37 research outputs found
Stimulus Size Dependence of Information Transfer from Retina to Thalamus
Relay cells in the mammalian lateral geniculate nucleus (LGN) are driven primarily by single retinal ganglion cells (RGCs). However, an LGN cell responds typically to less than half of the spikes it receives from the RGC that drives it, and without retinal drive the LGN is silent (Kaplan and Shapley, 1984). Recent studies, which used stimuli restricted to the receptive field (RF) center, show that despite the great loss of spikes, more than half of the information carried by the RGC discharge is typically preserved in the LGN discharge (Sincich et al., 2009), suggesting that the retinal spikes that are deleted by the LGN carry less information than those that are transmitted to the cortex. To determine how LGN relay neurons decide which retinal spikes to respond to, we recorded extracellularly from the cat LGN relay cell spikes together with the slow synaptic (âSâ) potentials that signal the firing of retinal spikes. We investigated the influence of the inhibitory surround of the LGN RF by stimulating the eyes with spots of various sizes, the largest of which covered the center and surround of the LGN relay cell's RF. We found that for stimuli that activated mostly the RF center, each LGN spike delivered more information than the retinal spike, but this difference was reduced as stimulus size increased to cover the RF surround. To evaluate the optimality of the LGN editing of retinal spikes, we created artificial spike trains from the retinal ones by various deletion schemes. We found that single LGN cells transmitted less information than an optimal detector could
Transient Detection and Classification
I provide an incomplete inventory of the astronomical variability that will
be found by next-generation time-domain astronomical surveys. These phenomena
span the distance range from near-Earth satellites to the farthest Gamma Ray
Bursts. The surveys that detect these transients will issue alerts to the
greater astronomical community; this decision process must be extremely robust
to avoid a slew of ``false'' alerts, and to maintain the community's trust in
the surveys. I review the functionality required of both the surveys and the
telescope networks that will be following them up, and the role of VOEvents in
this process. Finally, I offer some ideas about object and event
classification, which will be explored more thoroughly by other articles in
these proceedings.Comment: 4 pages. In Proceedings of Hot-wiring the Transient Universe (HTU)
2007. To be published in Astronomische Nachrichten, March 200
The POINT-AGAPE Survey: Comparing Automated Searches of Microlensing Events toward M31
Searching for microlensing in M31 using automated superpixel surveys raises a
number of difficulties which are not present in more conventional techniques.
Here we focus on the problem that the list of microlensing candidates is
sensitive to the selection criteria or "cuts" imposed and some subjectivity is
involved in this. Weakening the cuts will generate a longer list of
microlensing candidates but with a greater fraction of spurious ones;
strengthening the cuts will produce a shorter list but may exclude some genuine
events. We illustrate this by comparing three analyses of the same data-set
obtained from a 3-year observing run on the INT in La Palma. The results of two
of these analyses have been already reported: Belokurov et al. (2005) obtained
between 3 and 22 candidates, depending on the strength of their cuts, while
Calchi Novati et al. (2005) obtained 6 candidates. The third analysis is
presented here for the first time and reports 10 microlensing candidates, 7 of
which are new. Only two of the candidates are common to all three analyses. In
order to understand why these analyses produce different candidate lists, a
comparison is made of the cuts used by the three groups...Comment: 28 pages, 24 figures, 9 table
The Angstrom Project: a microlensing survey of the structure and composition of the bulge of the Andromeda galaxy
The Andromeda Galaxy Stellar Robotic Microlensing Project (The Angstrom
Project) aims to use stellar microlensing events to trace the structure and
composition of the inner regions of the Andromeda Galaxy (M31). We present
microlensing rate and timescale predictions and spatial distributions for
stellar and sub-stellar lens populations in combined disk and barred bulge
models of M31. We show that at least half of the stellar microlenses in and
around the bulge are expected to have characteristic durations between 1 and 10
days, rising to as much as 80% for brown-dwarf dominated mass functions. These
short-duration events are mostly missed by current microlensing surveys that
are looking for Macho candidates in the M31 dark matter halo. Our models
predict that an intensive monitoring survey programme such as Angstrom, which
will be able to detect events of durations upwards of a day, could detect
around 30 events per season within ~5 arcminutes of the M31 centre, due to
ordinary low-mass stars and remnants. This yield increases to more than 60
events for brown-dwarf dominated mass functions. The overall number of events
and their average duration are sensitive diagnostics of the bulge mass, in
particular the contribution of low-mass stars and brown dwarfs. The combination
of an inclined disk, an offset bar-like bulge, and differences in the bulge and
disk luminosity functions results in a four-way asymmetry in the number of
events expected in each quadrant defined by the M31 disk axes. The asymmetry is
sensitive to the bar prolongation, orientation and mass.Comment: 9 pages, submitted to MNRA
Optical afterglows of gamma-ray bursts: a bimodal distribution?"
The luminosities of the optical afterglows of Gamma Ray Bursts, 12 hours
(rest frame time) after the trigger, show a surprising clustering, with a
minority of events being at a significant smaller luminosity. If real, this
dichotomy would be a crucial clue to understand the nature of optically dark
afterglows, i.e. bursts that are detected in the X-ray band, but not in the
optical. We investigate this issue by studying bursts of the pre-Swift era,
both detected and undetected in the optical. The limiting magnitudes of the
undetected ones are used to construct the probability that a generic bursts is
observed down to a given magnitude limit. Then, by simulating a large number of
bursts with pre-assigned characteristics, we can compare the properties of the
observed optical luminosity distribution with the simulated one. Our results
suggest that the hints of bimodality present in the observed distribution
reflects a real bimodality: either the optical luminosity distributions of
bursts is intrinsically bimodal, or there exists a population of bursts with a
quite significant grey absorption, i.e. wavelength independent extinction. This
population of intrinsically weak or grey-absorbed events can be associated to
dark bursts.Comment: 10 pages, 5 figures, accepted for publication in MNRA
GRB 991216 Joins the Jet Set: Discovery and Monitoring of its Optical Afterglow
The optical light curve of the energetic gamma-ray burst GRB 991216 is
consistent with jet-like behavior in which a power-law decay steepens from
t**(-1.22 +/- 0.04) at early times to t**(-1.53 +/- 0.05) in a gradual
transition at around 2 d. The derivation of the late-time decay slope takes
into account the constant contribution of a host or intervening galaxy which
was measured 110 d after the event at R = 24.56 +/- 0.14, although the light
curve deviates from a single power law whether or not a constant term is
included. The early-time spectral energy distribution of the afterglow can be
described as F_nu ~ nu**(-0.74 +/- 0.05) or flatter between optical and X-ray,
which, together with the slow initial decay, is characteristic of standard
adiabatic evolution in a uniformly dense medium. Assuming that a reported
absorption-line redshift of 1.02 is correct, the apparent isotropic energy of
6.7 x 10**53 erg is reduced by a factor of ~ 200 in the jet model, and the
initial half-opening angle is ~ 6 deg. GRB 991216 is the third good example of
a jet-like afterglow (following GRB 990123 and GRB 990510), supporting a trend
in which the apparently most energetic gamma-ray events have the narrowest
collimation and a uniform ISM environment. This, plus the absence of evidence
for supernovae associated with jet-like afterglows, suggests that these events
may originate from a progenitor in which angular momentum plays an important
role but a massive stellar envelope or wind does not, e.g., the coalescence of
a compact binary.Comment: 19 pages, accepted by The Astrophysical Journa
Monte Carlo analysis of the MEGA microlensing events towards M31
We perform an analytical study and a Monte Carlo (MC) analysis of the main
features for microlensing events in pixel lensing observations towards M31. Our
main aim is to investigate the lens nature and location of the 14 candidate
events found by the MEGA collaboration. Assuming a reference model for the mass
distribution in M31 and the standard model for our galaxy, we estimate the
MACHO-to-self lensing probability and the event time duration towards M31.
Reproducing the MEGA observing conditions, as a result we get the MC event
number density distribution as a function of the event full-width half-maximum
duration and the magnitude at maximum . For a
MACHO mass of we find typical values of day
and , for both MACHO-lensing and self-lensing
events occurring beyond about 10 arcminutes from the M31 center. A comparison
of the observed features ( and ) with our MC
results shows that for a MACHO mass the four innermost MEGA
events are most likely self-lensing events, whereas the six outermost events
must be genuine MACHO-lensing events.Comment: in press on A&
First Microlensing Events From The MEGA Survey Of M31
We present the first M31 candidate microlensing events from the Microlensing
Exploration of the Galaxy and Andromeda (MEGA) survey. MEGA uses several
telescopes to detect microlensing towards the nearby Andromeda galaxy, M31, in
order to establish whether massive compact objects are a significant
contribution to the mass budget of the dark halo of M31. The results presented
here are based on observations with the Isaac Newton Telescope on La Palma,
during the 1999/00 and 2000/01 observing seasons. In this data set, 14 variable
sources consistent with microlensing have been detected, 12 of which are new
and 2 have been reported previously by the POINT-AGAPE group. A preliminary
analysis of the spatial and timescale distributions of the candidate events
support their microlensing nature. We compare the spatial distributions of the
candidate events and of long-period variable stars, assuming the chances of
finding a long-period variable and a microlensing event are comparable. The
spatial distribution of our candidate microlensing events is more far/near side
asymmetric than expected from the detected long-period variable distribution.
The current analysis is preliminary and the asymmetry not highly significant,
but the spatial distribution of candidate microlenses is suggestive of the
presence of a microlensing halo.Comment: revised version, 16 pages, 12 figures, submitted to Astronomy &
Astrophysic