86 research outputs found
Contemporaneous XMM-Newton investigation of a giant X-ray flare and quiescent state from a cool M-class dwarf in the local cavity
We report the serendipitous detection of a giant X-ray flare from the source
2XMM J043527.2-144301 during an XMM-Newton observation of the high latitude
molecular cloud MBM20. The source has not been previously studied at any
wavelength. The X-ray flux increases by a factor of more than 52 from quiescent
state to peak of flare. A 2MASS counterpart has been identified (2MASS
J04352724-1443017), and near-infrared colors reveal a spectral type of M8-M8.5
and a distance of (67\pm 13) pc, placing the source in front of MBM20. Spectral
analysis and source luminosity are also consistent with this conclusion. The
measured distance makes this object the most distant source (by about a factor
of 4) at this spectral type detected in X-rays. The X-ray flare was
characterized by peak X-ray luminosity of ~8.2E28 erg s-1 and integrated X-ray
energy of ~2.3E32 erg. The flare emission has been characterized with a
2-temperature model with temperatures of ~10 and 46 MK (0.82 and 3.97 keV), and
is dominated by the higher temperature component.Comment: 19 pages, 5 figures; Accepted for publication on Ap
Suzaku Discovery of the Strong Radiative Recombination Continuum of Iron from the Supernova Remnant W49B
We present a hard X-ray spectrum of unprecedented quality of the Galactic
supernova remnant W49B obtained with the Suzaku satellite. The spectrum
exhibits an unusual structure consisting of a saw-edged bump above 8 keV. This
bump cannot be explained by any combination of high-temperature plasmas in
ionization equilibrium. We firmly conclude that this bump is caused by the
strong radiative recombination continuum (RRC) of iron, detected for the first
time in a supernova remnant. The electron temperature derived from the
bremsstrahlung continuum shape and the slope of the RRC is 1.5 keV. On the
other hand, the ionization temperature derived from the observed intensity
ratios between the RRC and K-alpha lines of iron is 2.7 keV. These results
indicate that the plasma is in a highly overionized state. Volume emission
measures independently determined from the fluxes of the thermal and RRC
components are consistent with each other, suggesting the same origin of these
components.Comment: 5 pages,4 figures, accepted for publication in ApJ Lette
Powerful Winds from Low-Mass Stars: V374 Peg
The rapid rotation (P=0.44 d) of the M dwarf V374Peg (M4) along with its
intense magnetic field point toward magneto-centrifugal acceleration of a
coronal wind. In this work, we investigate the structure of the wind of V374Peg
by means of 3D magnetohydrodynamical (MHD) numerical simulations. For the first
time, an observationally derived surface magnetic field map is implemented in
MHD models of stellar winds for a low mass star. We show that the wind of
V374Peg deviates greatly from a low-velocity, low-mass-loss rate solar-type
wind. We find general scaling relations for the terminal velocities, mass-loss
rates, and spin-down times of highly magnetized M dwarfs. In particular, for
V374Peg, our models show that terminal velocities across a range of stellar
latitudes reach ~(1500-2300) n_{12}^{-1/2} km/s, where n_{12} is the coronal
wind base density in units of 10^{12} cm^{-3}, while the mass-loss rates are
about 4 x 10^{-10} n_{12}^{1/2} Msun/yr. We also evaluate the angular-momentum
loss of V374Peg, which presents a rotational braking timescale ~28
n_{12}^{-1/2} Myr. Compared to observationally derived values from period
distributions of stars in open clusters, this suggests that V374Peg may have
low coronal base densities (< 10^{11} cm^{-3}). We show that the wind ram
pressure of V374Peg is about 5 orders of magnitude larger than for the solar
wind. Nevertheless, a small planetary magnetic field intensity (~ 0.1G) is able
to shield a planet orbiting at 1 AU against the erosive effects of the stellar
wind. However, planets orbiting inside the habitable zone of V374Peg, where the
wind ram pressure is higher, might be facing a more significant atmospheric
erosion. In that case, higher planetary magnetic fields of, at least, about
half the magnetic field intensity of Jupiter, are required to protect the
planet's atmosphere.Comment: 13 pages, 5 figures, 1 table. MNRAS in pres
Structure of the X-ray Emission from the Jet of 3C 273
We present images from five observations of the quasar 3C 273 with the
Chandra X-ray Observatory. The jet has at least four distinct features which
are not resolved in previous observations. The first knot in the jet (A1) is
very bright in X-rays. Its X-ray spectrum is well fitted with a power law with
alpha = 0.60 +/- 0.05. Combining this measurement with lower frequency data
shows that a pure synchrotron model can fit the spectrum of this knot from
1.647 GHz to 5 keV (over nine decades in energy) with alpha = 0.76 +/- 0.02,
similar to the X-ray spectral slope. Thus, we place a lower limit on the total
power radiated by this knot of 1.5e43 erg/s; substantially more power may be
emitted in the hard X-ray and gamma-ray bands.
Knot A2 is also detected and is somewhat blended with knot B1. Synchrotron
emission may also explain the X-ray emission but a spectral bend is required
near the optical band. For knots A1 and B1, the X-ray flux dominates the
emitted energy. For the remaining optical knots (C through H), localized X-ray
enhancements that might correspond to the optical features are not clearly
resolved. The position angle of the jet ridge line follows the optical shape
with distinct, aperiodic excursions of +/-1 deg from a median value of
-138.0deg. Finally, we find X-ray emission from the ``inner jet'' between 5 and
10" from the core.Comment: 10 pages, 5 figures; accepted for publication in the Astrophysical
Journal Letters. For the color image, see fig1.ps or
http://space.mit.edu/~hermanm/papers/3c273/fig1.jp
First Light Measurements of Capella with the Low Energy Transmission Grating Spectrometer aboard the Chandra X-ray Observatory
We present the first X-ray spectrum obtained by the Low Energy Transmission
Grating Spectrometer (LETGS) aboard the Chandra X-ray Observatory. The spectrum
is of Capella and covers a wavelength range of 5-175 A (2.5-0.07 keV). The
measured wavelength resolution, which is in good agreement with ground
calibration, is 0.06 A (FWHM). Although in-flight
calibration of the LETGS is in progress, the high spectral resolution and
unique wavelength coverage of the LETGS are well demonstrated by the results
from Capella, a coronal source rich in spectral emission lines. While the
primary purpose of this letter is to demonstrate the spectroscopic potential of
the LETGS, we also briefly present some preliminary astrophysical results. We
discuss plasma parameters derived from line ratios in narrow spectral bands,
such as the electron density diagnostics of the He-like triplets of carbon,
nitrogen, and oxygen, as well as resonance scattering of the strong Fe XVII
line at 15.014 A.Comment: 4 pages (ApJ letter LaTeX), 2 PostScript figures, accepted for
publication in ApJ Letters, 200
Multi-wavelength observations of Proxima Centauri
We report simultaneous observations of the nearby flare star Proxima Centauri
with VLT/UVES and XMM-Newton over three nights in March 2009. Our optical and
X-ray observations cover the star's quiescent state, as well as its flaring
activity and allow us to probe the stellar atmospheric conditions from the
photosphere into the chromosphere, and then the corona during its different
activity stages. Using the X-ray data, we investigate variations in coronal
densities and abundances and infer loop properties for an intermediate-sized
flare. The optical data are used to investigate the magnetic field and its
possible variability, to construct an emission line list for the chromosphere,
and use certain emission lines to construct physical models of Proxima
Centauri's chromosphere.
We report the discovery of a weak optical forbidden Fe xiii line at 3388 AA
during the more active states of Proxima Centauri. For the intermediate flare,
we find two secondary flare events that may originate in neighbouring loops,
and discuss the line asymmetries observed during this flare in H i, He i, and
Ca ii lines. The high time-resolution in the H alpha line highlights strong
temporal variations in the observed line asymmetries, which re-appear during a
secondary flare event. We also present theoretical modelling with the stellar
atmosphere code PHOENIX to construct flaring chromospheric models.Comment: 19 pages, 22 figures, accepted by A&
Perspectives on Astrophysics Based on Atomic, Molecular, and Optical (AMO) Techniques
About two generations ago, a large part of AMO science was dominated by
experimental high energy collision studies and perturbative theoretical
methods. Since then, AMO science has undergone a transition and is now
dominated by quantum, ultracold, and ultrafast studies. But in the process, the
field has passed over the complexity that lies between these two extremes. Most
of the Universe resides in this intermediate region. We put forward that the
next frontier for AMO science is to explore the AMO complexity that describes
most of the Cosmos.Comment: White paper submission to the Decadal Assessment and Outlook Report
on Atomic, Molecular, and Optical (AMO) Science (AMO 2020
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