130 research outputs found
ORFEUS-II Far-Ultraviolet Observations of 3C273: 1. Interstellar and Intergalactic Absorption Lines
We present the first intermediate-resolution (lambda / 3000) spectrum of the
bright quasi-stellar object 3C273 at wavelengths between 900 and 1200 A.
Observations were performed with the Berkeley spectrograph aboard the
ORFEUS-SPAS II mission. We detect Lyman beta counterparts to
previously-identified intergalactic Lyman-alpha features at cz = 19900, 1600,
and 1000 km/s; counterparts to other putative Lyman-alpha clouds along the
sight line are below our detection limit. The strengths of the two very low
redshift Lyman-beta features, which are believed to arise in Virgo intracluster
gas, exceed preflight expectations, suggesting that the previous determination
of the cloud parameters may underestimate the true column densities. A
curve-of-growth analysis sets a minimum H I column density of 4 E14/cm^2 for
the 1600 km/s cloud. We find marginally significant evidence for Galactic H_2
along the sight line, with a total column density of about 1 E15/cm^2. We
detect the stronger interstellar O VI doublet member unambiguously; the weaker
member is blended with other features. If the Doppler b value for O VI is
comparable to that determined for N V then the O VI column density is 7 +/- 2
E14/cm^2, significantly above the only previous estimate. The O VI / N V ratio
is about 10, consistent with the low end of the range observed in the disk.
Additional interstellar species detected for the first time toward 3C273 (at
modest statistical significance) include P II, Fe III, Ar I, and S III.Comment: LaTeX file, 11 pages, 4 encapsulated PostScript figures. Uses
aaspp4.sty and astrobib.sty. (Astrobib is available from
http://www.stsci.edu/software/TeX.html .) The ORFEUS telescope is described
at http://sag-www.ssl.berkeley.edu/orfeus/ . To appear in ApJ (Letters
Identification of selected sources from the ROSAT Galactic Plane Survey - I
We report on optical searches in the error circles of 93 ROSAT survey sources
located at low galactic latitudes (|b| < 20 deg). These sources were extracted
from the ROSAT Galactic Plane Survey using various selection criteria on
hardness ratio, X-ray and optical brightness and integrated galactic absorption
in the direction of the source. We find optical identifications in 76 cases,
among which are 25 new AGN, 6 new CVs and a new Be/X-ray binary. In order to
illustrate the relevance of the source selections applied here, we
cross-correlated the ROSAT all-sky survey bright source list with SIMBAD.
Different classes of X-ray emitters populate distinct regions of a multi
dimensional parameter space involving flux ratios, galactic latitude and Nh.
This relatively good segregation offers the possibility to build source samples
with enhanced probability of identification with a given class. Complete
optical identification of such subsamples could eventually be used to compute
meaningful probabilities of identification for all sources using as basis a
restricted set of multi-wavelength information.Comment: 18 pages, 7 figures, Postscript. Optical spectra and finding charts
available at ftp://cdsarc.u-strasbg.fr/pub/xray/rgps_idI.ps.gz Submitted to
Astronomy & Astrophysics, Supplement Serie
The harmonic power spectrum of the soft X-ray background I. The data analysis
Fluctuations of the soft X-ray background are investigated using harmonic
analysis. A section of the ROSAT All-Sky Survey around the north galactic pole
is used. The flux distribution is expanded into a set of harmonic functions and
the power spectrum is determined. Several subsamples of the RASS have been used
and the spectra for different regions and energies are presented. The effects
of the data binning in pixels are assessed and taken into account. The spectra
of the analyzed samples reflect both small scale effects generated by strong
discrete sources and the large scale gradients of the XRB distribution. Our
results show that the power spectrum technique can be effectively used to
investigate anisotropy of the XRB at various scales. This statistics will
become a useful tool in the investigation of various XRB components.Comment: 12 pages, A&A accepte
The ORFEUS II Echelle Spectrometer: Instrument description, performance and data reduction
During the second flight of the ORFEUS-SPAS mission in November/December
1996, the Echelle spectrometer was used extensively by the Principal and Guest
Investigator teams as one of the two focal plane instruments of the ORFEUS
telescope. We present the in-flight performance and the principles of the data
reduction for this instrument. The wavelength range is 90 nm to 140 nm, the
spectral resolution is significantly better than lambda/(Delta lambda) = 10000,
where Delta lambda is measured as FWHM of the instrumental profile. The
effective area peaks at 1.3 cm^2 near 110 nm. The background is dominated by
straylight from the Echelle grating and is about 15% in an extracted spectrum
for spectra with a rather flat continuum. The internal accuracy of the
wavelength calibration is better than +/- 0.005 nm.Comment: 8 pages, 8 figure
The Extraordinary X-ray Light Curve of the Classical Nova V1494 Aquilae (1999 #2) in Outburst: The Discovery of Pulsations and a "Burst"
V1494 Aql (Nova Aql 1999 No. 2) was discovered on 2 December 1999. We
obtained Chandra ACIS-I spectra on 15 April and 7 June 2000 which appear to
show only emission lines. Our third observation, on 6 August, showed that its
spectrum had evolved to that characteristic of a Super Soft X-ray Source. We
then obtained Chandra LETG+HRC-S spectra on 28 September (8 ksec) and 1 October
(17 ksec). We analyzed the X-ray light curve of our grating observations and
found both a short time scale ``burst'' and oscillations. Neither of these
phenomena have previously been seen in the light curve of a nova in outburst.
The ``burst'' was a factor of 10 rise in X-ray counts near the middle of the
second observation, and which lasted about 1000 sec; it exhibited at least two
peaks, in addition to other structure. Our time series analysis of the combined
25 ksec observation shows a peak at 2500 s which is present in independent
analyses of both the zeroth order image and the dispersed spectrum and is not
present in similar analyses of grating data for HZ 43 and Sirius B. Further
analyses of the V1494 Aql data find other periods present which implies that we
are observing non-radial g+ modes from the pulsating, rekindled white dwarf.Comment: ApJ accepte
Exploring the supernova remnant G308.4-1.4
Aims: We present a detailed X-ray and radio wavelength study of G308.4-1.4, a
candidate supernova remnant (SNR) in the ROSAT All Sky Survey and the MOST
supernova remnant catalogue, in order to identify it as a SNR.
Methods: The SNR candidate and its central sources were studied using
observations from the Chandra X-ray Observatory, SWIFT, the Australian
Telescope Compact Array (ATCA) at 1.4 and 2.5 GHz and WISE infrared observation
at 24 m.
Results: We conclude that G308.4-1.4 is indeed a supernova remnant by means
of its morphology matching at X-ray, radio and infrared wavelength, its
spectral energy distribution in the X-ray band and its emission characteristics
in the radio band. G308.4-1.4 is a shell-type SNR. X-ray, radio and infrared
emission is seen only in the eastern part of the remnant. The X-ray emission
can best be described by an absorbed non-equilibrium collisional plasma with a
hydrogen density of cm, a
plasma temperature of million Kelvin and an under-abundance
of Iron, Neon and Magnesium, as well as an overabundance in Sulfur with respect
to the solar values. The SNR has a spectral index in the radio band of
. A detailed analysis revealed that the remnant is at a
distance of 6 to 12 kpc and the progenitor star exploded to 7500
years ago. Two faint X-ray point sources located near to the remnant's
geometrical center are detected. Both sources have no counterpart at other
wavelengths, leaving them as candidates for the compact remnant of the
supernova explosion.Comment: 12 pages, 11 figures, accepted by A&A, revised draf
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