8 research outputs found
The rapidly pulsating sdO star, SDSS J160043.6+074802.9
A spectroscopic analysis of SDSS J160043.6+074802.9, a binary system
containing a pulsating subdwarf-O (sdO) star with a late-type companion, yields
Teff = 70 000 +/- 5000 K and log g = 5.25 +/- 0.30, together with a most likely
type of K3V for the secondary star. We compare our results with atmospheric
parameters derived by Fontaine et al. (2008) and in the context of existing
evolution models for sdO stars. New and more extensive photometry is also
presented which recovers most, but not all, frequencies found in an earlier
paper. It therefore seems probable that some pulsation modes have variable
amplitudes. A non-adiabatic pulsation analysis of uniform metallicity sdO
models show those having log g > 5.3 to be more likely to be unstable and
capable of driving pulsation in the observed frequency range.Comment: 14 pages, 12 figures, accepted for publication in MNRAS, 2009
September
A radio-pulsing white dwarf binary star
White dwarfs are compact stars, similar in size to Earth but ~200,000 times more massive. Isolated white dwarfs emit most of their power from ultraviolet to near-infrared wavelengths, but when in close orbits with less dense stars, white dwarfs can strip material from their companions, and the resulting mass transfer can generate atomic line and X-ray emission, as well as near- and mid-infrared radiation if the white dwarf is magnetic. However, even in binaries, white dwarfs are rarely detected at far-infrared or radio frequencies. Here we report the discovery of a white dwarf / cool star binary that emits from X-ray to radio wavelengths. The star, AR Scorpii (henceforth AR Sco), was classified in the early 1970s as a delta-Scuti star, a common variety of periodic variable star. Our observations reveal instead a 3.56 hr period close binary, pulsing in brightness on a period of 1.97 min. The pulses are so intense that AR Sco's optical flux can increase by a factor of four within 30 s, and they are detectable at radio frequencies, the first such detection for any white dwarf system. They reflect the spin of a magnetic white dwarf which we find to be slowing down on a 10^7 yr timescale. The spin-down power is an order of magnitude larger than that seen in electromagnetic radiation, which, together with an absence of obvious signs of accretion, suggests that AR Sco is primarily spin-powered. Although the pulsations are driven by the white dwarf's spin, they originate in large part from the cool star. AR Sco's broad-band spectrum is characteristic of synchrotron radiation, requiring relativistic electrons. These must either originate from near the white dwarf or be generated in situ at the M star through direct interaction with the white dwarf's magnetosphere
The Peculiar Binary System AE Aquarii from its Characteristic Multi-wavelength Emission
The multi-wavelength properties of the novalike variable system AE Aquarii are discussed in terms of the interaction between the accretion inflow from a late-type main sequence star and the magnetosphere of a fast rotating white dwarf. This results in an efficient magnetospheric propeller process and particle acceleration. The spin-down of the white dwarf at a period rate of 5.64Ă10â14 s sâ1 results in a huge spin-down luminosity of Lsâd â 6 10Ă33 erg sâ1. Hence, the observed non-thermal hard X-ray emission and VHE and TeV gamma-ray emission may suggest that AE Aquarii can be placed in the category of spin-powered pulsars. Besides, observed hard X-ray luminosity of LX,hard †5 Ă 1030 erg sâ1 constitutes 0.1 % of the total spin-down luminosity of the white dwarf. This paper will discuss some recent theoretical studies and data analysis of the system
Accreting Pulsars: Mixing-up Accretion Phases in Transitional Systems
In the last 20 years our understanding of the millisecond pulsar (MSP)
population changed dramatically. Thanks to RXTE, we discovered that neutron
stars in LMXBs spins at 200-750 Hz frequencies, and indirectly confirmed the
recycling scenario, according to which neutron stars are spun up to ms periods
during the LMXB-phase. In the meantime, the continuous discovery of
rotation-powered MSPs in binary systems in the radio and gamma-ray band (mainly
with the Fermi LAT) allowed us to classify these sources into two "spiders"
populations, depending on the mass of their companion stars: Black Widow, with
very low-mass companion stars, and Redbacks, with larger companions possibly
filling their Roche lobes but without accretion. It was soon regained that MSPs
in short orbital period LMXBs are the progenitors of the spider populations of
rotation-powered MSPs, although a direct link between accretion- and
rotation-powered MSPs was still missing. In 2013 XMM-Newton spotted the X-ray
outburst of a new accreting MSP (IGR J18245-2452) in a source that was
previously classified as a radio MSP. Follow up observations of the source when
it went back to X-ray quiescence showed that it was able to swing between
accretion- to rotation-powered pulsations in a relatively short timescale (few
days), promoting this source as the direct link between the LMXB and the radio
MSP phases. Following discoveries showed that there exists a bunch of sources,
which alternates X-ray activity phases, showing X-ray pulsations, to radio-loud
phases, showing radio pulsations, establishing a new class of MSPs: the
Transitional MSP. In this review we describe these exciting discoveries and the
properties of accreting and transitional MSPs, highlighting what we know and
what we have still to learn about in order to fully understand the (sometime
puzzling) behavior of these systems and their evolutive connection (abridged)
The role of arbuscular mycorrhizal fungi in grain production and nutrition of sorghum genotypes: Enhancing sustainability through plant-microbial partnership
Effect of community-based soil and water conservation practices on arbuscular mycorrhizal fungi types, spore densities, root colonization, and soil nutrients in the northern highlands of Ethiopia
Soil management practices affect arbuscular mycorrhizal fungi propagules, root colonization and growth of rainfed maize
Paving the way to simultaneous multi-wavelength astronomy
Whilst astronomy as a science is historically founded on observations at optical wavelengths, studying the Universe in other bands has yielded remarkable discoveries, from pulsars in the radio, signatures of the Big Bang at submm wavelengths, through to high energy emission from accreting, gravitationally-compact objects and the discovery of gamma-ray bursts. Unsurprisingly, the result of combining multiple wavebands leads to an enormous increase in diagnostic power, but powerful insights can be lost when the sources studied vary on timescales shorter than the temporal separation between observations in different bands. In July 2015, the workshop "Paving the way to simultaneous multi-wavelength astronomy" was held as a concerted effort to address this at the Lorentz Center, Leiden. It was attended by 50 astronomers from diverse fields as well as the directors and staff of observatories and spaced-based missions. This community white paper has been written with the goal of disseminating the findings of that workshop by providing a concise review of the field of multi-wavelength astronomy covering a wide range of important source classes, the problems associated with their study and the solutions we believe need to be implemented for the future of observational astronomy. We hope that this paper will both stimulate further discussion and raise overall awareness within the community of the issues faced in a developing, important field