210 research outputs found
X-ray and optical observations of the unique binary system HD49798/RXJ0648.0-4418
We report the results of XMM-Newton observations of HD49798/RXJ0648.0-4418,
the only known X-ray binary consisting of a hot sub-dwarf and a white dwarf.
The white dwarf rotates very rapidly (P=13.2 s) and has a dynamically measured
mass of 1.28+/-0.05 M_sun. Its X-ray emission consists of a strongly pulsed,
soft component, well fit by a blackbody with kT~40 eV, accounting for most of
the luminosity, and a fainter hard power-law component (photon index ~1.6). A
luminosity of ~10^{32} erg/s is produced by accretion onto the white dwarf of
the helium-rich matter from the wind of the companion, which is one of the few
hot sub-dwarfs showing evidence of mass-loss. A search for optical pulsations
at the South African Astronomical Observatory 1.9-m telescope gave negative
results. X-rays were detected also during the white dwarf eclipse. This
emission, with luminosity 2x10^{30} erg/s, can be attributed to HD 49798 and
represents the first detection of a hot sub-dwarf star in the X-ray band.
HD49798/RXJ0648.0-4418 is a post-common envelope binary which most likely
originated from a pair of stars with masses ~8-10 M_sun. After the current
He-burning phase, HD 49798 will expand and reach the Roche-lobe, causing a
higher accretion rate onto the white dwarf which can reach the Chandrasekhar
limit. Considering the fast spin of the white dwarf, this could lead to the
formation of a millisecond pulsar. Alternatively, this system could be a Type
Ia supernova progenitor with the appealing characteristic of a short time
delay, being the descendent of relatively massive stars.Comment: Accepted for publication on The Astrophysical Journa
High Speed Photometry of SDSS J013701.06-091234.9
We present high speed photometry of the Sloan Digital Sky Survey cataclysmic
variable SDSS J013701.06-091234.9 in quiescence and during its 2003 December
superoutburst. The orbital modulation at 79.71\pm0.01 min is double humped; the
superhump period is 81.702\pm0.007 min. Towards the end of the outburst late
superhumps with a period of 81.29\pm0.01 min were observed. We argue that this
is a system of very low mass transfer rate, and that it probably has a long
outburst interval.Comment: 5 pages, 8 figures. Accepted for publication in MNRA
The OmegaWhite Survey for Short-Period Variable Stars IV: Discovery of the warm DQ white dwarf OW J175358.85-310728.9
We present the discovery and follow-up observations of the second known
variable warm DQ white dwarf OW J175358.85-310728.9 (OW J1753-3107). OW
J1753-3107 is the brightest of any of the currently known warm or hot DQ and
was discovered in the OmegaWhite Survey as exhibiting optical variations on a
period of 35.5452 (2) mins, with no evidence for other periods in its light
curves. This period has remained constant over the last two years and a
single-period sinusoidal model provides a good fit for all follow-up light
curves. The spectrum consists of a very blue continuum with strong absorption
lines of neutral and ionised carbon, a broad He I 4471 A line, and possibly
weaker hydrogen lines. The C I lines are Zeeman split, and indicate the
presence of a strong magnetic field. Using spectral Paschen-Back model
descriptions, we determine that OW J1753-3107 exhibits the following physical
parameters: T_eff = 15430 K, log(g) = 9.0, log(N(C)/N(He)) = -1.2, and the mean
magnetic field strength is B_z =2.1 MG. This relatively low temperature and
carbon abundance (compared to the expected properties of hot DQs) is similar to
that seen in the other warm DQ SDSS J1036+6522. Although OW J1753-3107 appears
to be a twin of SDSS J1036+6522, it exhibits a modulation on a period slightly
longer than the dominant period in SDSS J1036+6522 and has a higher carbon
abundance. The source of variations is uncertain, but they are believed to
originate from the rotation of the magnetic white dwarf.Comment: 11 pages, 8 figures, 7 tables. Accepted for publication by MNRA
A return to strong radio flaring by Circinus X-1 observed with the Karoo Array Telescope test array KAT-7
Circinus X-1 is a bright and highly variable X-ray binary which displays
strong and rapid evolution in all wavebands. Radio flaring, associated with the
production of a relativistic jet, occurs periodically on a ~17-day timescale. A
longer-term envelope modulates the peak radio fluxes in flares, ranging from
peaks in excess of a Jansky in the 1970s to an historic low of milliJanskys
during the years 1994 to 2007. Here we report first observations of this source
with the MeerKAT test array, KAT-7, part of the pathfinder development for the
African dish component of the Square Kilometre Array (SKA), demonstrating
successful scientific operation for variable and transient sources with the
test array. The KAT-7 observations at 1.9 GHz during the period 13 December
2011 to 16 January 2012 reveal in temporal detail the return to the
Jansky-level events observed in the 1970s. We compare these data to
contemporaneous single-dish measurements at 4.8 and 8.5 GHz with the HartRAO
26-m telescope and X-ray monitoring from MAXI. We discuss whether the overall
modulation and recent dramatic brightening is likely to be due to an increase
in the power of the jet due to changes in accretion rate or changing Doppler
boosting associated with a varying angle to the line of sight.Comment: 7 pages, 5 figures, accepted for publication in MNRAS 14 May 201
Radio Frequency Models of Novae in eruption. I. The Free-Free Process in Bipolar Morphologies
Observations of novae at radio frequencies provide us with a measure of the
total ejected mass, density profile and kinetic energy of a nova eruption. The
radio emission is typically well characterized by the free-free emission
process. Most models to date have assumed spherical symmetry for the eruption,
although it has been known for as long as there have been radio observations of
these systems, that spherical eruptions are to simplistic a geometry. In this
paper, we build bipolar models of the nova eruption, assuming the free-free
process, and show the effects of varying different parameters on the radio
light curves. The parameters considered include the ratio of the minor- to
major-axis, the inclination angle and shell thickness (further parameters are
provided in the appendix). We also show the uncertainty introduced when fitting
spherical model synthetic light curves to bipolar model synthetic light curves.
We find that the optically thick phase rises with the same power law () for both the spherical and bipolar models. In the bipolar case
there is a "plateau" phase -- depending on the thickness of the shell as well
as the ratio of the minor- to major-axis -- before the final decline, that
follows the same power law () as in the spherical case.
Finally, fitting spherical models to the bipolar model synthetic light curves
requires, in the worst case scenario, doubling the ejected mass, more than
halving the electron temperature and reducing the shell thickness by nearly a
factor of 10. This implies that in some systems we have been over predicting
the ejected masses and under predicting the electron temperature of the ejecta.Comment: 9 pages, 6 figures, accepted for publication in ApJ, accompanying
movie to figure 3 available at
http://www.ast.uct.ac.za/~valerio/papers/radioI
A Strong Jet Signature in the Late-Time Lightcurve of GW170817
We present new 0.6-10 GHz observations of the binary neutron star merger
GW170817 covering the period up to 300 days post-merger, taken with the Karl G.
Jansky Very Large Array, the Australia Telescope Compact Array, the Giant
Metrewave Radio Telescope and the MeerKAT telescope. We use these data to
precisely characterize the decay phase of the late-time radio light curve. We
find that the temporal decay is consistent with a power-law slope of t^-2.2,
and that the transition between the power-law rise and decay is relatively
sharp. Such a slope cannot be produced by a quasi-isotropic (cocoon-dominated)
outflow, but is instead the classic signature of a relativistic jet. This
provides strong observational evidence that GW170817 produced a successful jet,
and directly demonstrates the link between binary neutron star mergers and
short-hard GRBs. Using simple analytical arguments, we derive constraints on
the geometry and the jet opening angle of GW170817. These results are
consistent with those from our companion Very Long Baseline Interferometry
(VLBI) paper, reporting superluminal motion in GW170817.Comment: 11 pages, 3 figures, 3 tables. Accepted for publication in ApJ
Letter
The hidden HI-massive LIRG HIZOA J0836-43: Inside-out galaxy formation
HIZOA J0836-43 is an extreme gas-rich (=7.5\times10^{10}
M_{\sun}) disk galaxy which lies hidden behind the strongly obscuring Vela
region of the Milky Way. Utilizing observations from the {\it Spitzer Space
Telescope}, we have found it to be a luminous infrared starburst galaxy with a
star formation rate of \sim 21 M_{\sun} \rm{yr^{-1}}, arising from
exceptionally strong molecular PAH emission (L_{7.7\micron} = 1.50 \times
10^{9} L_{\odot}) and far-infrared emission from cold dust. The galaxy
exhibits a weak mid-infrared continuum compared to other starforming galaxies
and U/LIRGs. This relative lack of emission from small grains suggests atypical
interstellar medium conditions compared to other starbursts. We do not detect
significant Ne {\sc v} or O {\sc iv}, which implies an absent or
very weak AGN. The galaxy possesses a prominent bulge of evolved stars and a
stellar mass of 4.4(1.4)\times10^{10} M_{\sun}. With its plentiful gas
supply and current star formation rate, a doubling of stellar mass would occur
on a timescale of 2 Gyr. Compared to local galaxies, HIZOA J0836-43
appears to be a "scaled-up" spiral undergoing inside-out formation, possibly
resembling stellar disk building processes at intermediate redshifts.Comment: 5 pages, 3 figures, 1 table; Accepted for publication in ApJL: August
25 2008. A version with full resolution figures is available at
http://spider.ipac.caltech.edu/staff/jarrett/Cluver_ApJL.pd
Hunting For Eclipses: High Speed Observations of Cataclysmic Variables
We present new time-resolved photometry of 74 cataclysmic variables (CVs), 47 of which are eclipsing. 13 of these eclipsing systems are newly discovered. For all 47 eclipsing systems we show high cadence (1-20 seconds) light curves obtained with the high-speed cameras ultracam and ultraspec. We provide new or refined ephemerides, and supply mid-eclipse times for all observed eclipses. We assess the potential for light curve modelling of all 47 eclipsing systems to determine their system parameters, finding 20 systems which appear to be suitable for future study
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