218 research outputs found
Gaia Data Release 1: Pre-processing and source list creation
Context. The first data release from the Gaia mission contains accurate positions and magnitudes for more than a billion sources, and proper motions and parallaxes for the majority of the 2.5 million HIPPARCOS and Tycho-2 stars.
Aims. We describe three essential elements of the initial data treatment leading to this catalogue: the image analysis, the construction of a source list, and the near real-time monitoring of the payload health. We also discuss some weak points that set limitations for the attainable precision at the present stage of the mission.
Methods. Image parameters for point sources are derived from one-dimensional scans, using a maximum likelihood method, under the assumption of a line spread function constant in time, and a complete modelling of bias and background. These conditions are, however, not completely fulfilled. The Gaia source list is built starting from a large ground-based catalogue, but even so a significant number of new entries have been added, and a large number have been removed. The autonomous onboard star image detection will pick up many spurious images, especially around bright sources, and such unwanted detections must be identified. Another key step of the source list creation consists in arranging the more than 10^(10) individual detections in spatially isolated groups that can be analysed individually.
Results. Complete software systems have been built for the Gaia initial data treatment, that manage approximately 50 million focal plane transits daily, giving transit times and fluxes for 500 million individual CCD images to the astrometric and photometric processing chains. The software also carries out a successful and detailed daily monitoring of Gaia health
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
The Broad Absorption Line Tidal Disruption Event iPTF15af: Optical and Ultraviolet Evolution
We present multi-wavelength observations of the tidal disruption event (TDE)
iPTF15af, discovered by the intermediate Palomar Transient Factory (iPTF)
survey at redshift . The optical and ultraviolet (UV) light curves
of the transient show a slow decay over five months, in agreement with previous
optically discovered TDEs. It also has a comparable black-body peak luminosity
of erg/s. The inferred temperature
from the optical and UV data shows a value of (35) K. The
transient is not detected in X-rays up to erg/s within
the first five months after discovery. The optical spectra exhibit two distinct
broad emission lines in the He II region, and at later times also H
emission. Additionally, emission from [N III] and [O III] is detected, likely
produced by the Bowen fluorescence effect. UV spectra reveal broad emission and
absorption lines associated with high-ionization states of N V, C IV, Si IV,
and possibly P V. These features, analogous to those of broad absorption line
quasars (BAL QSOs), require an absorber with column densities cm. This optically thick gas would also explain the
non-detection in soft X-rays. The profile of the absorption lines with the
highest column density material at the largest velocity is opposite that of BAL
QSOs. We suggest that radiation pressure generated by the TDE flare at early
times could have provided the initial acceleration mechanism for this gas.
Spectral UV line monitoring of future TDEs could test this proposal.Comment: 20 pages, 12 figures, published in Ap
Sifting for Sapphires: Systematic Selection of Tidal Disruption Events in iPTF
We present results from a systematic selection of tidal disruption events
(TDEs) in a wide-area (4800~deg), band, Intermediate Palomar
Transient Factory (iPTF) experiment. Our selection targets typical
optically-selected TDEs: bright (60\% flux increase) and blue transients
residing in the center of red galaxies. Using photometric selection criteria to
down-select from a total of 493 nuclear transients to a sample of 26 sources,
we then use follow-up UV imaging with the Neil Gehrels Swift Telescope,
ground-based optical spectroscopy, and light curve fitting to classify them as
14 Type Ia supernovae (SNe Ia), 9 highly variable active galactic nuclei
(AGNs), 2 confirmed TDEs, and 1 potential core-collapse supernova. We find it
possible to filter AGNs by employing a more stringent transient color cut ( 0.2 mag); further, UV imaging is the best discriminator for filtering
SNe, since SNe Ia can appear as blue, optically, as TDEs in their early phases.
However, when UV-optical color is unavailable, higher precision astrometry can
also effectively reduce SNe contamination in the optical. Our most stringent
optical photometric selection criteria yields a 4.5:1 contamination rate,
allowing for a manageable number of TDE candidates for complete spectroscopic
follow-up and real-time classification in the ZTF era. We measure a TDE per
galaxy rate of 1.7 10 gal yr (90\%
CL in Poisson statistics). This does not account for TDEs outside our selection
criteria, thus may not reflect the total TDE population, which is yet to be
fully mapped.Comment: 24 pages, 21 figures. Accepted for publication in the Astrophysical
Journal Supplement Serie
Effects of a localized beam on the dynamics of excitable cavity solitons
We study the dynamical behavior of dissipative solitons in an optical cavity
filled with a Kerr medium when a localized beam is applied on top of the
homogeneous pumping. In particular, we report on the excitability regime that
cavity solitons exhibits which is emergent property since the system is not
locally excitable. The resulting scenario differs in an important way from the
case of a purely homogeneous pump and now two different excitable regimes, both
Class I, are shown. The whole scenario is presented and discussed, showing that
it is organized by three codimension-2 points. Moreover, the localized beam can
be used to control important features, such as the excitable threshold,
improving the possibilities for the experimental observation of this
phenomenon.Comment: 9 Pages, 12 figure
Revisiting Optical Tidal Disruption Events with iPTF16axa
We report the discovery by the intermediate Palomar Transient Factory (iPTF) of a candidate tidal disruption event (TDE) iPTF16axa at z = 0.108 and present its broadband photometric and spectroscopic evolution from three months of follow-up observations with ground-based telescopes and Swift. The light curve is well fitted with a t^(−5/3) decay, and we constrain the rise time to peak to be <49 rest-frame days after disruption, which is roughly consistent with the fallback timescale expected for the ~5 × 10^6 M_⊙ black hole inferred from the stellar velocity dispersion of the host galaxy. The UV and optical spectral energy distribution is well described by a constant blackbody temperature of T ~ 3 × 10^4 K over the monitoring period, with an observed peak luminosity of 1.1 × 10^(44) erg s^(−1). The optical spectra are characterized by a strong blue continuum and broad He ii and Hα lines, which are characteristic of TDEs. We compare the photometric and spectroscopic signatures of iPTF16axa with 11 TDE candidates in the literature with well-sampled optical light curves. Based on a single-temperature fit to the optical and near-UV photometry, most of these TDE candidates have peak luminosities confined between log(L [erg s^(−1)]) = 43.4–44.4, with constant temperatures of a few ×10^4 K during their power-law declines, implying blackbody radii on the order of 10 times the tidal disruption radius, that decrease monotonically with time. For TDE candidates with hydrogen and helium emission, the high helium-to-hydrogen ratios suggest that the emission arises from high-density gas, where nebular arguments break down. We find no correlation between the peak luminosity and the black hole mass, contrary to the expectations for TDEs to have M ∝ M_(BH)^(-1/2)
Early Observations of the Type Ia Supernova iPTF 16abc: A Case of Interaction with Nearby, Unbound Material and/or Strong Ejecta Mixing
Early observations of Type Ia supernovae (SNe Ia) provide a unique probe of
their progenitor systems and explosion physics. Here we report the intermediate
Palomar Transient Factory (iPTF) discovery of an extraordinarily young SN Ia,
iPTF 16abc. By fitting a power law to our early light curve, we infer that
first light for the SN, that is when the SN could have first been detected by
our survey, occurred only days before our first
detection. In the 24 hr after discovery, iPTF 16abc rose by 2 mag,
featuring a near-linear rise in flux for 3 days. Early spectra show
strong C II absorption, which disappears after 7 days. Unlike the
extensivelyobserved SN Ia SN 2011fe, the colors of iPTF 16abc are
blue and nearly constant in the days after explosion. We show that our early
observations of iPTF 16abc cannot be explained by either SN shock breakout and
the associated, subsequent cooling or the SN ejecta colliding with a stellar
companion. Instead, we argue that the early characteristics of iPTF 16abc,
including (i) the rapid, near-linear rise, (ii) the nonevolving blue colors,
and (iii) the strong C II absorption, are the result of either ejecta
interaction with nearby, unbound material or vigorous mixing of radioactive
Ni in the SN ejecta, or a combination of the two. In the next few years,
dozens of very young \textit{normal} SNe Ia will be discovered, and
observations similar to those presented here will constrain the white dwarf
explosion mechanism.Comment: 18 pages, 12 figures, accepted by Ap
A Tale of Two Transients: GW 170104 and GRB 170105A
We present multi-wavelength follow-up campaigns by the AstroSat CZTI and GROWTH collaborations in search of an electromagnetic counterpart to the gravitational wave event GW 170104. At the time of the GW 170104 trigger, the AstroSat CZTI field of view covered 50.3% of the sky localization. We do not detect any hard X-ray (>100 keV) signal at this time, and place an upper limit of ≈4.5 x 10^(-7) erg cm^(-2) s^(-1), for a 1 s timescale. Separately, the ATLAS survey reported a rapidly fading optical source dubbed ATLAS17aeu in the error circle of GW 170104. Our panchromatic investigation of ATLAS17aeu shows that it is the afterglow of an unrelated long, soft GRB 170105A, with only a fortuitous spatial coincidence with GW 170104. We then discuss the properties of this transient in the context of standard long GRB afterglow models
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