129 research outputs found
The X-ray properties of the magnetic cataclysmic variable UUColumbae
Aims. XMM-Newton observations to determine for the first time the broad-band X-ray properties of the faint, high galactic latitude intermediate polar UUCol are presented.
Methods. We performed X-ray timing analysis in different energy ranges of the EPIC cameras, which reveals the dominance of the 863 s white dwarf rotational period. The spin pulse is strongly energy dependent. Weak variabilities at the beat 935 s and at the 3.5 h orbital periods are also observed, but the orbital modulation is detected only below 0.5 keV. Simultaneous UV and optical photometry shows that the spin pulse is anti-phased with respect to the hard X-rays. Analysis of the EPIC and RGS spectra reveals
the complexity of the X-ray emission, which is composed of a soft 50 eV blackâbody component and two optically thin emission components at 0.2 keV and 11 keV strongly absorbed by dense material with an equivalent hydrogen column density of 1023 cmâ2 that partially (50%) covers the X-ray source.
Results. The complex X-ray and UV/optical temporal behaviour indicates that accretion occurs predominantly (âŒ80%) via a disc with a partial contribution (âŒ20%) directly from the stream. The main accreting pole dominates at high energies whilst the secondary pole mainly contributes in the soft X-rays and at lower energies. The bolometric flux ratio of the soft-to-hard X-ray emissions is found to be consistent with the prediction of the standard accretion shock model. We find the white dwarf in UUCol accretes at a low rate and possesses a low magnetic moment. It is therefore unlikely that UUCol will evolve into a moderate field strength polar, so that the soft X-ray intermediate polars still remain an enigmatic small group of magnetic cataclysmic variables
Analysis of the NGXO Telescope X-Ray Hartmann Data
Next Generation X-Ray Optics (NGXO) team at the Goddard Space Flight Center (GSFC) has been developing a new silicon-based grazing incidence mirror technology for future high resolution x-ray astronomical missions. Recently, the GSFC team completed the construction of first few mirror modules that contain one pair of mirrors. One of the mirror pairs was tested in GSFC 600-m long beamline facility and Panter (Neuried, Germay) 120-m long x-ray beamline facility. Both full aperture x-ray tests, Hartmann tests, and focal plane sweeps were completed. In this paper we present the data analysis process and compare the results from our models to measured x-ray centroid data, x-ray performance data, and out of focus images of the mirror pair
A deep Large Binocular Telescope view of the Canes Venatici I dwarf galaxy
We present the first deep color-magnitude diagram of the Canes Venatici I
(CVnI) dwarf galaxy from observations with the wide field Large Binocular
Camera on the Large Binocular Telescope. Reaching down to the main-sequence
turnoff of the oldest stars, it reveals a dichotomy in the stellar populations
of CVnI: it harbors an old (> 10 Gyr), metal-poor ([Fe/H] ~ -2.0) and spatially
extended population along with a much younger (~ 1.4-2.0 Gyr), 0.5 dex more
metal-rich, and spatially more concentrated population. These young stars are
also offset by 64_{-20}^{+40} pc to the East of the galaxy center. The data
suggest that this young population, which represent ~ 3-5 % of the stellar mass
of the galaxy within its half-light radius, should be identified with the
kinematically cold stellar component found by Ibata et al. (2006). CVnI
therefore follows the behavior of the other remote MW dwarf spheroidals which
all contain intermediate age and/or young populations: a complex star formation
history is possible in extremely low-mass galaxies.Comment: 4 pages, 3 figures, accepted for publication in ApJL. Minor changes,
conclusions unchange
The importance of Rydberg orbitals in dissociative ionization of small hydrocarbon molecules in intense laser fields
Much of our intuition about strong-field processes is built upon studies of diatomic molecules, which typically have electronic states that are relatively well separated in energy. In polyatomic molecules, however, the electronic states are closer together, leading to more complex interactions. A combined experimental and theoretical investigation of strong-field ionization followed by hydrogen elimination in the hydrocarbon series C2D2, C2D4 and C2D6 reveals that the photofragment angular distributions can only be understood when the field-dressed orbitals rather than the field-free orbitals are considered. Our measured angular distributions and intensity dependence show that these field-dressed orbitals can have strong Rydberg character for certain orientations of the molecule relative to the laser polarization and that they may contribute significantly to the hydrogen elimination dissociative ionization yield. These findings suggest that Rydberg contributions to field-dressed orbitals should be routinely considered when studying polyatomic molecules in intense laser fields
Effects of color superconductivity on the structure and formation of compact stars
We show that if color superconducting quark matter forms in hybrid or quark
stars it is possible to satisfy most of recent observational boundaries on
masses and radii of compact stellar objects. An energy of the order of
erg is released in the conversion from a (metastable) hadronic star
into a (stable) hybrid or quark star in presence of a color superconducting
phase. If the conversion occurs immediately after the deleptonization of the
proto-neutron star, the released energy can help Supernovae to explode. If the
conversion is delayed the energy released can power a Gamma Ray Burst. A delay
between the Supernova and the subsequent Gamma Ray Burst is possible, in
agreement with the delay proposed in recent analysis of astrophysical data.Comment: 4 pages, 2 figures. To be published in Phys.Rev.
Spitzer Space Telescope observations of magnetic cataclysmic variables: possibilities for the presence of dust in polars
We present Spitzer Space Telescope photometry of six short-period polars, EF
Eri, V347 Pav, VV Pup, V834 Cen, GG Leo, and MR Ser. We have combined the
Spitzer Infrared Array Camera (3.6 -8.0 microns) data with the 2MASS J, H, K_s
photometry to construct the spectral energy distributions of these systems from
the near- to mid-IR (1.235 - 8 microns). We find that five out of the six
polars have flux densities in the mid-IR that are substantially in excess of
the values expected from the stellar components alone. We have modeled the
observed SEDs with a combination of contributions from the white dwarf,
secondary star, and either cyclotron emission or a cool, circumbinary dust disk
to fill in the long-wavelength excess. We find that a circumbinary dust disk is
the most likely cause of the 8 micron excess in all cases, but we have been
unable to rule out the specific (but unlikely) case of completely optically
thin cyclotron emission as the source of the observed 8 micron flux density.
While both model components can generate enough flux at 8 microns, neither dust
nor cyclotron emission alone can match the excess above the stellar components
at all wavelengths. A model combining both cyclotron and dust contributions,
possibly with some accretion-generated flux in the near-IR, is probably
required, but our observed SEDs are not sufficiently well-sampled to constrain
such a complicated model. If the 8 micron flux density is caused by the
presence of a circumbinary dust disk, then our estimates of the masses of these
disks are many orders of magnitude below the mass required to affect CV
evolution.Comment: 58 pages, 14 figures, ApJ accepte
New solar axion search in CAST with He filling
The CERN Axion Solar Telescope (CAST) searches for conversion in
the 9 T magnetic field of a refurbished LHC test magnet that can be directed
toward the Sun. Two parallel magnet bores can be filled with helium of
adjustable pressure to match the X-ray refractive mass to the axion
search mass . After the vacuum phase (2003--2004), which is optimal for
eV, we used He in 2005--2007 to cover the mass range of
0.02--0.39 eV and He in 2009--2011 to scan from 0.39--1.17 eV. After
improving the detectors and shielding, we returned to He in 2012 to
investigate a narrow range around 0.2 eV ("candidate setting" of our
earlier search) and 0.39--0.42 eV, the upper axion mass range reachable with
He, to "cross the axion line" for the KSVZ model. We have improved the
limit on the axion-photon coupling to (95% C.L.), depending on the pressure settings. Since 2013, we
have returned to vacuum and aim for a significant increase in sensitivity.Comment: CAST Collaboration 6 pages 3 figure
Restricting quark matter models by gravitational wave observation
We consider the possibilities for obtaining information about the equation of
state for quark matter by using future direct observational data on
gravitational waves. We study the nonradial oscillations of both fluid and
spacetime modes of pure quark stars. If we observe the and the lowest
modes from quark stars, by using the simultaneously obtained
radiation radius we can constrain the bag constant with reasonable
accuracy, independently of the quark mass.Comment: To appear in Phys. Rev.
The spin periods and magnetic moments of white dwarfs in magnetic cataclysmic variables
We have used a model of magnetic accretion to investigate the rotational
equilibria of magnetic cataclysmic variables (mCVs). The results of our
numerical simulations demonstrate that there is a range of parameter space in
the P_spin / P_orb versus mu_1 plane at which rotational equilibrium occurs.
This has allowed us to calculate the theoretical histogram describing the
distribution of magnetic CVs as a function of P_spin / P_orb. We show that this
agrees with the observed distribution assuming that the number of systems as a
function of white dwarf magnetic moment is distributed approximately according
to N(mu_1) d mu_1 proportional to 1/mu_1 d mu_1. The rotational equilibria also
allow us to infer approximate values for the magnetic moments of all known
intermediate polars. We predict that intermediate polars with mu_1 > 5 x 10^33
G cm^3 and P_orb > 3h will evolve into polars, whilst those with mu_1 < 5 x
10^33 G cm^3 and P_orb > 3h will either evolve into low field strength polars
which are (presumably) unobservable, and possibly EUV emitters, or, if their
fields are buried by high accretion rates, evolve into conventional polars once
their magnetic fields re-surface when the mass accretion rate reduces. We
speculate that EX Hya-like systems may have low magnetic field strength
secondaries and so avoid synchronisation. Finally we note that the equilibria
we have investigated correspond to a variety of different types of accretion
flow, including disc-like accretion at small P_spin / P_orb values, stream-like
accretion at intermediate P_spin / P_orb values, and accretion fed from a ring
at the outer edge of the white dwarf Roche lobe at higher P_spin / P_orb
values.Comment: Accepted for publication in ApJ. Figre 3 included here as a low
resolution gif onl
First Light Measurements of Capella with the Low Energy Transmission Grating Spectrometer aboard the Chandra X-ray Observatory
We present the first X-ray spectrum obtained by the Low Energy Transmission
Grating Spectrometer (LETGS) aboard the Chandra X-ray Observatory. The spectrum
is of Capella and covers a wavelength range of 5-175 A (2.5-0.07 keV). The
measured wavelength resolution, which is in good agreement with ground
calibration, is 0.06 A (FWHM). Although in-flight
calibration of the LETGS is in progress, the high spectral resolution and
unique wavelength coverage of the LETGS are well demonstrated by the results
from Capella, a coronal source rich in spectral emission lines. While the
primary purpose of this letter is to demonstrate the spectroscopic potential of
the LETGS, we also briefly present some preliminary astrophysical results. We
discuss plasma parameters derived from line ratios in narrow spectral bands,
such as the electron density diagnostics of the He-like triplets of carbon,
nitrogen, and oxygen, as well as resonance scattering of the strong Fe XVII
line at 15.014 A.Comment: 4 pages (ApJ letter LaTeX), 2 PostScript figures, accepted for
publication in ApJ Letters, 200
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