2,975 research outputs found
Testing EUV/X-ray Atomic Data for the Solar Dynamics Observatory
The Atmospheric Imaging Assembly (AIA) and the Exteme-ultraviolet Variability
Experiment (EVE) onboard the Solar Dynamics Observatory include spectral
windows in the X-ray/EUV band. Accuracy and completeness of the atomic data in
this wavelength range is essential for interpretation of the spectrum and
irradiance of the solar corona, and of SDO observations made with the AIA and
EVE instruments. Here we test the X-ray/EUV data in the CHIANTI database to
assess their completeness and accuracy in the SDO bands, with particular focus
on the 94A and 131A AIA passbands. Given the paucity of solar observations
adequate for this purpose, we use high-resolution X-ray spectra of the
low-activity solar-like corona of Procyon obtained with the Chandra Low Energy
Transmission Grating Spectrometer (LETGS). We find that while spectral models
overall can reproduce quite well the observed spectra in the soft X-ray range
ll 130A, they significantly underestimate
the observed flux in the 50-130A wavelength range. The model underestimates the
observed flux by a variable factor ranging from \approx 1.5, at short
wavelengths below \sim50A, up to \approx5-7 in the \sim 70-125A range. In the
AIA bands covered by LETGS, i.e. 94A and 131A, we find that the observed flux
can be underestimated by large factors (\sim 3 and \sim 1.9 respectively, for
the case of Procyon presented here). We discuss the consequences for analysis
of AIA data and possible empirical corrections to the AIA responses to model
more realistically the coronal emission in these passbands.Comment: 11 pages, 9 figures, accepted for publication on Ap
Study of fuel cells using storable rocket propellants quarterly report no. 2, 18 may - 17 aug. 1965
Catalysts for Aerozine-50 reforming and nitrogen tetroxide decomposition for development of rocket fuel cells operating on storable propellan
Optical Discovery of Probable Stellar Tidal Disruption Flares
Using archival Sloan Digital Sky Survey (SDSS) multi-epoch imaging data (Stripe 82), we have searched for the tidal disruption of stars by supermassive black holes in non-active galaxies. Two candidate tidal disruption events (TDEs) are identified. The TDE flares have optical blackbody temperatures of 2 × 10^4 K and observed peak luminosities of M_g = –18.3 and –20.4 (νL_ν = 5 × 10^(42), 4 × 10^(43) erg s^(–1), in the rest frame); their cooling rates are very low, qualitatively consistent with expectations for tidal disruption flares. The properties of the TDE candidates are examined using (1) SDSS imaging to compare them to other flares observed in the search, (2) UV emission measured by GALEX, and (3) spectra of the hosts and of one of the flares. Our pipeline excludes optically identifiable AGN hosts, and our variability monitoring over nine years provides strong evidence that these are not flares in hidden AGNs. The spectra and color evolution of the flares are unlike any SN observed to date, their strong late-time UV emission is particularly distinctive, and they are nuclear at high resolution arguing against these being first cases of a previously unobserved class of SNe or more extreme examples of known SN types. Taken together, the observed properties are difficult to reconcile with an SN or an AGN-flare explanation, although an entirely new process specific to the inner few hundred parsecs of non-active galaxies cannot be excluded. Based on our observed rate, we infer that hundreds or thousands of TDEs will be present in current and next-generation optical synoptic surveys. Using the approach outlined here, a TDE candidate sample with O(1) purity can be selected using geometric resolution and host and flare color alone, demonstrating that a campaign to create a large sample of TDEs, with immediate and detailed multi-wavelength follow-up, is feasible. A by-product of this work is quantification of the power spectrum of extreme flares in AGNs
From X-ray dips to eclipse: Witnessing disk reformation in the recurrent nova USco
The 10th recorded outburst of the recurrent eclipsing nova USco was observed
simultaneously in X-ray, UV, and optical by XMM-Newton on days 22.9 and 34.9
after outburst. Two full passages of the companion in front of the nova ejecta
were observed, witnessing the reformation of the accretion disk. On day 22.9,
we observed smooth eclipses in UV and optical but deep dips in the X-ray light
curve which disappeared by day 34.9, then yielding clean eclipses in all bands.
X-ray dips can be caused by clumpy absorbing material that intersects the line
of sight while moving along highly elliptical trajectories. Cold material from
the companion could explain the absence of dips in UV and optical light. The
disappearance of X-ray dips before day 34.9 implies significant progress in the
formation of the disk. The X-ray spectra contain photospheric continuum
emission plus strong emission lines, but no clear absorption lines. Both
continuum and emission lines in the X-ray spectra indicate a temperature
increase from day 22.9 to day 34.9. We find clear evidence in the spectra and
light curves for Thompson scattering of the photospheric emission from the
white dwarf. Photospheric absorption lines can be smeared out during scattering
in a plasma of fast electrons. We also find spectral signatures of resonant
line scattering that lead to the observation of the strong emission lines.
Their dominance could be a general phenomenon in high-inclination systems such
as Cal87.Comment: Submitted to ApJ. 16 pages, 16 figure
Chandra/ACIS-I study of the X-ray properties of the NGC 6611 and M16 stellar population
Mechanisms regulating the origin of X-rays in YSOs and the correlation with
their evolutionary stage are under debate. Studies of the X-ray properties in
young clusters allow to understand these mechanisms. One ideal target for this
analysis is the Eagle Nebula (M16), with its central cluster NGC6611. At 1750
pc from the Sun, it harbors 93 OB stars, together with a population of low-mass
stars from embedded protostars to disk-less Class III objects, with age <=
3Myrs. We study an archival 78 ksec Chandra/ACIS-I observation of NGC6611, and
two new 80ksec observations of the outer region of M16, one centered on the
Column V, and one on a region of the molecular cloud with ongoing
star-formation. We detect 1755 point sources, with 1183 candidate cluster
members (219 disk-bearing and 964 disk-less). We study the global X-ray
properties of M16 and compare them with those of the Orion Nebula Cluster. We
also compare the level of X-ray emission of Class II and Class III stars, and
analyze the X-ray spectral properties of OB stars. Our study supports the lower
level of X-ray activity for the disk-bearing stars with respect to the
disk-less members. The X-ray Luminosity Function (XLF) of M16 is similar to
that of Orion, supporting the universality of the XLF in young clusters. 85% of
the O stars of NGC6611 have been detected in X-rays. With only one possible
exception, they show soft spectra with no hard component, indicating that
mechanisms for the production of hard X-ray emission in O stars are not
operating in NGC 6611.Comment: Accepted in Ap
Stellar X-ray sources in the Chandra COSMOS survey
We present an analysis of the X-ray properties of a sample of solar- and
late-type field stars identified in the Chandra Cosmic Evolution Survey
(COSMOS), a deep (160ks) and wide (0.9 deg2) extragalactic survey. The sample
of 60 sources was identified using both morphological and photometric
star/galaxy separation methods. We determine X-ray count rates, extract spectra
and light curves and perform spectral fits to determine fluxes and plasma
temperatures. Complementary optical and near-IR photometry is also presented
and combined with spectroscopy for 48 of the sources to determine spectral
types and distances for the sample. We find distances ranging from 30pc to
~12kpc, including a number of the most distant and highly active stellar X-ray
sources ever detected. This stellar sample extends the known coverage of the
L_X-distance plane to greater distances and higher luminosities, but we do not
detect as many intrinsically faint X-ray sources compared to previous surveys.
Overall the sample is typically more luminous than the active Sun, representing
the high-luminosity end of the disk and halo X-ray luminosity functions. The
halo population appears to include both low-activity spectrally hard sources
that may be emitting through thermal bremsstrahlung, as well as a number of
highly active sources in close binaries.Comment: 13 pages, 5 figures, accepted for publication in Ap
The EUVE point of view of AD Leo
All the Extreme Ultraviolet Explorer (EUVE) observations of AD Leo, totalling
1.1 Ms of exposure time, have been employed to analyze the corona of this
single M dwarf. The light curves show a well defined quiescent stage, and a
distribution of amplitude of variability following a power law with a ~-2.4
index. The flaring behavior exhibits much similarity with other M active stars
like FK Aqr or YY Gem, and flares behave differently from late type active
giants and subgiants. The Emission Measure Distribution (EMD) of the summed
spectrum, as well as that of quiescent and flaring stages, were obtained using
a line-based method. The average EMD is dominated by material at log T(K)~6.9,
with a second peak around log T(K)~6.3, and a large increase in the amount of
material with log T(K)>~7.1 during flares, material almost absent during
quiescence. The results are interpreted as the combination of three families of
loops with maximum temperatures at log T(K)~6.3, ~6.9 and somewhere beyond log
T(K)>~7.1. A value of the abundance of [Ne/Fe]=1.05+-0.08 was measured at log
T(K)~5.9. No significative increment of Neon abundance was detected between
quiescence and flaring states.Comment: Full PS version can be found also at
http://www.astropa.unipa.it/~jsanz/papers0002.htm
SN 2008iy: An Unusual Type IIn Supernova with an Enduring 400 Day Rise Time
We present spectroscopic and photometric observations of the Type IIn
supernova (SN) 2008iy. SN 2008iy showed an unprecedentedly long rise time of
~400 days, making it the first SN to take significantly longer than 100 days to
reach peak optical luminosity. The peak absolute magnitude of SN 2008iy was M_r
~ -19.1 mag, and the total radiated energy over the first ~700 days was ~2 x
10^50 erg. Spectroscopically, SN 2008iy is very similar to the Type IIn SN
1988Z at late times, and, like SN 1988Z, it is a luminous X-ray source (both
supernovae had an X-ray luminosity L_ X > 10^41 erg/s). The Halpha emission
profile of SN 2008iy shows a narrow P Cygni absorption component, implying a
pre-SN wind speed of ~100 km/s. We argue that the luminosity of SN 2008iy is
powered via the interaction of the SN ejecta with a dense, clumpy circumstellar
medium. The ~400 day rise time can be understood if the number density of
clumps increases with distance over a radius ~1.7 x 10^16 cm from the
progenitor. This scenario is possible if the progenitor experienced an episodic
phase of enhanced mass-loss < 1 century prior to explosion or the progenitor
wind speed increased during the decades before core collapse. We favour the
former scenario, which is reminiscent of the eruptive mass-loss episodes
observed for luminous blue variable (LBV) stars. The progenitor wind speed and
increased mass-loss rates serve as further evidence that at least some, and
perhaps all, Type IIn supernovae experience LBV-like eruptions shortly before
core collapse. We also discuss the host galaxy of SN 2008iy, a subluminous
dwarf galaxy, and offer a few reasons why the recent suggestion that unusual,
luminous supernovae preferentially occur in dwarf galaxies may be the result of
observational biases.Comment: 15 pages, 5 figures, MNRAS accepte
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