297 research outputs found
Diffuse radio sources in the cluster of galaxies Abell 548b
We report extensive VLA and ATCA observations of the two diffuse radio
sources in the cluster of galaxies Abell 548b, which confirm their
classification as relics. The two relics (named A and B) show similar flux
density, extent, shape, polarization and spectral index and are located at
projected distances of about 430 and 500 kpc from the cluster center, on the
same side of the cluster's X-ray peak. On the basis of spectral indices of
discrete radio sources embedded within the diffuse features, we have attempted
to distinguish emission peaks of the diffuse sources from unrelated sources. We
have found that both relics, in particular the B-relic, show possible fine
structure, when observed at high resolution. Another diffuse source (named C)
is detected close in projection to the cluster center. High-resolution images
show that it contains two discrete radio sources and a diffuse component, which
might be a candidate for a small relic source. The nature and properties of the
diffuse radio sources are discussed. We conclude that they are likely related
to the merger activity in the cluster.Comment: Accepted for publication in MNRAS. 11 pages, 7 figures. Some figures
are degraded to reduce their size. A version with high resolution images is
available at http://www.ira.inaf.it/~lferetti/OUTGOING/papA548b.ps.g
A Search for Very Active Stars in the Galaxy
We report the first results of a systematic search near the plane of the
Galaxy for the so called very active stars (VAS), which are characterized by a
hard X-ray spectrum and activity in the radio domain. Candidates with hard
X-ray binary-like spectra have been selected from the Bright ROSAT Source
Catalogue in the Zone of Avoidance () and were tentatively
identified in GB6/PMM/NVSS radio surveys. Most of them were observed with the
ATCA and VLA. Precise radio coordinates have led to unambiguous optical
identification for 60 candidates, and a sub-sample of five of themhas been
observed with the VLT. Also some discovery and confirmatory spectra were
obtained with the AAT (4-m) and BTA (6-m). Spectroscopy with moderate
dispersion, made with the FORS1 spectrograph of the VLT has revealed two
stellar objects (one of them, VASC J1628-41, is definitivelya binary VAS), one
new AGN and two featureless spectrum sources. One of these objects, VASC
J1353-66, shows a marginal evidence of proper motion, which, if confirmed,
would imply the discovery of a new type of galactic source.Comment: to appear in A&A, 7 figure
On the binary nature of 1RXS J162848.1-415241
We present spectroscopy of the optical counterpart to 1RXS J162848.1-41524,
also known as the microquasar candidate MCQC J162847-4152. All the data
indicate that this X-ray source is not a microquasar, and that it is a
single-lined chromospherically active binary system with a likely orbital
period of 4.9 days. Our analysis supports a K3IV spectral classification for
the star, which is dominant at optical wavelengths. The unseen binary component
is most likely a late-type (K7-M) dwarf or a white dwarf. Using the high
resolution spectra we have measured the K3 star's rotational broadening to be
vsini = 43 +/- 3 km/s and determined a lower limit to the binary mass ratio of
q(=M2/M1)>2.0. The high rotational broadening together with the strong CaII H &
K / Halpha emission and high-amplitude photometric variations indicate that the
evolved star is very chromospherically active and responsible for the
X-ray/radio emission.Comment: 15 pages, 5 figures, accepted for publication in Ap
A Radio Survey of Seven Southern X-ray Luminous Clusters of Galaxies
The Australia Telescope Compact Array (ATCA) has been used at 1.38 and 2.38
GHz to survey seven southern Abell clusters of galaxies with high X-ray
luminosities: A2746, A2837, A3126, A3216, A3230, A3827 and A3836. The clusters
have also been surveyed at 0.843 GHz with the Molonglo Observatory Synthesis
Telescope (MOST). We have listed a complete 1.38-GHz sample of 149 radio
sources within the Abell circles centred on their X-ray centroids. We compare
their identification fractions, emitted 1.38-GHz and optical powers, radio
spectral indices and radial variation in projected source density with those of
the radio-selected samples of Slee et al. (1998). We compare our fractional
radio luminosity function with that of the radio-selected samples of Ledlow and
Owen (1996) and Slee et al. (1998). Three significant differences are noted
between X-ray and radio-selected samples of clusters; (1) the X-ray sample has
an excess of flat-spectrum radio sources; (2) the fractional radio luminosity
function for the FR I sources in the X-ray selected sample is much steeper,
implying that fewer of their cluster galaxies become hosts for the stronger FR
I radio galaxies; (3) a complete absence of FR II radio galaxies in the X-ray
selected sample. The average excess projected density of radio sources near our
cluster centres is approx. 5 times the background source density.Comment: 24 pages, 8 figures, plus 6 figures to be published online only;
accepted to appear in MNRA
Does Collinder 236 host a Cepheid calibrator?
Photoelectric UBV photometry and star counts are presented for the previously
unstudied open cluster Collinder 236, supplemented by observations for stars
near the Cepheid WZ Car. Collinder 236 is typical of groups associated with
Cepheids, with an evolutionary age of (3.4+-1.1)x10^7 years, but it is 1944+-71
pc distant, only half the predicted distance to WZ Car. The cluster is reddened
by E(B-V)~0.26, and has nuclear and coronal radii of rn~2 arcmin (1.1 pc) and
Rc~8 arcmin (4.5 pc), respectively. The Cepheid is not a member of Collinder
236 on the basis of location beyond the cluster tidal radius and implied
distance, but its space reddening can be established as E(B-V)=0.268+-0.006
s.e. from 5 adjacent stars. Period changes in WZ Car studied with the aid of
archival data are revised. The period of WZ Car is increasing, its rate of
+8.27+-0.19 s yr^(-1) being consistent with a third crossing of the instability
strip.Comment: Accepted for publication (MNRAS
Assessing potential cluster Cepheids from a new distance and reddening parameterization and 2MASS photometry
A framework is outlined to assess Cepheids as potential cluster members from
readily available photometric observations. A relationship is derived to
estimate colour excess and distance for individual Cepheids through a
calibration involving recently published HST parallaxes and a cleaned sample of
established cluster Cepheids. Photometric (V-J) colour is found to be a viable
parameter for approximating a Cepheid's reddening. The non-universal nature of
the slope of the Cepheid PL relation for BV photometry is confirmed. By
comparison, the slopes of the VJ and VI relations seem relatively unaffected by
metallicity. A new Galactic Cepheid confirmed here, GSC 03729-01127 (F6-G1 Ib),
is sufficiently coincident with the coronal regions of Tombaugh 5 to warrant
follow-up radial velocity measures to assess membership. CCD photometry and
O--C diagrams are presented for GSC 03729-01127 and the suspected cluster
Cepheids AB Cam and BD Cas. Fourier analysis of the photometry for BD Cas and
recent estimates of its metallicity constrain it to be a Population I overtone
pulsator rather than a Type II s-Cepheid. AB Cam and BD Cas are not physically
associated with the spatially-adjacent open clusters Tombaugh 5 and King 13,
respectively, the latter being much older (log t ~ 9) than believed previously.
Rates of period change are determined for the three Cepheids from archival and
published data. GSC 03729-01127 and AB Cam exhibit period increases, implying
fifth and third crossings of the instability strip, respectively, while BD Cas
exhibits a period decrease, indicating a second crossing, with possible
superposed trends unrelated to binarity. More importantly, the observed rates
of period change confirm theoretical predictions. The challenges and prospects
for future work in this area of research are discussed.Comment: accepted for publication (MNRAS
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