502B was formed in this starburst episode, the time of the GRB explosion
since birth is tau = 1.3 +- 0.2 Gyr and the minimum kick velocity of the SHB
progenitor is v_kick = 55 +- 15 km/s.Comment: Accepted to the Astrophysical Journal. 18 pages, 3 figures; expanded
discussion of alternative host possibilitie
Hat-P-25b: A Hot-Jupiter Transiting a Moderately Faint G Star
We report the discovery of HAT-P-25b, a transiting extrasolar planet orbiting the V = 13.19 G5 dwarf star GSC 1788-01237, with a period P = 3.652836 ± 0.000019 days, transit epoch T_c = 2455176.85173 ± 0.00047 (BJDâbarycentric Julian dates throughout the paper are calculated from Coordinated Universal Time, UTC), and transit duration 0.1174 ± 0.0017 days. The host star has a mass of 1.01 ± 0.03 M_â, radius of 0.96^(+0.05)_(â 0.04) R_â, effective temperature 5500 ± 80 K, and metallicity [Fe/H] = +0.31 ± 0.08. The planetary companion has a mass of 0.567 ± 0.022 M_J and radius of 1.190^(+0.081)_(â0.056) R_J yielding a mean density of 0.42 ± 0.07 g cm^(â3)
DASCH Discovery of A Possible Nova-like Outburst in A Peculiar Symbiotic Binary
We present photometric and spectroscopic observations of a peculiar variable
(designated DASCH J075731.1+201735 or J0757) discovered from our DASCH project
using the digitized Harvard College Observatory archival photographic plates.
It brightened by about 1.5 magnitudes in B within a year starting in 1942, and
then slowly faded back to its pre-outburst brightness from 1943 to the 1950s.
The mean brightness level was stable before and after the outburst, and
ellipsoidal variations with a period of days are seen,
suggesting that the star is tidally distorted. Radial-velocity measurements
indicate that the orbit is nearly circular () with a
spectroscopic period that is the same as the photometric period. The binary
consists of a M0III star, and a
companion, very likely a white dwarf (WD). Unlike other symbiotic binaries,
there is no sign of emission lines or a stellar wind in the spectra. With an
outburst timescale of ~10 years and estimated B band peak luminosity M_B~0.7,
J0757 is different from any other known classic or symbiotic novae. The most
probable explanation of the outburst is Hydrogen shell-burning on the WD,
although an accretion-powered flare cannot be ruled out.Comment: 12 pages, 6 figures, accepted for publication in Ap
The Troublesome Broadband Evolution of GRB 061126: Does a Grey Burst Imply Grey Dust?
We report on observations of a gamma-ray burst (GRB 061126) with an extremely
bright (R ~ 12 mag at peak) early-time optical afterglow. The optical afterglow
is already fading as a power law 22 seconds after the trigger, with no
detectable prompt contribution in our first exposure, which was coincident with
a large prompt-emission gamma-ray pulse. The optical--infrared photometric
spectral energy distribution is an excellent fit to a power law, but it
exhibits a moderate red-to-blue evolution in the spectral index at about 500 s
after the burst. This color change is contemporaneous with a switch from a
relatively fast decay to slower decay. The rapidly decaying early afterglow is
broadly consistent with synchrotron emission from a reverse shock, but a bright
forward-shock component predicted by the intermediate- to late-time X-ray
observations under the assumptions of standard afterglow models is not
observed. Indeed, despite its remarkable early-time brightness, this burst
would qualify as a dark burst at later times on the basis of its nearly flat
optical-to-X-ray spectral index. Our photometric spectral energy distribution
provides no evidence of host-galaxy extinction, requiring either large
quantities of grey dust in the host system (at redshift 1.1588 +/- 0.0006,
based upon our late-time Keck spectroscopy) or separate physical origins for
the X-ray and optical afterglows.Comment: Revised version submitted to ApJ. Contains significantly expanded
discussion, an additional figure, and numerous other change
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