17 research outputs found
Multi-frequency constraints on the non-thermal pressure in galaxy clusters
The origin of radio halos in galaxy clusters is still unknown and is the
subject of a vibrant debate both from the observational and theoretical point
of view. In particular the amount and the nature of non-thermal plasma and of
the magnetic field energy density in clusters hosting radio halos is still
unclear. The aim of this paper is to derive an estimate of the pressure ratio X
between the non-thermal and thermal plasma in radio halo clusters that have
combined radio, X-ray and SZ effect observations. From the simultaneous
P_{1.4}-L_X and P_{1.4}-Y_{SZ} correlations for a sample of clusters observed
with Planck, we derive a correlation between Y_{SZ} and L_X that we use to
derive a value for X. This is possible since the Compton parameter Y_{SZ} is
proportional to the total plasma pressure in the cluster (that we characterize
as the sum of the thermal and non-thermal pressure) while the X-ray luminosity
L_X is proportional only to the thermal pressure of the intracluster plasma.
Our results indicate that the average (best fit) value of the pressure ratio in
a self-similar cluster formation model is X =0.55 \pm 0.05 in the case of an
isothermal beta-model with beta=2/3 and a core radius r_c = 0.3 R_{500} holding
on average for the cluster sample. We also show that the theoretical prediction
for the Y_{SZ}-L_X correlation in this model has a slope that is steeper than
the best fit value for the available data. The agreement with the data can be
recovered if the pressure ratio X decreases with increasing X-ray luminosity as
L_X^{-0.96}. We conclude that the available data on radio halo clusters
indicate a substantial amount of non-thermal pressure in cluster atmospheres
whose value must decrease with increasing X-ray luminosity, or increasing
cluster mass (temperature). (abridged)Comment: A&A, in press; 10 pages; 10 figure
Spectroscopic observations of the intermediate polar EX Hydrae in quiescence
Results from spectroscopic observations of the intermediate polar (IP) EX Hya in quiescence during 1991 and 2001 are presented. Spin-modulated radial velocities consistent with an outer disc origin were detected for the first time in an IP. The spin pulsation was modulated with velocities near ∼500–600 km s−1. These velocities are consistent with those of the material circulating at the outer edge of the accretion disc, suggesting co-rotation of the accretion curtain with the material near the Roche lobe radius. Furthermore, spin Doppler tomograms have revealed evidence of the accretion curtain emission extending from velocities of ∼500 to ∼1000 km s−1. These findings have confirmed the theoretical model predictions for EX Hya, which predict large accretion curtains that extend to a distance close to the Roche lobe radius in this system.
Evidence for overflow stream of material falling on to the magnetosphere was observed, confirming the result of Belle et al. that disc overflow in EX Hya is present during quiescence as well as outburst.
It appears that the Hβ and Hγ spin radial velocities originated from the rotation of the funnel at the outer disc edge, while those of Hα were produced due to the flow of the material along the field lines far from the white dwarf (narrow component) and close to the white dwarf (broad-base component), in agreement with the accretion curtain model
Outbursts of EX Hydrae Revisited
We present optical spectroscopy of EX Hya during its 1991 outburst. This
outburst is characterised by strong irradiation of the front face of the
secondary star by the white dwarf, an overflowing stream which is seen strongly
in HeII and by a dip in the light curves, which extends from 0.1-0.6 in the
binary and spin phases. Strong irradiation of the accretion curtain and that of
the inner regions of the disc led to strong emission of HeII and to the
suppression of the Hg and Hb emission.
Disc overflow was observed in quiescence in earlier studies, where the
overflow stream material was modulated at high velocities close to 1000 km/s.
In outburst, the overflowing material is modulated at even higher velocities
(~1500 km/s). These are streaming velocities down the field lines close to the
white dwarf. Evidence for material collecting near the outer edge of the disc
and corotating with the accretion curtain was observed. In decline, this
material and the accretion curtain obscured almost all the emission near binary
phase 0.4, causing a dip. The dip minimum nearly corresponds with spin pulse
minimum. This has provided additional evidence for an extended accretion
curtain, and for the corotation of material with the accretion curtain at the
outer edge of the disc. From these observations we suggest that a mechanism
similar to that of Spruit & Taam, where outbursts result due to the storage and
release of matter outside the magnetosphere, triggers the outbursts of EX Hya.
This is followed by the irradiation of the secondary star due to accretion
induced radiation.Comment: 12 pages, 14 figures, 1 table. Figures 6, 7, 8 and 11 at low
resolution. Paper accepted by the Monthly Notices of the Royal Astronomical
Societ
The discovery of a persistent quasi-periodic oscillation in the intermediate polar TX Col
We report on the detection of an ∼5900 s quasi-periodic variation in the extensive photometry of TX Col spanning 12 yr. We discuss five different models to explain this period. We favour a mechanism where the quasi-periodic variation results from the beating of the Keplerian frequency of the ‘blobs’ orbiting in the outer accretion disc with the spin frequency and from modulated accretion of these ‘blobs’ taking place in a shocked region near the disc/magnetosphere boundary
Search for hyperdeformation in U isotopes
The U-232 nucleus was studied in order to search for a hyperdeformed band built upon the third minimum of the fission barrier. Upper limits for the percentage population of a hypothetical hyperdeformed band relative to the ground state band are given
The MeerKAT Galaxy Cluster Legacy Survey: I. Survey overview and highlights
Please abstract in the article.The South African Radio Astronomy Observatory (SARAO), the National Research Foundation (NRF), the National Radio Astronomy Observatory, US National Science Foundation, the South African Research Chairs Initiative of the DSI/NRF, the SARAO HCD programme, the South African Research Chairs Initiative of the Department of Science and Innovation.http://www.aanda.orghj2022Physic
The state of space science in Africa
There has been an increase in the number of space science activities and facilities in Africa in the last 15 years. This increase, however, is not proportionate to the indigenous user community for these activities and facilities. In this paper, I discuss these activities and their benefits for the African region, and point out some of the challenges for space science in Africa
Revealing the unusual structure of the KAT-7-discovered giant radio galaxy J0133-1302
We present a new study of the 1.7 Mpc KAT-7-discovered giant radio galaxy,
J01331302, which was carried out using GMRT data at 323 and 608 MHz. This
source is located at RA and Dec
and has a photometric redshift of
0.3. We discovered unusual morphological properties of the source which
include lobes that are exceptionally asymmetric, where the upper lobe is much
further from the core when compared to the lower lobe, and a complex structure
of the upper lobe. This complex structure of the upper lobe hints at the
presence of another source, in close proximity to the edge of the lobe, which
resembles a bent-double, or distorted bent tail (DBT) radio galaxy. Both the
upper lobe and the lower lobe have a steep spectrum, and the synchrotron age of
the lower lobe should be less than about 44 Myr. The core has an inverted
spectrum, and our results suggest that the parent galaxy in J01331302 is
starting a new jet activity. Our spectral analysis indicates that this source
could be a GigaHertz Peaked Spectrum (GPS) radio galaxy.Comment: 13 pages, 15 figures. To be published in the Monthly Notices of the
Royal Astronomical Society (MNRAS