42 research outputs found
Ultraviolet spectroscopy of the brightest supergiants in M31 and M33
Ultraviolet spectroscopy from the IUE, in combination with groundbased visual and infrared photometry, are to determine the energy distributions of the luminous blue variables, the Hubble-Sandage variables, in M31 and M33. The observed energy distributions, especially in the ultraviolet, show that these stars are suffering interstellar reddening. When corrected for interstellar extinction, the integrated energy distributions yield the total luminosities and black body temperatures of the stars. The resulting bolometric magnitudes and temperatures confirm that these peculiar stars are indeed very luminous, hot stars. They occupy the same regions of the sub B01 vs. log T sub e diagram as do eta Car, P Cyg and S Dor in our galaxy and the LMC. Many of the Hubble-Sandage variables have excess infrared radiation which is attributed to free-free emission from their extended atmospheres. Rough mass loss estimates from the infrared excess yield rates of 0.00001 M sub annual/yr. The ultraviolet spectra of the H-S variables are also compared with similar spectra of eta Car, P Cyg and S For
Supernova Remnants in the Magellanic Clouds. IV. X-Ray Emission from the Largest SNR in the LMC
We present the first X-ray detection of SNR 0450-70.9 the largest known
supernova remnant (SNR) in the Large Magellanic Cloud. To study the physical
conditions of this SNR, we have obtained XMM-Newton X-ray observations, optical
images and high-dispersion spectra, and radio continuum maps. Optical images of
SNR 0450-70.9 show a large, irregular elliptical shell with bright filaments
along the eastern and western rims and within the shell interior. The interior
filaments have higher [S II]/Halpha ratios and form an apparent inner shell
morphology. The X-ray emission region is smaller than the full extent of the
optical shell, with the brightest X-ray emission found within the small
interior shell and on the western rim of the large shell. The expansion
velocity of the small shell is ~220 km/s, while the large shell is ~120 km/s.
The radio image shows central brightening and a fairly flat radio spectral
index over the SNR. However, no point X-ray or radio source corresponding to a
pulsar is detected and the X-ray emission is predominantly thermal. Therefore,
these phenomena can be most reasonably explained in terms of the advanced age
of the large SNR. Using hydrodynamic models combined with a nonequilibrium
ionization model for thermal X-ray emission, we derived a lower limit on the
SNR age of about 45,000 yr, well into the later stages of SNR evolution.
Despite this, the temperature and density derived from spectral fits to the
X-ray emission indicate that the remnant is still overpressured, and thus that
the development is largely driven by hot gas in the SNR interior.Comment: Accepted for publication in The Astrophysical Journa
A Synoptic X-ray Study of M31 with the Chandra-HRC
We have obtained 17 epochs of Chandra High Resolution Camera (HRC) snapshot
images, each covering most of the M31 disk. The data cover a total baseline of
2.5 years and contain a mean effective exposure of 17 ks. We have measured the
mean fluxes and long-term lightcurves for 166 objects detected in these data.
At least 25% of the sources show significant variability. The cumulative
luminosity function (CLF) of the disk sources is well-fit by a power-law with a
slope comparable to those observed in typical elliptical galaxies. The CLF of
the bulge is a broken power law similar to measurements made by previous
surveys. We note several sources in the southwestern disk with L_X > 10^{37}
erg/s . We cross-correlate all of our sources with published optical and radio
catalogs, as well as new optical data, finding counterpart candidates for 55
sources. In addition, 17 sources are likely X-ray transients. We analyze
follow-up HST WFPC2 data of two X-ray transients, finding F336W (U-band
equivalent) counterparts. In both cases, the counterparts are variable. In one
case, the optical counterpart is transient with F336W = 22.3 +/- 0.1 mag. The
X-ray and optical properties of this object are consistent with a ~10 solar
mass black hole X-ray nova with an orbital period of ~20 days. In the other
case, the optical counterpart varies between F336W = 20.82 +/- 0.06 mag and
F336W = 21.11 +/- 0.02 mag. Ground-based and HST observations show this object
is bright (V = 18.8 +/- 0.1) and slightly extended. Finally, the frequency of
bright X-ray transients in the M31 bulge suggests that the ratio of neutron
star to black hole primaries in low-mass X-ray binaries (NS/BH) is ~1.Comment: 68 pages (27 text), 8 tables, 16 figures, 1 appendix, accepted by
ApJ; accepted version contains reorganized text, new tables and figures, and
updated result
Optical Spectra of SNR Candidates in NGC 300
We present moderate-resolution (<5A) long-slit optical spectra of 51 nebular
objects in the nearby Sculptor Group galaxy NGC 300 obtained with the 2.3 meter
Advanced Technology Telescope at Siding Spring Observatory, Australia. Adopting
the criterion of [SII]/Ha>=0.4 to confirm supernova remnants (SNRs) from
optical spectra, we find that of 28 objects previously proposed as SNRs from
optical observations, 22 meet this criterion with six showing [SII]/Ha of less
than 0.4. Of 27 objects suggested as SNRs from radio data, four are associated
with the 28 previously proposed SNRs. Of these four, three (included in the 22
above) meet the criterion. In all, 22 of the 51 nebular objects meet the
[SII]/Ha criterion as SNRs while the nature of the remaining 29 objects remains
undetermined by these observations.Comment: Accepted for publication in Astrophysics & Space Scienc
Long-term X-ray Variability Study of IC342 from XMM-Newton Observations
We presented the results of an analysis of four XMM-Newton observations of
the starburst galaxy IC342 taken over a four-year span from 2001 to 2005, with
an emphasis on investigating the long-term flux and spectral variability of the
X-ray point sources. We detected a total of 61 X-ray sources within 35'
30' of the galaxy down to a luminosity of (1-2)1037 erg s-1
depending on the local background. We found that 39 of the 61 detected sources
showed long-term variability, in which 26 of them were classified as X-ray
transients. We also found 19 sources exhibiting variations in hardness ratios
or undergoing spectral transitions among observations, and were identified as
spectral variables. In particular, 8 of the identified X-ray transients showed
spectral variability in addition to flux variability. The diverse patterns of
variability observed is indicative of a population of X-ray binaries. We used
X-ray colors, flux and spectral variability, and in some cases the optical or
radio counterparts to classify the detected X-ray sources into several stellar
populations. We identified a total of 11 foreground stars, 1 supersoft sources
(SSS), 3 quasisoft sources (QSS), and 2 supernova remnants (SNR). The
identified SSS/QSS are located near or on the spiral arms, associate with young
stellar populations; the 2 SNR are very close to the starburst nucleus where
current star formation activities are dominated. We also discovered a spectral
change in the nuclear source of IC342 for the first time by a series of X-ray
spectrum analysis.Comment: 45 pages, 6 figures accepted by Ap
High-resolution radio continuum survey of M33 II. Thermal and nonthermal emission
We determine the variation in the nonthermal radio spectral index in the
nearby spiral galaxy M33 at a linear resolution of 360 pc. We separate the
thermal and nonthermal components of the radio continuum emission without the
assumption of a constant nonthermal spectral index. Using the Spitzer FIR data
at 70 and 160 m and a standard dust model, we deredden the H
emission. The extinction corrected H emission serves as a template for
the thermal free-free radio emission. Subtracting from the observed 3.6 cm and
20 cm emission (Effelsberg and the VLA) this free-free emission, we obtain the
nonthermal maps. A constant electron temperature used to obtain the thermal
radio intensity seems appropriate for M~33 which, unlike the Milky Way, has a
shallow metallicity gradient. For the first time, we derive the distribution of
the nonthermal spectral index across a galaxy, M33. We detect strong nonthermal
emission from the spiral arms and star-forming regions. Wavelet analysis shows
that at 3.6 cm the nonthermal emission is dominated by contributions from
star-forming regions, while it is smoothly distributed at 20 cm. For the whole
galaxy, we obtain thermal fractions of 51% and 18% at 3.6 cm and 20 cm,
respectively. The thermal emission is slightly stronger in the southern than in
the northern half of the galaxy. We find a clear radial gradient of mean
extinction in the galactic plane. The nonthermal spectral index map indicates
that the relativistic electrons suffer energy-loss when diffusing from their
origin in star-forming regions towards interarm regions and the outer parts of
the galaxy. We also conclude that the radio emission is mostly nonthermal at R
5 kpc in M33.Comment: 15 pages, 14 figures, accepted for publication in the Astronomy and
Astrophysics journa
Galactic and Extragalactic Samples of Supernova Remnants: How They Are Identified and What They Tell Us
Supernova remnants (SNRs) arise from the interaction between the ejecta of a
supernova (SN) explosion and the surrounding circumstellar and interstellar
medium. Some SNRs, mostly nearby SNRs, can be studied in great detail. However,
to understand SNRs as a whole, large samples of SNRs must be assembled and
studied. Here, we describe the radio, optical, and X-ray techniques which have
been used to identify and characterize almost 300 Galactic SNRs and more than
1200 extragalactic SNRs. We then discuss which types of SNRs are being found
and which are not. We examine the degree to which the luminosity functions,
surface-brightness distributions and multi-wavelength comparisons of the
samples can be interpreted to determine the class properties of SNRs and
describe efforts to establish the type of SN explosion associated with a SNR.
We conclude that in order to better understand the class properties of SNRs, it
is more important to study (and obtain additional data on) the SNRs in galaxies
with extant samples at multiple wavelength bands than it is to obtain samples
of SNRs in other galaxiesComment: Final 2016 draft of a chapter in "Handbook of Supernovae" edited by
Athem W. Alsabti and Paul Murdin. Final version available at
https://doi.org/10.1007/978-3-319-20794-0_90-