241 research outputs found
Chandra Observations of a Young Embedded Magnetic B Star in the rho Ophiuchus Cloud
This paper reports on an analysis of two Chandra X-ray observations of the
young magnetic B star rho Ophiuchus S1. X-ray emission from the star was
detected in both observations. The average flux was almost the same in both,
but during each observation the flux showed significant time variations by a
factor of two on timescales of 20-40 ks. Each spectrum could be fit by either
an absorbed power-law model with a photon index of ~3 or a thin-thermal plasma
model with a temperature of ~2 keV and an extremely low metal abundance (<~0.1
solar). The spectrum of the first observation has a weak-line feature at about
6.8 keV, which might correspond to highly ionized iron Kalpha. In contrast, the
spectrum of the second observation apparently shows a weak edge absorption
component at E~4 keV. The continuum emission and log LX/Lbol ~ -6 are similar
to those of young intermediate-mass stars (Herbig Ae/Be stars), although the
presence of a strong magnetic field (inferred from the detection of non-thermal
radio emission) has drawn an analogy between rho Ophiuchus S1 and magnetic
chemically peculiar (MCP) stars. If the X-ray emission is thermal, the small
abundances that we derived might be related to the inverse first-ionization
potential (FIP) effect, though there is no significant trend as a function of
FIP from our model fits. If the emission is non-thermal, it might be produced
by high-energy electrons in the magnetosphere.Comment: 13 pages, 6 figures, To appear in the October issue of PASJ (vol.55,
No. 5
Eclipse and Collapse of the Colliding Wind X-ray Emission from Eta Carinae
X-ray emission from the massive stellar binary system, Eta Carinae, drops strongly around periastron passage; the event is called the X-ray minimum. We launched a focused observing campaign in early 2009 to understand the mechanism of causing the X-ray minimum. During the campaign, hard X-ray emission (<10 keV) from Eta Carinae declined as in the previous minimum, though it recovered a month earlier. Extremely hard X-ray emission between 15-25 keV, closely monitored for the first time with the Suzaku HXD/PIN, decreased similarly to the hard X-rays, but it reached minimum only after hard X-ray emission from the star had already began to recover. This indicates that the X-ray minimum is produced by two composite mechanisms: the thick primary wind first obscured the hard, 210 keV thermal X-ray emission from the wind-wind collision (WWC) plasma; the WWC activity then decays as the two stars reach periastron
Centimeter Polarimetry of the R Coronae Australis Region
Circularly polarized 3.5 cm continuum emission was detected toward three
radio sources in the R CrA region using the Very Large Array. The Class I
protostar IRS 5b persistently showed polarized radio emission with a constant
helicity over 8 yr, which suggests that its magnetosphere has a stable
configuration. There is a good correlation between the Stokes I and Stokes V
fluxes, and the fractional polarization is about 0.17. During active phases the
fractional polarization is a weakly decreasing function of Stokes I flux, which
suggests that IRS 5b is phenomenologically similar to other types of flare
stars such as RS CVn binaries. The variability timescale of the polarized flux
is about a month, and the magnetosphere of IRS 5b must be very large in size.
The Class I protostar IRS 7A was detected once in circularly polarized radio
emission, even though IRS 7A drives a thermal radio jet. This detection implies
that the radio emission from the magnetosphere of a young protostar can escape
the absorption by the partially ionized wind at least once in a while. The
properties of IRS 7A and IRS 5b suggests that Class I protostars have organized
peristellar magnetic fields of a few kilogauss and that the detectability of
magnetospheric emission may depend on the evolutionary status of protostar.
Also reported is the detection of circularly polarized radio emission toward
the variable radio source B5.Comment: To appear in the Astrophysical Journa
The Orbital Variation of Non-Thermal X-Ray Emission from eta Carinae
The collision of strong stellar winds in massive binary systems creates powerful shocks,which accelerates a small amount of particles to relativistic energies at the shock interface.This process is important in fundamental astrophysics in two ways;i) some particles may contribute to cosmic-rays observed around the Earth, whose origin is not well known, andii) it provides a good laboratory for particle acceleration physics as the shock occurs steadily in a predictable environment.Some relativistic particles collide with stellar photons or ambient material near the binary systemand emit non-thermal X-rays and gamma-rays, which are good probes of particle acceleration.This emission had been searched for decades but not found convincingly without sensitive high-energy telescopes.The NuSTAR telescope observed the enigmatic supermassive binary system eta Carinae multiple times after 2014 and found conclusive evidence of non-thermal emission from the star in the extremely hard X-ray band.This emission is prominent between 20-50 keV, below which thermal emission from shock colliding plasma dominates.It is relatively stable throughout the binary orbit, but it disappears near periastron when the wind colliding activity shuts off.This variation indicates that the non-thermal X-ray emission originates from the head-on wind-wind collision.The flat spectrum is consistent with inverse-Compton of stellar UV photons by accelerated electrons.The spectrum smoothly connects to a gamma-ray spectrum of a Fermi source detected around eta Carinae,suggesting that acceleration occurs up to the GeV energy.This result provide the strongest evidence so far that particle acceleration occurs at the wind colliding shock of a massive colliding wind binary system
The Suzaku Observations of SS Cygni in Quiescence and Outburst
We present results from the Suzaku observations of the dwarf nova SS Cyg in
quiescence and outburst in 2005 November. Owing to high sensitivity of the HXD
PIN detector and high spectral resolution of the XIS, we have determined
parameters of the plasma with unprecedented precision. The maximum temperature
of the plasma in quiescence 20.4 +4.0-2.6 (stat.) +/- 3.0 (sys.) keV is
significantly higher than that in outburst 6.0 +0.2-1.3 keV. The elemental
abundances are close to the solar ones for the medium-Z elements (Si, S, Ar)
whereas they decline both in lighter and heavier elements. Those of oxygen and
iron are 0.46 and 0.37 solar, respectively. That of carbon is exceptionally
high and 2 solar at least. The solid angle of the reflector subtending over the
optically thin thermal plasma is Omega/2\pi = 1.7+/-0.2 (stat.) +/-0.1 (sys.)
in quiescence. A 6.4 keV iron Ka line is resolved into a narrow and broad
components. These facts indicate that both the white dwarf and the accretion
disk contribute to the continuum reflection and the 6.4 keV iron Ka line. We
consider the standard optically thin boundary layer as the most plausible
picture for the plasma configuration in quiescence. The solid angle of the
reflector in outburst Omega/2\pi = 0.9 +0.5-0.4 and a broad 6.4 keV iron line
indicates that the reflection in outburst originates from the accretion disk
and an equatorial accretion belt. From the energy width of the 6.4 keV line, we
consider the optically thin thermal plasma in outburst as being distributed on
the accretion disk like solar coronae.Comment: 28 pages, 15 figures, accepted for publication in PASJ Suzaku 3rd
special issue Pdf of this paper can be downloaded from
http://www.astro.isas.jaxa.jp/~ishida/Papers/sscyg_sub2.pd
On the Weak-Wind Problem in Massive Stars: X-ray Spectra Reveal a Massive Hot Wind in \mu\ Columbae
\mu\ Columbae is a prototypical weak-wind O-star for which we have obtained a
high-resolution X-ray spectrum with the Chandra LETG/ACIS-S instrument and a
low resolution spectrum with Suzaku. This allows us, for the first time, to
investigate the role of X-rays on the wind structure in a bona fide weak-wind
system and to determine whether there actually is a massive, hot wind. The
X-ray emission measure indicates that the outflow is an order of magnitude
greater than that derived from UV lines and is commensurate with the nominal
wind-luminosity relationship for O-stars. Therefore, the ``weak-wind
problem''---identified from cool wind UV/optical spectra---is largely resolved
by accounting for the hot wind seen in X-rays. From X-ray line profiles,
Doppler shifts, and relative strengths, we find that this weak-wind star is
typical of other late O dwarfs. The X-ray spectra do not suggest a magnetically
confined plasma---the spectrum is soft and lines are broadened; Suzaku spectra
confirm the lack of emission above 2 keV. Nor do the relative line shifts and
widths suggest any wind decoupling by ions. The He-like triplets indicate that
the bulk of the X-ray emission is formed rather close to the star, within 5
stellar radii. Our results challenge the idea that some OB stars are
``weak-wind'' stars that deviate from the standard wind-luminosity
relationship. The wind is not weak, but it is hot and its bulk is only
detectable in X-rays.Comment: Accepted for publication in ApJ Letter
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