640 research outputs found
The Fading of Supernova Remnant Cassiopeia A from 38 MHz to 16.5 GHz from 1949 to 1999 with New Observations at 1405 MHz
We report 1405 MHz measurements of the flux density of the approximately 320 year old supernova remnant Cassiopeia A, relative to the flux density of Cygnus A, made between 1995 and 1999. When compared to measurements made between 1957 and 1976, we find that the rate at which Cassiopeia A has been fading at this and nearby frequencies has changed from approximately 0.9 % yr^-1 in the 1960s to approximately 0.6 - 0.7 % yr^-1 now. Furthermore, we have collected from the literature measurements of this fading rate at lower (38 - 300 MHz) and higher (7.8 - 16.5 GHz) frequencies. We show that the fading rate has dropped by a factor of approximately 3 over the past 50 years at the lower frequencies, while remaining relatively constant at the higher frequencies, which is in agreement with the findings of others. Our findings at 1405 MHz, in conjunction with a measurement of the fading rate at the nearby frequency of 927 MHz by Vinyajkin (1997), show an intermediate behavior at intermediate frequencies. We also find that Cassiopeia A, as of approximately 1990, was fading at about the same rate, approximately 0.6 - 0.7 % yr^-1, at all of these frequencies. Future measurements are required to determine whether the fading rate will continue to decrease at the lower frequencies, or whether Cassiopeia A will now fade at a relatively constant rate at all of these frequencies
Are there cosmological evolution trends on Gamma-Ray Burst features?
The variability of gamma-ray burst (GRB) is thought to be correlated with its
absolute peak luminosity, and this relation had been used to derive an estimate
of the redshifts of GRBs. Recently Amati et al. presented the results of
spectral and energetic properties of several GRBs with known redshifts. Here we
analyse the properties of two group GRBs, one group with known redshift from
afterglow observation, and another group with redshift derived from the
luminosity - variability relation. We study the redshift dependence of various
GRBs features in their cosmological rest frames, including the burst duration,
the isotropic luminosity and radiated energy, and the peak energy of spectra. We find that, for these two group GRBs, their properties are
all redshift dependent, i.e. their intrinsic duration, luminosity, radiated
energy and peak energy , are all correlated with the redshift, which means
that there are cosmological evolution effects on gamma-ray bursts features, and
this can provide an interesting clue to the nature of GRBs. If this is true,
then the results also imply that the redshift derived from the luminosity -
variability relation may be reliable.Comment: Latex, 11 pages. Discussion of the selection effects have been added.
Accepted for publication in MNRA
The discovery of the electromagnetic counterpart of GW170817: kilonova AT 2017gfo/DLT17ck
During the second observing run of the Laser Interferometer gravitational-
wave Observatory (LIGO) and Virgo Interferometer, a gravitational-wave signal
consistent with a binary neutron star coalescence was detected on 2017 August
17th (GW170817), quickly followed by a coincident short gamma-ray burst trigger
by the Fermi satellite. The Distance Less Than 40 (DLT40) Mpc supernova search
performed pointed follow-up observations of a sample of galaxies regularly
monitored by the survey which fell within the combined LIGO+Virgo localization
region, and the larger Fermi gamma ray burst error box. Here we report the
discovery of a new optical transient (DLT17ck, also known as SSS17a; it has
also been registered as AT 2017gfo) spatially and temporally coincident with
GW170817. The photometric and spectroscopic evolution of DLT17ck are unique,
with an absolute peak magnitude of Mr = -15.8 \pm 0.1 and an r-band decline
rate of 1.1mag/d. This fast evolution is generically consistent with kilonova
models, which have been predicted as the optical counterpart to binary neutron
star coalescences. Analysis of archival DLT40 data do not show any sign of
transient activity at the location of DLT17ck down to r~19 mag in the time
period between 8 months and 21 days prior to GW170817. This discovery
represents the beginning of a new era for multi-messenger astronomy opening a
new path to study and understand binary neutron star coalescences, short
gamma-ray bursts and their optical counterparts.Comment: ApJL in press, 4 figure
A search for rapidly pulsating hot subdwarf stars in the GALEX survey
NASA's Galaxy Evolution Explorer (GALEX) provided near- and far-UV
observations for approximately 77 percent of the sky over a ten-year period;
however, the data reduction pipeline initially only released single NUV and FUV
images to the community. The recently released Python module gPhoton changes
this, allowing calibrated time-series aperture photometry to be extracted
easily from the raw GALEX data set. Here we use gPhoton to generate light
curves for all hot subdwarf B (sdB) stars that were observed by GALEX, with the
intention of identifying short-period, p-mode pulsations. We find that the
spacecraft's short visit durations, uneven gaps between visits, and dither
pattern make the detection of hot subdwarf pulsations difficult. Nonetheless,
we detect UV variations in four previously known pulsating targets and report
their UV pulsation amplitudes and frequencies. Additionally, we find that
several other sdB targets not previously known to vary show promising signals
in their periodograms. Using optical follow-up photometry with the Skynet
Robotic Telescope Network, we confirm p-mode pulsations in one of these
targets, LAMOST J082517.99+113106.3, and report it as the most recent addition
to the sdBVr class of variable stars.Comment: 11 Pages, 8 Figures, Accepted for publication in the Astrophysical
Journa
SDSS-RASS: Next Generation of Cluster-Finding Algorithms
We outline here the next generation of cluster-finding algorithms. We show
how advances in Computer Science and Statistics have helped develop robust,
fast algorithms for finding clusters of galaxies in large multi-dimensional
astronomical databases like the Sloan Digital Sky Survey (SDSS). Specifically,
this paper presents four new advances: (1) A new semi-parametric algorithm -
nicknamed ``C4'' - for jointly finding clusters of galaxies in the SDSS and
ROSAT All-Sky Survey databases; (2) The introduction of the False Discovery
Rate into Astronomy; (3) The role of kernel shape in optimizing cluster
detection; (4) A new determination of the X-ray Cluster Luminosity Function
which has bearing on the existence of a ``deficit'' of high redshift, high
luminosity clusters. This research is part of our ``Computational
AstroStatistics'' collaboration (see Nichol et al. 2000) and the algorithms and
techniques discussed herein will form part of the ``Virtual Observatory''
analysis toolkit.Comment: To appear in Proceedings of MPA/MPE/ESO Conference "Mining the Sky",
July 31 - August 4, 2000, Garching, German
The Broadband Afterglow of GRB980329
We present radio observations of the afterglow of the bright gamma-ray burst
GRB980329 made between one month and several years after the burst, a
re-analysis of previously published submillimeter data, and late-time optical
and near-infrared (NIR) observations of the host galaxy. From the absence of a
spectral break in the optical/NIR colors of the host galaxy, we exclude the
earlier suggestion that GRB980329 lies at a redshift of z >~5. We combine our
data with the numerous multi-wavelength observations of the early afterglow,
fit a comprehensive afterglow model to the entire broadband dataset, and derive
fundamental physical parameters of the blast-wave and its host environment.
Models for which the ejecta expand isotropically require both a high
circumburst density and extreme radiative losses from the shock. No low density
model (n << 10 cm^{-3}) fits the data. A burst with a total energy of ~ 10^{51}
erg, with the ejecta narrowly collimated to an opening angle of a few degrees,
driven into a surrounding medium with density ~ 20 cm^{-3}, provides a
satisfactory fit to the lightcurves over a range of redshifts.Comment: 27 pages, incl. 6 figures, minor revisions (e.g. added/updated
references) Accepted by Ap
The progenitor and early evolution of the Type IIb SN 2016gkg
We report initial observations and analysis on the Type IIb SN~2016gkg in the
nearby galaxy NGC~613. SN~2016gkg exhibited a clear double-peaked light curve
during its early evolution, as evidenced by our intensive photometric follow-up
campaign. SN~2016gkg shows strong similarities with other Type IIb SNe, in
particular with respect to the \he~emission features observed in both the
optical and near infrared. SN~2016gkg evolved faster than the prototypical
Type~IIb SN~1993J, with a decline similar to that of SN~2011dh after the first
peak. The analysis of archival {\it Hubble Space Telescope} images indicate a
pre-explosion source at SN~2016gkg's position, suggesting a progenitor star
with a mid F spectral type and initial mass \msun, depending on
the distance modulus adopted for NGC~613. Modeling the temperature evolution
within of explosion, we obtain a progenitor radius of
\rsun, smaller than that obtained from the analysis of the
pre-explosion images (\rsun).Comment: 7 pages, 5 figures. Submitted to ApJ Letter
A New Frequency-Luminosity Relation for Long GRBs?
We have studied power density spectra (PDS) of 206 long Gamma-Ray Bursts
(GRBs). We fitted the PDS with a simple power-law and extracted the exponent of
the power-law (alpha) and the noise-crossing threshold frequency (f_th). We
find that the distribution of the extracted alpha peaks around -1.4 and that of
f_th around 1 Hz. In addition, based on a sub-set of 58 bursts with known
redshifts, we show that the redshift-corrected threshold frequency is
positively correlated with the isotropic peak luminosity. The correlation
coefficient is 0.57 +/- 0.03.Comment: 9 pages, 17 figures, 1 table; Accepted for publication in MNRA
IGR J19552+0044: A new asynchronous short period polar: "Filling the gap between intermediate and ordinary polars"
Based on XMM--Newton X-ray observations IGR J19552+0044 appears to be either
a pre-polar or an asynchronous polar. We conducted follow-up optical
observations to identify the sources and periods of variability precisely and
to classify this X-ray source correctly. Extensive multicolor photometric and
medium- to high-resolution spectroscopy observations were performed and period
search codes were applied to sort out the complex variability of the object. We
found firm evidence of discording spectroscopic (81.29+/-0.01m) and photometric
(83.599+/-0.002m) periods that we ascribe to the white dwarf (WD)\ spin period
and binary orbital period, respectively. This confirms that IGR J19552+0044 is
an asynchronous polar. Wavelength-dependent variability and its continuously
changing shape point at a cyclotron emission from a magnetic WD with a
relatively low magnetic field below 20 MG.
The difference between the WD spin period and the binary orbital period
proves that IGR J19552+0044 is a polar with the largest known degree of
asynchronism (0.97 or 3%).Comment: 9 pages, 10 figures, A&A accepte
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