282 research outputs found
The Patterns of High-Level Magnetic Activity Occurring on the Surface of V1285 Aql: The OPEA Model of Flares and DFT Models of Stellar Spots
Statistically analyzing Johnson UBVR observations of V1285 Aql during the
three observing seasons, both activity level and behavior of the star are
discussed in respect to obtained results. We also discuss the out-of-flare
variation due to rotational modulation. Eighty-three flares were detected in
the U-band observations of season 2006 . First, depending on statistical
analyses using the independent samples t-test, the flares were divided into two
classes as the fast and the slow flares. According to the results of the test,
there is a difference of about 73 s between the flare-equivalent durations of
slow and fast flares. The difference should be the difference mentioned in the
theoretical models. Second, using the one-phase exponential association
function, the distribution of the flare-equivalent durations versus the flare
total durations was modeled. Analyzing the model, some parameters such as
plateau, half-life values, mean average of the flare-equivalent durations,
maximum flare rise, and total duration times are derived. The plateau value,
which is an indicator of the saturation level of white-light flares, was
derived as 2.421{\pm}0.058 s in this model, while half-life is computed as 201
s. Analyses showed that observed maximum value of flare total duration is 4641
s, while observed maximum flare rise time is 1817 s. According to these
results, although computed energies of the flares occurring on the surface of
V1285 Aql are generally lower than those of other stars, the length of its
flaring loop can be higher than those of more active stars.Comment: 44 pages, 10 figures, 5 tables, 2011PASP..123..659
X-Ray Spectroscopy of II Pegasi: Coronal Temperature Structure, Abundances, and Variability
We have obtained high resolution X-ray spectra of the coronally active
binary, II Pegasi (HD 224085), covering the wavelength range of 1.5-25
Angstroms. For the first half of our 44 ksec observation, the source was in a
quiescent state with constant X-ray flux, after which it flared, reaching twice
the quiescent flux in 12 ksec, then decreasing. We analyze the emission-line
spectrum and continuum during quiescent and flaring states. The differential
emission measure derived from lines fluxes shows a hot corona with a continuous
distribution in temperature. During the non-flare state, the distribution peaks
near log T = 7.2, and when flaring, near 7.6. High-temperature lines are
enhanced slightly during the flare, but most of the change occurs in the
continuum. Coronal abundance anomalies are apparent, with iron very deficient
relative to oxygen and significantly weaker than expected from photospheric
measurements, while neon is enhanced relative to oxygen. We find no evidence of
appreciable resonant scattering optical depth in line ratios of iron and
oxygen. The flare light curve is consistent with Solar two-ribbon flare models,
but with a very long reconnection time-constant of about 65 ks. We infer loop
lengths of about 0.05 stellar radii, to about 0.25 in the flare, if the flare
emission originated from a single, low-density loop.Comment: 25 pages, 5 figures, 3 tables, accepted by ApJ (scheduled for the
v559 n2 p1 Oct 1, 2001 issue
Where Are The M Dwarf Disks Older Than 10 Million Years?
We present 11.7-micron observations of nine late-type dwarfs obtained at the
Keck I 10-meter telescope in December 2002 and April 2003. Our targets were
selected for their youth or apparent IRAS 12-micron excess. For all nine
sources, excess infrared emission is not detected. We find that stellar wind
drag can dominate the circumstellar grain removal and plausibly explain the
dearth of M Dwarf systems older than 10 Myr with currently detected infrared
excesses. We predict M dwarfs possess fractional infrared excess on the order
of L_{IR}/L_{*}\sim10^{-6} and this may be detectable with future efforts.Comment: 24 pages, 2 figures, accepted to Ap
The Sun as an X-ray Star: III. Flares
In previous works we have developed a method to convert solar X-ray data,
collected with the Yohkoh/SXT, into templates of stellar coronal observations.
Here we apply the method to several solar flares, for comparison with stellar
X-ray flares. Eight flares, from weak (GOES class C5.8) to very intense ones
(X9) are selected as representative of the flaring Sun. The emission measure
distribution vs. temperature, EM(T), of the flaring regions is derived from
Yohkoh/SXT observations in the rise, peak and decay of the flares. The EM(T) is
rather peaked and centered around K for most of the time.
Typically, it grows during the rise phase of the flare, and then it decreases
and shifts toward lower temperatures during the decay, more slowly if there is
sustained heating. The most intense flare we studied shows emission measure
even at very high temperature ( K). Time-resolved X-ray spectra
both unfiltered and filtered through the instrumental responses of the
non-solar instruments ASCA/SIS and ROSAT/PSPC are then derived. Synthesized
ASCA/SIS and ROSAT/PSPC spectra are generally well fitted with single thermal
components at temperatures close to that of the EM(T) maximum, albeit two
thermal components are needed to fit some flare decays. ROSAT/PSPC spectra show
that solar flares are in a two-orders of magnitude flux range (
erg cm s) and a narrow PSPC hardness ratio range, however higher
than that of typical non-flaring solar-like stars.Comment: 32 pages, 8 figures, 3 table
A spatially resolved limb flare on Algol B observed with XMM-Newton
We report XMM-Newton observations of the eclipsing binary Algol A (B8V) and B
(K2III). The XMM-Newton data cover the phase interval 0.35 - 0.58, i.e.,
specifically the time of optical secondary minimum, when the X-ray dark B-type
star occults a major fraction of the X-ray bright K-type star. During the
eclipse a flare was observed with complete light curve coverage. The decay part
of the flare can be well described with an exponential decay law allowing a
rectification of the light curve and a reconstruction of the flaring plasma
region. The flare occurred near the limb of Algol B at a height of about 0.1R
with plasma densities of a few times 10^11 cm^-3 consistent with spectroscopic
density estimates. No eclipse of the quiescent X-ray emission is observed
leading us to the conclusion that the overall coronal filling factor of Algol B
is small.Comment: 8 pages, 7 figures, accepted by A&
Bayesian Estimation of Hardness Ratios: Modeling and Computations
A commonly used measure to summarize the nature of a photon spectrum is the
so-called Hardness Ratio, which compares the number of counts observed in
different passbands. The hardness ratio is especially useful to distinguish
between and categorize weak sources as a proxy for detailed spectral fitting.
However, in this regime classical methods of error propagation fail, and the
estimates of spectral hardness become unreliable. Here we develop a rigorous
statistical treatment of hardness ratios that properly deals with detected
photons as independent Poisson random variables and correctly deals with the
non-Gaussian nature of the error propagation. The method is Bayesian in nature,
and thus can be generalized to carry out a multitude of
source-population--based analyses. We verify our method with simulation
studies, and compare it with the classical method. We apply this method to real
world examples, such as the identification of candidate quiescent Low-mass
X-ray binaries in globular clusters, and tracking the time evolution of a flare
on a low-mass star.Comment: 43 pages, 10 figures, 3 tables; submitted to Ap
Flare energetics: analysis of a large flare on YZ Canis Minoris observed simultaneously in the ultraviolet, optical and radio.
The results of coordinated observations of the dMe star YZ CMi at optical, UV and radio wavelengths during 3-7 February 1983 are presented. YZ CMi showed repeated optical flaring with the largest flare having a magnitude of 3.8 in the U-band. This flare coincided with an IUE exposure which permits a comparison of the emission measure curves of YZ CMi in its flaring and quiescent state. During the flare a downward shift of the transition zone is observed while the radiative losses in the range 10^4^-10^7^K strongly increase. The optical flare is accompanied with a radio flare at 6cm, while at 20cm no emission is detected. The flare is interpreted in terms of optically thick synchrotron emission. We present a combined interpretation of the optical/radio flare and show that the flare can be interpreted within the context of solar two-ribbon/white-light flares. Special attention is paid to the bombardment of dMe atmospheres by particle beams. We show that the characteristic temperature of the heated atmosphere is almost independent of the beam flux and lies within the range of solar white-light flare temperatures. We also show that it is unlikely that stellar flares emit black-body spectra. The fraction of accelerated particles, as follows from our combined optical/radio interpretation is in good agreement with the fraction determined by two-ribbon flare reconnection models
A Search for the Near-Infrared Counterpart to GCRT J1745-3009
We present an optical/near-infrared search for a counterpart to the
perplexing radio transient GCRT J1745-3009, a source located ~1 degree from the
Galactic Center. Motivated by some similarities to radio bursts from nearby
ultracool dwarfs, and by a distance upper limit of 70 pc for the emission to
not violate the 1e12 K brightness temperature limit for incoherent radiation,
we searched for a nearby star at the position of GCRT J1745-3009. We found only
a single marginal candidate, limiting the presence of any late-type star to >1
kpc (spectral types earlier than M9), >200 pc (spectral types L and T0-T4), and
>100 pc (spectral types T4-T7), thus severely restricting the possible local
counterparts to GCRT J1745-3009. We also exclude any white dwarf within 1 kpc
or a supergiant star out to the distance of the Galactic Center as possible
counterparts. This implies that GCRT J1745-3009 likely requires a coherent
emission process, although whether or not it reflects a new class of sources is
unclear.Comment: 10 pages, 5 figures. Accepted for publication in the Astrophysical
Journa
X-Ray Flaring on the dMe Star, Ross 154
We present results from two Chandra imaging observations of Ross 154, a
nearby flaring M dwarf star. During a 61-ks ACIS-S exposure, a very large flare
occurred (the equivalent of a solar X3400 event, with L_X = 1.8x10^30 ergs/s)
in which the count rate increased by a factor of over 100. The early phase of
the flare shows evidence for the Neupert effect, followed by a further rise and
then a two-component exponential decay. A large flare was also observed at the
end of a later 48-ks HRC-I observation. Emission from the non-flaring phases of
both observations was analyzed for evidence of low level flaring. From these
temporal studies we find that microflaring probably accounts for most of the
`quiescent' emission, and that, unlike for the Sun and the handful of other
stars that have been studied, the distribution of flare intensities does not
appear to follow a power-law with a single index. Analysis of the ACIS spectra,
which was complicated by exclusion of the heavily piled-up source core,
suggests that the quiescent Ne/O abundance ratio is enhanced by a factor of
~2.5 compared to the commonly adopted solar abundance ratio, and that the Ne/O
ratio and overall coronal metallicity during the flare appear to be enhanced
relative to quiescent abundances. Based on the temperatures and emission
measures derived from the spectral fits, we estimate the length scales and
plasma densities in the flaring volume and also track the evolution of the
flare in color-intensity space. Lastly, we searched for a stellar-wind
charge-exchange X-ray halo around the star but without success; because of the
relationship between mass-loss rate and the halo surface brightness, not even
an upper limit on the stellar mass-loss rate can be determined.Comment: 20 pages, 12 figures (4 color), accepted by ApJ, expected publication
April 1, 200
Detection of hard X-rays from a Class I protostar in the HH24-26 region in the Orion Molecular Cloud
We observed the HH24-26 region in the L1630 Orion molecular cloud complex
with the X-ray observatory ASCA in the 0.510 keV band. X-ray emission was
detected from the T Tauri star SSV61 and from the region where the Class I
protostars
SSV63E and SSV63W are located (hereafter SSV63E+W). The spectra of both
SSV63E+W and SSV61 are well explained by an optically thin thermal plasma
model. The spectrum of the T Tauri star SSV61 has a low temperature of
(0.71.2) keV and a moderate absorption of (0.91.7)
cm, while that of the protostar SSV63E+W has a high
temperature of (3.37.9) keV and a heavy absorption of
(1.21.8) cm. The X-ray light curve
of SSV63E+W showed a flare during the observation. The peak flux reached about
9 times that of the quiescent flux. The temperature and the absorption column
density do not change conspicuously during the flare. The 0.510 keV
luminosity of SSV63E+W was about erg s in the quiescent
state. The present detection of hard X-rays from SSV63E+W is remarkable,
because this is the first X-ray detection of a protostar in Orion.Comment: 14 pages, 3 postscript figures, uses aasms4.st
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