4,258 research outputs found
Supernova 2002ap - The First Month
Supernova (SN) 2002ap in M74 was discovered on January 29, 2002. Being one of
the nearest (10 Mpc) SN events in the last decades, and spectroscopically
similar to the so-called ``hypernovae'' 1997ef and 1998bw, both possibly
associated with gamma-ray bursts (GRBs), it is of great interest. Shortly after
its discovery, we launched an intensive photometric and spectroscopic
monitoring campaign of this event, and here we report the results of the first
month of observations. We use our UBVRI photometry to estimate the magnitudes
at, and dates of, peak brightness. Our data suggest that this object reached
its peak B-band luminosity on Feb. 7.1(-1.3)(+2) UT. Based on its similarity to
SN 1998bw, we estimate the range of possible dates for a GRB that may have been
associated with SN 2002ap. We find that it may include dates outside the time
frame for which all available gamma-ray data have been intensively scanned,
according to recent reports. The absolute magnitude at peak brightness of SN
2002ap (M_B = -16.9) shows that it was significantly fainter than SN 1998bw, or
normal type-Ia SNe, but similar to SN 1997ef. Our spectroscopic observations
confirm that SN 2002ap is strikingly similar to SNe 1998bw and 1997ef. We
briefly describe the spectral evolution of this object. To assist other
observers and to stimulate theoretical models, we make our entire data set
publicly available in digital form.Comment: 5 pages, including 4 figures. MNRAS (pink pages) in press. Data
available electronically from http://wise-obs.tau.ac.il/~avishay/local.htm
A spectroscopic search for White Dwarf companions to 101 nearby M dwarfs
Recent studies of the stellar population in the solar neighborhood (<20 pc)
suggest that there are undetected white dwarfs (WDs) in multiple systems with
main sequence companions. Detecting these hidden stars and obtaining a more
complete census of nearby WDs is important for our understanding of binary and
galactic evolution, as well as the study of explosive phenomena. In an attempt
to uncover these hidden WDs, we present intermediate resolution spectroscopy
over the wavelength range 3000-25000 \AA\ of 101 nearby M dwarfs (dMs),
observed with the Very Large Telescope X-Shooter spectrograph. For each star we
search for a hot component superimposed on the dM spectrum. X-Shooter has
excellent blue sensitivity and thus can reveal a faint hot WD despite the
brightness of its red companion. Visual examination shows no clear evidence of
a WD in any of the spectra. We place upper limits on the effective temperatures
of WDs that may still be hiding by fitting dM templates to the spectra, and
modeling WD spectra. On average our survey is sensitive to WDs hotter than
about 5300 K. This suggests that the frequency of WD companions of T<5300 K
with separation of order <50 AU among the local dM population is <3% at the 95%
confidence level. The reduced spectra are made available on via WISeREP
repository.Comment: 41 pages, 105 figures, 2 tables. Submitted to AAS journal
Follow-up question handling in the IMIX and Ritel systems: A comparative study
One of the basic topics of question answering (QA) dialogue systems is how follow-up questions should be interpreted by a QA system. In this paper, we shall discuss our experience with the IMIX and Ritel systems, for both of which a follow-up question handling scheme has been developed, and corpora have been collected. These two systems are each other's opposites in many respects: IMIX is multimodal, non-factoid, black-box QA, while Ritel is speech, factoid, keyword-based QA. Nevertheless, we will show that they are quite comparable, and that it is fruitful to examine the similarities and differences. We shall look at how the systems are composed, and how real, non-expert, users interact with the systems. We shall also provide comparisons with systems from the literature where possible, and indicate where open issues lie and in what areas existing systems may be improved. We conclude that most systems have a common architecture with a set of common subtasks, in particular detecting follow-up questions and finding referents for them. We characterise these tasks using the typical techniques used for performing them, and data from our corpora. We also identify a special type of follow-up question, the discourse question, which is asked when the user is trying to understand an answer, and propose some basic methods for handling it
Real-time Detection and Rapid Multiwavelength Follow-up Observations of a Highly Subluminous Type II-P Supernova from the Palomar Transient Factory Survey
The Palomar Transient Factory (PTF) is an optical wide-field variability survey carried out using a camera with a 7.8 deg^2 field of view mounted on the 48 inch Oschin Schmidt telescope at Palomar Observatory. One of the key goals of this survey is to conduct high-cadence monitoring of the sky in order to detect optical transient sources shortly after they occur. Here, we describe the real-time capabilities of the PTF and our related rapid multiwavelength follow-up programs, extending from the radio to the γ-ray bands. We present as a case study observations of the optical transient PTF10vdl (SN 2010id), revealed to be a very young core-collapse (Type II-P) supernova having a remarkably low luminosity. Our results demonstrate that the PTF now provides for optical transients the real-time discovery and rapid-response follow-up capabilities previously reserved only for high-energy transients like gamma-ray bursts
Orphan GRB radio afterglows: Candidates and constraints on beaming
The number of orphan radio afterglows associated with gamma-ray bursts (GRBs)
that should be detected by a flux limited radio survey, is calculated. It is
shown that for jetted GRBs this number is smaller for smaller jet opening angle
(theta), contrary to naive expectation. For a beaming factor
f_b^{-1}=(theta^2/2)^{-1} = 500, roughly the value inferred by Frail et al.
(2001) from analysis of afterglow light curves, we predict that between several
hundreds to several thousands orphan radio afterglows should be detectable
(over all sky) above 1 mJy at GHz frequencies at any given time. This orphan
population is dominated by sources lying at distances of a few hundred Mpc, and
having an age of ~1 yr. A search for point-like radio transients with flux
densities greater than 6 mJy was conducted using the FIRST and NVSS surveys,
yielding a list of 25 orphan candidates. We argue that most of the candidates
are unlikely to be radio supernovae. However, the possibility that they are
radio loud AGNs cannot be ruled out without further observations. Our analysis
sets an upper limit for the all sky number of radio orphans, which corresponds
to a lower limit f_b^{-1}>10 on the beaming factor. Rejection of all candidates
found in our search would imply f_b^{-1}>100. This, and the fact that some
candidates may indeed be radio afterglows, strongly motivate further
observations of these transients.Comment: 18 pages, including 2 figure
A Very Large Array Search for 5 GHz Radio Transients and Variables at Low Galactic Latitudes
We present the results of a 5 GHz survey with the Very Large Array (VLA) and the expanded VLA, designed to search for short-lived (≾1 day) transients and to characterize the variability of radio sources at milli-Jansky levels. A total sky area of 2.66 deg^2, spread over 141 fields at low Galactic latitudes (b≅6-8 deg), was observed 16 times with a cadence that was chosen to sample timescales of days, months, and years. Most of the data were reduced, analyzed, and searched for transients in near real-time. Interesting candidates were followed up using visible light telescopes (typical delays of 1-2 hr) and the X-ray Telescope on board the Swift satellite. The final processing of the data revealed a single possible transient with a peak flux density of f_ν≅2.4 mJy. This implies a transient's sky surface density of κ(f_ν > 1.8 mJy) = 0.039^(+0.13 +0.18)_(–0.032,–0.038) deg^(–2) (1σ, 2σ confidence errors). This areal density is roughly consistent with the sky surface density of transients from the Bower et al. survey extrapolated to 1.8 mJy. Our observed transient areal density is consistent with a neutron star's origin for these events. Furthermore, we use the data to measure the source variability on timescales of days to years, and we present the variability structure function of 5 GHz sources. The mean structure function shows a fast increase on ≈1 day timescale, followed by a slower increase on timescales of up to 10 days. On timescales between 10 and 60 days, the structure function is roughly constant. We find that ≳30% of the unresolved sources brighter than 1.8 mJy are variables at the >4σ confidence level, presumably mainly due to refractive scintillation
Longtime behavior of nonlocal Cahn-Hilliard equations
Here we consider the nonlocal Cahn-Hilliard equation with constant mobility
in a bounded domain. We prove that the associated dynamical system has an
exponential attractor, provided that the potential is regular. In order to do
that a crucial step is showing the eventual boundedness of the order parameter
uniformly with respect to the initial datum. This is obtained through an
Alikakos-Moser type argument. We establish a similar result for the viscous
nonlocal Cahn-Hilliard equation with singular (e.g., logarithmic) potential. In
this case the validity of the so-called separation property is crucial. We also
discuss the convergence of a solution to a single stationary state. The
separation property in the nonviscous case is known to hold when the mobility
degenerates at the pure phases in a proper way and the potential is of
logarithmic type. Thus, the existence of an exponential attractor can be proven
in this case as well
Far-Ultraviolet to Near-Infrared Spectroscopy of A Nearby Hydrogen Poor Superluminous Supernova Gaia16apd
We report the first maximum-light far-Ultraviolet to near-infrared spectra
(1000A - 1.62um, rest) of a H-poor superluminous supernova, Gaia16apd. At
z=0.1018, it is one of the closest and the UV brightest such events, with 17.4
(AB) magnitude in Swift UV band (1928A) at -11days pre-maximum. Assuming an
exponential form, we derived the rise time of 33days and the peak bolometric
luminosity of 3x10^{44}ergs^-1. At maximum light, the estimated photospheric
temperature and velocity are 17,000K and 14,000kms^-1 respectively. The
inferred radiative and kinetic energy are roughly 1x10^{51} and 2x10^{52}erg.
Gaia16apd is extremely UV luminous, emitting 50% of its total luminosity at
1000 - 2500A. Compared to the UV spectra (normalized at 3100A) of well studied
SN1992A (Ia), SN2011fe(Ia), SN1999em (IIP) and SN1993J (IIb), it has orders of
magnitude more far-UV emission. This excess is interpreted primarily as a
result of weaker metal line blanketing due to much lower abundance of
iron-group elements in the outer ejecta. Because these elements originate
either from the natal metallicity of the star, or have been newly produced, our
observation provides direct evidence that little of these freshly synthesized
material, including 56Ni, was mixed into the outer ejecta, and the progenitor
metallicity is likely sub-solar. This disfavors Pair-Instability Supernova
(PISN) models with Helium core masses >=90Msun, where substantial 56Ni material
is produced. Higher photospheric temperature of Gaia16apd than that of normal
SNe may also contribute to the observed far-UV excess. We find some indication
that UV luminous SLSNe-I like Gaia16apd could be common. Using the UV spectra,
we show that WFIRST could detect SLSNe-I out to redshift of 8.Comment: 19 pages. Match with the version accepted in Ap
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