1,332 research outputs found
A systematic search for novae in M31 on a large set of digitized archival Schmidt plates
This paper reports on the detection of optical novae in our neighbour galaxy
M31 based on digitized historical Tautenburg Schmidt plates. The accurate
positions of the detected novae lead to a much larger database when searching
for recurrent novae in M31. We conducted a systematic search for novae on 306
digitized Tautenburg Schmidt plates covering a time span of 36 years from 1960
to 1996. From the database of both ~ 300 000 light curves and about one million
detections on only one plate per colour band, nova candidates were efficiently
selected by automated algorithms and subsequently individually inspected by
eye. We report the detection of 84 nova candidates. We found 55 nova candidates
from the automated analysis of the light curves. Among these, 22 were
previously unknown, 12 were known but not identified on Tautenburg Schmidt
plates before, and 21 novae had been previously discovered on Tautenburg
plates. An additional 29 known novae could be confirmed by the detailed
investigation of single detections. One of our newly discovered nova candidates
shows a high position coincidence with a nova detected about 30 years earlier.
Therefore, this object is likely to be a recurrent nova. Furthermore, we
re-investigated all 41 nova candidates previously found on Tautenburg plates
and confirm all but two. Positions are given for all nova candidates with a
typical accuracy of ~ 0.4 arcsec. We present light curves and finding charts as
online material. The analysis of the plates has shown the wealth of information
still buried in old plate archives. Extrapolating from this survey,
digitization of other historical M31 plate archives (e.g. from the Mount Wilson
or Asiago observatories) for a systematic nova search looks very promising.Comment: 29 pages, 14 figures, 19 tables, accepted for publication in A&A.
Figs 6-14 are reduced in resolution due to the restrictions on space
available on astro-ph; v2: minor grammatical change
Development of the adjoint of GEOS-Chem
We present the adjoint of the global chemical transport model GEOS-Chem, focusing on the chemical and thermodynamic relationships between sulfate – ammonium – nitrate aerosols and their gas-phase precursors. The adjoint model is constructed from a combination of manually and automatically derived discrete adjoint algorithms and numerical solutions to continuous adjoint equations. Explicit inclusion of the processes that govern secondary formation of inorganic aerosol is shown to afford efficient calculation of model sensitivities such as the dependence of sulfate and nitrate aerosol concentrations on emissions of SOx, NOx, and NH3. The adjoint model is extensively validated by comparing adjoint to finite difference sensitivities, which are shown to agree within acceptable tolerances; most sets of comparisons have a nearly 1:1 correlation and R2>0.9. We explore the robustness of these results, noting how insufficient observations or nonlinearities in the advection routine can degrade the adjoint model performance. The potential for inverse modeling using the adjoint of GEOS-Chem is assessed in a data assimilation framework through a series of tests using simulated observations, demonstrating the feasibility of exploiting gas- and aerosol-phase measurements for optimizing emission inventories of aerosol precursors
Global modeling of secondary organic aerosol formation from aromatic hydrocarbons: high- vs low-yield pathways
Formation of SOA from the aromatic species toluene, xylene, and, for the first time, benzene, is added to a global chemical transport model. A simple mechanism is presented that accounts for competition between low and high-yield pathways of SOA formation, wherein secondary gas-phase products react further with either nitrogen oxide (NO) or hydroperoxy radical (HO2) to yield semi- or non-volatile products, respectively. Aromatic species yield more SOA when they react with OH in regions where the [NO]/[HO2] ratios are lower. The SOA yield thus depends upon the distribution of aromatic emissions, with biomass burning emissions being in areas with lower [NO]/[HO2] ratios, and the reactivity of the aromatic with respect to OH, as a lower initial reactivity allows transport away from industrial source regions, where [NO]/[HO2] ratios are higher, to more remote regions, where this ratio is lower and, hence, the ultimate yield of SOA is higher. As a result, benzene is estimated to be the most important aromatic species with regards to formation of SOA, with a total production nearly equal that of toluene and xylene combined. In total, while only 39% percent of the aromatic species react via the low-NOx pathway, 72% of the aromatic SOA is formed via this mechanism. Predicted SOA concentrations from aromatics in the Eastern United States and Eastern Europe are actually largest during the summer, when the [NO]/[HO2] ratio is lower. Global production of SOA from aromatic sources is estimated at 3.5 Tg/yr, resulting in a global burden of 0.08 Tg, twice as large as previous estimates. The contribution of these largely anthropogenic sources to global SOA is still small relative to biogenic sources, which are estimated to comprise 90% of the global SOA burden, about half of which comes from isoprene. Compared to recent observations, it would appear there are additional pathways beyond those accounted for here for production of anthropogenic SOA. However, owing to differences in spatial distributions of sources and seasons of peak production, there are still regions in which aromatic SOA produced via the mechanisms identified here are predicted to contribute substantially to, and even dominate, the local SOA concentrations, such as outflow regions from North America and South East Asia during the wintertime, though total SOA concentrations there are small (~0.1 μg/m^³)
Development of the adjoint of GEOS-Chem
International audienceWe present the adjoint of the global chemical transport model GEOS-Chem, focusing on the chemical and thermodynamic relationships between sulfate ? ammonium ? nitrate aerosols and their gas-phase precursors. The adjoint model is constructed from a combination of manually and automatically derived discrete adjoint algorithms and numerical solutions to continuous adjoint equations. Explicit inclusion of the processes that govern secondary formation of inorganic aerosol is shown to afford efficient calculation of model sensitivities such as the dependence of sulfate and nitrate aerosol concentrations on emissions of SOx, NOx, and NH3. The accuracy of the adjoint model is extensively verified by comparing adjoint to finite difference sensitivities, which are shown to agree within acceptable tolerances. We explore the robustness of these results, noting how discontinuities in the advection routine hinder, but do not entirely preclude, the use of such comparisons for validation of the adjoint model. The potential for inverse modeling using the adjoint of GEOS-Chem is assessed in a data assimilation framework using simulated observations, demonstrating the feasibility of exploiting gas- and aerosol-phase measurements for optimizing emission inventories of aerosol precursors
J004457+4123 (Sharov 21): not a remarkable nova in M31 but a background quasar with a spectacular UV flare
We announce the discovery of a quasar behind the disk of M31, which was
previously classified as a remarkable nova in our neighbour galaxy. The paper
is primarily aimed at the outburst of J004457+4123 (Sharov 21), with the first
part focussed on the optical spectroscopy and the improvement in the
photometric database. Both the optical spectrum and the broad band spectral
energy distribution of Sharov 21 are shown to be very similar to that of
normal, radio-quiet type 1 quasars. We present photometric data covering more
than a century and resulting in a long-term light curve that is densely sampled
over the past five decades. The variability of the quasar is characterized by a
ground state with typical fluctuation amplitudes of ~0.2 mag around B~20.5,
superimposed by a singular flare of ~2 yr duration (observer frame) with the
maximum at 1992.81 where the UV flux has increased by a factor of ~20. The
total energy in the flare is at least three orders of magnitudes higher than
the radiated energy of the most luminous supernovae, provided that it comes
from an intrinsic process and the energy is radiated isotropically. The profile
of the flare light curve appears to be in agreement with the standard
predictions for a stellar tidal disruption event where a ~10 M_sun giant star
was shredded in the tidal field of a ~2...5 10^8 M_sun black hole. The short
fallback time derived from the light curve requires an ultra-close encounter
where the pericentre of the stellar orbit is deep within the tidal disruption
radius. Gravitational microlensing provides an alternative explanation, though
the probability of such a high amplification event is very low.Comment: Accepted for publication in Astronomy and Astrophysics, 14 pages, 11
figure
CXOM31 J004253.1+411422: The first ultra-luminous X-ray transient in M 31
We seek clarification of the nature of X-ray sources detected in M 31. Here
we focus on CXOM31 J004253.1+411422, the brightness of which suggests that it
belongs to the class of ultraluminous X-ray sources. We determine the X-ray
properties of sources detected in the XMM-Newton Chandra monitoring program. We
investigate spectral properties and search for periodic or quasi-periodic
oscillations. A multi-component model is applied to the spectra obtained from
XMM-Newton data to evaluate the relative contributions from thermal and
non-thermal emission. The time dependence of this ratio is evaluated over a
period of forty days. We simultaneously fit data from XMM-Newton EPIC-pn, MOS1
and MOS2 detectors with (non-thermal) powerlaw and (thermal) multicolored
blackbody. The X-ray spectrum is best fit by the combination of a thermal
component with kT ~ 1 keV and a powerlaw component with photon index
approximately 2.6. From combined analysis of Chandra, Swift and XMM-Newton
data, the unabsorbed total luminosity of this source decreases from ~ 3.8 x
10^{39} erg s^{-1} in the first observation to ~ 0.5 x 10^{39} ergs s^{-1} over
a period of three months. The decay closely follows an exponential decline with
a time constant of 32 days. The source spectrum evolves significantly,
exhibiting a faster decline of the thermal component. We do not find evidence
of any significant temporal features in the power density spectrum. The
presence of a thermal component at kT ~ 1 keV in conjunction with a non-thermal
high energy tail, is also consistent with spectral properties of other ULXs in
the "high state". Our analysis indicates that the underlying source of this
first ULX in M~31 is a black hole of mass, M > 13 M_{sun}, accreting near the
Eddington limit, that underwent a transient outburst followed by an exponential
decay reminiscent of transients associated with galactic X-ray novae.Comment: Accepted by Astronomy and Astrophysics. 6 pages, 8 figure
X-ray monitoring of classical novae in the central region of M 31. II. Autumn and winter 2007/2008 and 2008/2009
[Abridged] Classical novae (CNe) represent the major class of supersoft X-ray
sources (SSSs) in the central region of our neighbouring galaxy M 31. We
performed a dedicated monitoring of the M 31 central region with XMM-Newton and
Chandra between Nov 2007 and Feb 2008 and between Nov 2008 and Feb 2009
respectively, in order to find SSS counterparts of CNe, determine the duration
of their SSS phase and derive physical outburst parameters. We systematically
searched our data for X-ray counterparts of CNe and determined their X-ray
light curves and spectral properties. We detected in total 17 X-ray
counterparts of CNe in M 31, only four of which were known previously. These
latter sources are still active 12.5, 11.0, 7.4 and 4.8 years after the optical
outburst. From the 17 X-ray counterparts 13 were classified as SSSs. Four novae
displayed short SSS phases (< 100 d). Based on these results and previous
studies we compiled a catalogue of all novae with SSS counterparts in M 31
known so far. We used this catalogue to derive correlations between the
following X-ray and optical nova parameters: turn-on time, turn-off time,
effective temperature (X-ray), t2 decay time and expansion velocity of the
ejected envelope (optical). Furthermore, we found a first hint for the
existence of a difference between SSS parameters of novae associated with the
stellar populations of the M 31 bulge and disk. Additionally, we conducted a
Monte Carlo Markov Chain simulation on the intrinsic fraction of novae with SSS
phase. This simulation showed that the relatively high fraction of novae
without detected SSS emission might be explained by the inevitably incomplete
coverage with X-ray observations in combination with a large fraction of novae
with short SSS states, as expected from the WD mass distribution. In order to
verify our results with an increased sample further monitoring observations are
needed.Comment: 31 pages, 23 figures, 10 tables; submitted to A&
Recent discoveries of supersoft X-ray sources in M 31
Classical novae (CNe) have recently been reported to represent the major
class of supersoft X-ray sources (SSSs) in the central area of our neighbouring
galaxy M 31. This paper presents a review of results from recent X-ray
observations of M 31 with XMM-Newton and Chandra. We carried out a dedicated
optical and X-ray monitoring program of CNe and SSSs in the central area of M
31. We discovered the first SSSs in M 31 globular clusters (GCs) and their
connection to the very first discovered CN in a M 31 GC. This result may have
an impact on the CN rate in GCs. Furthermore, in our optical and X-ray
monitoring data we discovered the CN M31N 2007-11a, which shows a very short
SSS phase of 29 - 52 days. Short SSS states (durations < 100 days) of CNe
indicate massive white dwarfs (WDs) that are candidate progenitors of
supernovae type Ia. In the case of M31N 2007-11a, the optical and X-ray light
curves suggest a binary containing a WD with M_WD > 1.0 M_sun. Finally, we
present the discovery of the SSS counterpart of the CN M31N 2006-04a. The X-ray
light curve of M31N 2006-04a shows short-time variability, which might indicate
an orbital period of about 2 hours.Comment: 4 pages, 1 figure; Proc. of workshop "Supersoft X-ray Sources - New
Developments", ESAC, May 2009; accepted for publication in Astronomische
Nachrichte
Film calibration for the Skylab/ATM S-056 X-ray telescope
The sensitometry and film calibration effort for the Skylab/ATM S-056 X-ray telescope is summarized. The apparatus and procedures used are described together with the two types of flight film used, Kodak SO-212 and SO-242. The sensitometry and processing of the flight film are discussed, and the results are presented in the form of the characteristic curves and related data. The use of copy films is also discussed
- …