282 research outputs found
X-ray emission from the double-binary OB-star system QZ Car (HD 93206)
X-ray observations of the double-binary OB-star system QZ Car (HD 93206)
obtained with the Chandra X-ray Observatory over a period of roughly 2 years
are presented. The orbit of systems A (O9.7 I+b2 v, PA = 21 d) and B (O8 III+o9
v, PB = 6 d) are reasonably well sampled by the observations, allowing the
origin of the X-ray emission to be examined in detail. The X-ray spectra can be
well fitted by an attenuated three temperature thermal plasma model,
characterised by cool, moderate, and hot plasma components at kT ~ 0.2, 0.7,
and 2 keV, respectively, and a circumstellar absorption of ~ 0.2 x 10^22 cm-2.
Although the hot plasma component could be indicating the presence of wind-wind
collision shocks in the system, the model fluxes calculated from spectral fits,
with an average value of ~ 7 x 10^-13 erg s-1 cm-2, do not show a clear
correlation with the orbits of the two constituent binaries. A semi-analytical
model of QZ Car reveals that a stable momentum balance may not be established
in either system A or B. Yet, despite this, system B is expected to produce an
observed X-ray flux well in excess of the observations. If one considers the
wind of the O8 III star to be disrupted by mass transfer the model and
observations are in far better agreement, which lends support to the previous
suggestion of mass-transfer in the O8 III + o9 v binary. We conclude that the
X-ray emission from QZ Car can be reasonably well accounted for by a
combination of contributions mainly from the single stars and the mutual
wind-wind collision between systems A and B.Comment: 11 pages, 7 figures. Accepted for the ApJS Special Issue on the
Chandra Carina Complex Project (CCCP), scheduled for publication in May 2011.
All 16 CCCP Special Issue papers are available at
http://cochise.astro.psu.edu/Carina_public/special_issue.html through 2011 at
leas
Waiting in the Wings: Reflected X-ray Emission from the Homunculus Nebula
We report the first detection of X-ray emission associated with the
Homunculus Nebula which surrounds the supermassive star Eta Carinae. The
emission is characterized by a temperature in excess of 100 MK, and is
consistent with scattering of the time-delayed X-ray flux associated with the
star. The nebular emission is bright in the northwestern lobe and near the
central regions of the Homunculus, and fainter in the southeastern lobe. We
also report the detection of an unusually broad Fe K fluorescent line, which
may indicate fluorescent scattering off the wind of a companion star or some
other high velocity outflow. The X-ray Homunculus is the nearest member of the
small class of Galactic X-ray reflection nebulae, and the only one in which
both the emitting and reflecting sources are distinguishable.Comment: 17 pages, 5 figures, accepted by Ap
The UV Scattering Halo of the Central Source Associated with Eta Carinae
We have made an extensive study of the UV spectrum of Eta Carinae, and find
that we do not directly observe the star and its wind in the UV. Because of
dust along our line of sight, the UV light that we observe arises from
bound-bound scattering at large impact parameters. We obtain a reasonable fit
to the UV spectrum by using only the flux that originates outside 0.033". This
explains why we can still observe the primary star in the UV despite the large
optical extinction -- it is due to the presence of an intrinsic coronagraph in
the Eta Carinae system, and to the extension of the UV emitting region. It is
not due to peculiar dust properties alone. We have computed the spectrum of the
purported companion star, and show that it could only be directly detected in
the UV spectrum preferentially in the Far Ultraviolet Spectroscopic Explorer
(FUSE) spectral region (912-1175 Ang.). However, we find no direct evidence for
a companion star, with the properties indicated by X-ray studies and studies of
the Weigelt blobs, in UV spectra. This might be due to reprocessing of the
companion's light by the dense stellar wind of the primary. Broad FeII and
[FeII] emission lines, which form in the stellar wind, are detected in spectra
taken in the SE lobe, 0.2" from the central star. The wind spectrum shows some
similarities to the spectra of the B & D Weigelt blobs, but also shows some
marked differences in that high excitation lines, and lines pumped by Ly-alpha,
are not seen. The detection of the broad lines lends support to our
interpretation of the UV spectrum, and to our model for Eta Carinae.Comment: To appear in ApJ. 57 pages with 18 figure
Recent X-ray Variability of eta Carinae: the Quick Road to Recovery
We report continued monitoring of the superluminous binary system eta Car by the Proportional Counter Array on the Rossi X-ray Timing Observatory (RXTE) through the 2009 X-ray minimum. The RXTE campaign shows that the minimum began on 2009 January 16, consistent with the phasings of the two previous minima, and overall, the temporal behavior of the X-ray emission was similar to that observed by RXTE in the previous two cycles. However, important differences did occur. The 2-10 keV X-ray flux and X-ray hardness decreased in the 2.5-year interval leading up to the 2009 minimum compared to the previous cycle. Most intriguingly, the 2009 X-ray minimum was about one month shorter than either of the previous two minima. During the egress from the 2009 minimum the X-ray hardness increased markedly as it had during egress from the previous two minima, although the maximum X-ray hardness achieved was less than the maximum observed after the two previous recoveries. We suggest that the cycle-to-cycle variations, especially the unexpectedly early recovery from the 2009 X-ray minimum, might have been the result of a decline in eta Car's wind momentum flux produced by a drop in eta Car's mass loss rate or wind terminal velocity (or some combination), though if so the change in wind momentum flux required to match the X-ray variation is surprisingly large
Global X-ray properties of the O and B stars in Carina
The key empirical property of the X-ray emission from O stars is a strong
correlation between the bolometric and X-ray luminosities. In the framework of
the Chandra Carina Complex Project, 129 O and B stars have been detected as
X-ray sources; 78 of those, all with spectral type earlier than B3, have enough
counts for at least a rough X-ray spectral characterization. This leads to an
estimate of the Lx/Lbol ratio for an exceptional number of 60 O stars belonging
to the same region and triples the number of Carina massive stars studied
spectroscopically in X-rays. The derived log(Lx/Lbol) is -7.26 for single
objects, with a dispersion of only 0.21dex. Using the properties of hot massive
stars listed in the literature, we compare the X-ray luminosities of different
types of objects. In the case of O stars, the Lx/Lbol ratios are similar for
bright and faint objects, as well as for stars of different luminosity classes
or spectral types. Binaries appear only slightly harder and slightly more
luminous in X-rays than single objects; the differences are not formally
significant (at the 1% level), except for the Lx/Lbol ratio in the medium
(1.0--2.5keV) energy band. Weak-wind objects have similar X-ray luminosities
but they display slightly softer spectra compared to "normal" O stars with the
same bolometric luminosity. Discarding three overluminous objects, we find a
very shallow trend of harder emission in brighter objects. The properties of
the few B stars bright enough to yield some spectral information appear to be
different overall (constant X-ray luminosities, harder spectra), hinting that
another mechanism for producing X-rays, besides wind shocks, might be at work.
However, it must be stressed that the earliest and X-ray brightest amongst
these few detected objects are similar to the latest O stars, suggesting a
possibly smooth transition between the two processes.Comment: 30 pages, 9 figures. Accepted for the ApJS Special Issue on the
Chandra Carina Complex Project (CCCP), scheduled for publication in May 2011.
All 16 CCCP Special Issue papers are available at
http://cochise.astro.psu.edu/Carina_public/special_issue.html through 2011 at
leas
Detection of high-velocity material from the wind-wind collision zone of Eta Carinae across the 2009.0 periastron passage
We report near-IR spectroscopic observations of the Eta Carinae massive
binary system during 2008-2009 using VLT/CRIRES. We detect a strong, broad
absorption wing in He I 10833 extending up to -1900 km/s across the 2009.0
spectroscopic event. Archival HST/STIS ultraviolet and optical data shows a
similar high-velocity absorption (up to -2100 km/s) in the UV resonance lines
of Si IV 1394, 1403 across the 2003.5 event. UV lines from low-ionization
species, such as Si II 1527, 1533 and C II 1334, 1335, show absorption up to
-1200 km/s, indicating that the absorption with v from -1200 to -2100 km/s
originates in a region markedly faster and more ionized than the nominal wind
of the primary star. Observations obtained at the OPD/LNA during the last 4
spectroscopic cycles (1989-2009) also display high-velocity absorption in He I
10833 during periastron. Based on the OPD/LNA dataset, we determine that
material with v < -900 km/s is present in the phase range 0.976 < phi < 1.023
of the spectroscopic cycle, but absent in spectra taken at phi < 0.947 and phi
> 1.049. Therefore, we constrain the duration of the high-velocity absorption
to be 95 to 206 days (or 0.047 to 0.102 in phase). We suggest that the
high-velocity absorption originates from shocked gas in the wind-wind collision
zone, at distances of 15 to 45 AU in the line-of-sight to the primary star.
Using 3-D hydrodynamical simulations of the wind-wind collision zone, we find
that the dense high-velocity gas is in the line-of-sight to the primary star
only if the binary system is oriented in the sky so that the companion is
behind the primary star during periastron, corresponding to a longitude of
periastron of omega ~ 240 to 270 degrees. We study a possible tilt of the
orbital plane relative to the Homunculus equatorial plane and conclude that our
data are broadly consistent with orbital inclinations in the range i=40 to 60
degrees.Comment: 18 pages, 15 figures, accepted for publication in A&A;
high-resolution PDF version available also at
http://www.mpifr.de/staff/jgroh/etacar.htm
Chandra X-ray Spectroscopic Imaging of Sgr A* and the Central Parsec of the Galaxy
We present results of our Chandra observation with the ACIS-I instrument
centered on the position of Sagittarius A* (Sgr A*), the compact nonthermal
radio source associated with the massive black hole (MBH) at the dynamical
center of the Milky Way Galaxy. We have obtained the first
high-spatial-resolution (~1 arcsec), hard X-ray (0.5-7 keV) image of the
central 40 pc (17 arcmin) of the Galaxy and have discovered an X-ray source,
CXOGC J174540.0-290027, coincident with the radio position of Sgr A* to within
0.35 arcsec, corresponding to a maximum projected distance of 16 light-days for
an assumed distance to the center of the Galaxy of 8.0 kpc. We received 222
+/-17 (1 sigma) net counts from the source in 40.3 ks. Due to the low number of
counts, the spectrum is well fit either by an absorbed power-law model with
photon index Gamma = 2.7 (1.8-4.0) and column density NH = (9.8 [6.8-14.2]) x
10^22 cm^-2 (90% confidence interval) or by an absorbed optically thin thermal
plasma model with kT = 1.9 (1.4-2.8) keV and NH = (11.5 [8.4-15.9]) x 10^22
cm^-2. Using the power-law model, the measured (absorbed) flux in the 2-10 keV
band is (1.3 [1.1-1.7]) x 10^-13 ergs cm^-2 s^-1, and the absorption-corrected
luminosity is (2.4 [1.8-5.4]) x 10^33 ergs s^-1. We also briefly discuss the
complex structure of the X-ray emission from the Sgr A radio complex and along
the Galactic plane and present morphological evidence that Sgr A* and Sgr A
West lie within the hot plasma in the central cavity of Sgr A East.Comment: 33 pages, 10 figures (Figures 2-5 in color), LaTeX, emulateapj5.sty,
submitted to The Astrophysical Journal, version with full-resolution figures
available at http://space.mit.edu/~fkb/GC
Eta Carinae -- Physics of the Inner Ejecta
Eta Carinae's inner ejecta are dominated observationally by the bright
Weigelt blobs and their famously rich spectra of nebular emission and
absorption lines. They are dense (n_e ~ 10^7 to 10^8 cm^-3), warm (T_e ~ 6000
to 7000 K) and slow moving (~40 km/s) condensations of mostly neutral (H^0)
gas. Located within 1000 AU of the central star, they contain heavily
CNO-processed material that was ejected from the star about a century ago.
Outside the blobs, the inner ejecta include absorption-line clouds with similar
conditions, plus emission-line gas that has generally lower densities and a
wider range of speeds (reaching a few hundred km/s) compared to the blobs. The
blobs appear to contain a negligible amount of dust and have a nearly dust-free
view of the central source, but our view across the inner ejecta is severely
affected by uncertain amounts of dust having a patchy distribution in the
foreground. Emission lines from the inner ejecta are powered by photoionization
and fluorescent processes. The variable nature of this emission, occurring in a
5.54 yr event cycle, requires specific changes to the incident flux that hold
important clues to the nature of the central object.Comment: This is Chapter 5 in a book entitled: Eta Carinae and the Supernova
Impostors, Kris Davidson and Roberta M. Humphreys, editors Springe
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