1,188 research outputs found
EUVE J0425.6-5714: A Newly Discovered AM Herculis Star
We detected a new AM Her star serendipitously in a 25 day observation with
the EUVE satellite. A coherent period of 85.82 min is present in the EUVE Deep
Survey imager light curve of this source. A spectroscopic identification is
made with a 19th magnitude blue star that has H and He emission lines, and
broad cyclotron humps typical of a magnetic cataclysmic variable. A lower limit
to the polar magnetic field of 46 MG is estimated from the spacing of the
cyclotron harmonics. EUVE J0425.6-5714 is also detected in archival ROSAT HRI
observations spanning two months, and its stable and highly structured light
curve permits us to fit a coherent ephemeris linking the ROSAT and EUVE data
over a 1.3 yr gap. The derived period is 85.82107 +/- 0.00020 min, and the
ephemeris should be accurate to 0.1 cycles until the year 2005. A narrow but
partial X-ray eclipse suggests that this object belongs to the group of Am Her
stars whose viewing geometry is such that the accretion stream periodically
occults the soft X-ray emitting accretion spot on the surface of the white
dwarf. A non-detection of hard X-rays from ASCA observations that are
contemporaneous with the ROSAT HRI shows that the soft X-rays must dominate by
at least an order of magnitude, which is consistent with a known trend among AM
Her stars with large magnetic field. This object should not be confused with
the Seyfert galaxy 1H 0419-577 (= LB 1727), another X-ray/EUV source which lies
only 4' away, and was the principal target of these monitoring observations.Comment: 13 pages, 5 figures, to appear in PASP, Dec. 1998 issu
ORFEUS II Far-UV Spectroscopy of AM Herculis
Six high-resolution (\lambda/\Delta\lambda ~ 3000) far-UV (\lambda\lambda =
910-1210 \AA) spectra of the magnetic cataclysmic variable AM Herculis were
acquired in 1996 November during the flight of the ORFEUS-SPAS II mission. AM
Her was in a high optical state at the time of the observations, and the
spectra reveal emission lines of O VI \lambda\lambda 1032, 1038, C III \lambda
977, \lambda 1176, and He II \lambda 1085 superposed on a nearly flat
continuum. Continuum flux variations can be described as per Gansicke et al. by
a ~ 20 kK white dwarf with a ~ 37 kK hot spot covering a fraction f~0.15 of the
surface of the white dwarf, but we caution that the expected Lyman absorption
lines are not detected. The O VI emission lines have narrow and broad component
structure similar to that of the optical emission lines, with radial velocities
consistent with an origin in the irradiated face of the secondary and the
accretion funnel, respectively. The density of the narrow- and broad-line
regions is n_{nlr} ~ 3\times 10^{10} cm^{-3} and n_{blr} ~ 1\times 10^{12}
cm^{-3}, respectively, yet the narrow-line region is optically thick in the O
VI line and the broad-line region is optically thin; apparently, the velocity
shear in the broad-line region allows the O VI photons to escape, rendering the
gas effectively optically thin. Unexplained are the orbital phase variations of
the emission-line fluxes.Comment: 15 pages, 6 Postscript figures; LaTeX format, uses aaspp4.sty;
table2.tex included separately because it must be printed sideways - see
instructions in the file; accepted on April 17, 1998 for publication in The
Astrophysical Journa
Variability of the Accretion Stream in the Eclipsing Polar EP Dra
We present the first high time resolution light curves for six eclipses of
the magnetic cataclysmic variable EP Dra, taken using the superconducting
tunnel junction imager S-Cam2. The system shows a varying eclipse profile
between consecutive eclipses over the two nights of observation. We attribute
the variable stream eclipse after accretion region ingress to a variation in
the amount and location of bright material in the accretion stream. This
material creates an accretion curtain as it is threaded by many field lines
along the accretion stream trajectory. We identify this as the cause of
absorption evident in the light curves when the system is in a high accretion
state. We do not see direct evidence in the light curves for an accretion spot
on the white dwarf; however, the variation of the stream brightness with the
brightness of the rapid decline in flux at eclipse ingress indicates the
presence of some form of accretion region. This accretion region is most likely
located at high colatitude on the white dwarf surface, forming an arc shape at
the foot points of the many field lines channeling the accretion curtain.Comment: Accepted for publication in MNRAS (7 pages
A panchromatic analysis of starburst galaxy M82: Probing the dust properties
(Abridged) We combine NUV, optical and IR imaging of the nearby starburst
galaxy M82 to explore the properties of the dust both in the interstellar
medium of the galaxy and the dust entrained in the superwind. The three NUV
filters of Swift/UVOT enable us to probe in detail the properties of the
extinction curve in the region around the 2175A bump. The NUV colour-colour
diagram strongly rules out a Calzetti-type law, which can either reflect
intrinsic changes in the dust properties or in the star formation history
compared to starbursts well represented by such an attenuation law. We
emphasize that it is mainly in the NUV region where a standard Milky-Way-type
law is preferred over a Calzetti law. The age and dust distribution of the
stellar populations is consistent with the scenario of an encounter with M81 in
the recent 400 Myr. The radial gradients of the NUV and optical colours in the
superwind region support the hypothesis that the emission in the wind cone is
driven by scattering from dust grains entrained in the ejecta. The observed
wavelength dependence reveals either a grain size distribution , where is the size of the grain, or a flatter distribution with a
maximum size cutoff, suggesting that only small grains are entrained in the
supernovae-driven wind.Comment: 12 pages, 12 figures, 3 tables, MNRAS, in pres
EUVE Observations of the Magnetic Cataclysmic Variable QQ Vulpeculae
We present simultaneous X-ray (lambda_peak ~ 44A) and EUV (lambda_peak = 89A)
light curves for the magnetic cataclysmic variable QQ Vulpeculae, obtained with
the EUVE satellite. We find that the unique shape of the X-ray light curve is
different from previously obtained X-ray light curves of QQ Vul and provides
evidence for two-pole accretion. Detailed examination of the photometric data
indicates that QQ Vul undergoes a stellar eclipse of the X-ray emitting region,
indicative of a high binary inclination. We discuss possible implications for
the nature of this system given the observed shape of its EUV and X-ray light
curves.Comment: 12 pages including 4 figures, accepted to PAS
SN1993J VLBI (I): The Center of the Explosion and a Limit on Anisotropic Expansion
Phase-referenced VLBI observations of supernova 1993J at 24 epochs, from 50
days after shock breakout to the present, allowed us to determine the
coordinates of the explosion center relative to the quasi-stationary core of
the host galaxy M81 with an accuracy of 45 micro-arcsec, and to determine the
nominal proper motion of the geometric center of the radio shell with an
accuracy of 9micro-arcsec/yr. The uncertainties correspond to 160 AU for the
position and 160 km/s for the proper motion at the distance of the source of
3.63 Mpc. After correcting for the expected galactic proper motion of the
supernova around the core of M81 using HI rotation curves, we obtain a peculiar
proper motion of the radio shell center of only 320 +/- 160 km/s to the south,
which limits any possible one-sided expansion of the shell. We also find that
the shell is highly circular, the outer contours in fact being circular to
within 3%. Combining our proper motion values with the degree of circular
symmetry, we find that the expansion of the shockfront from the explosion
center is isotropic to within 5.5% in the plane of the sky. This is a more
fundamental result on isotropic expansion than can be derived from the
circularity of the images alone. The brightness of the radio shell, however,
varies along the ridge and systematically changes with time. The degree of
isotropy in the expansion of the shockfront contrasts with the asymmetries and
polarization found in optical spectral lines. Asymmetric density distributions
in the ejecta or more likely in the circumstellar medium, are favored to
reconcile the radio and optical results. We see no sign of any disk-like
density distribution of the circumstellar material, with the average axis ratio
of the radio shell of SN1993J being less than 1.04.Comment: 21 pages, LaTex + 5 Figures (encapulsated PostScript), Accepted for
Publication in the Astrophysical Journa
UBVRI photopolarimetry of the long period eclipsing AM Herculis binary V1309
We report simultaneous UBVRI photo-polarimetric observations of the long
period (7.98 h) AM Her binary V1309 Ori. The length and shape of the eclipse
ingress and egress varies from night to night. We suggest this is due to the
variation in the brightness of the accretion stream. By comparing the phases of
circular polarization zero-crossovers with previous observations, we confirm
that V1309 Ori is well synchronized, and find an upper limit of 0.002 percent
for the difference between the spin and orbital periods. We model the
polarimetry data using a model consisting of two cyclotron emission regions at
almost diametrically opposite locations, and centered at colatitude 35 (deg)
and 145 (deg) on the surface of the white dwarf. We also present archive X-ray
observations which show that the negatively polarised accretion region is X-ray
bright.Comment: 11 pages, 12 figures (2 colour), Fig1 and Fig 4 are in lower
resolution than in original paper, accepted for publication in Monthly
Notices of the Royal Astronomical Societ
The birthplace and age of the isolated neutron star RX J1856.5-3754
X-ray observations unveiled various types of radio-silent Isolated Neutron
Stars (INSs), phenomenologically very diverse, e.g. the Myr old X-ray Dim INS
(XDINSs) and the kyr old magnetars. Although their phenomenology is much
diverse, the similar periods (P=2--10 s) and magnetic fields (~10^{14} G)
suggest that XDINSs are evolved magnetars, possibly born from similar
populations of supermassive stars. One way to test this hypothesis is to
identify their parental star clusters by extrapolating backward the neutron
star velocity vector in the Galactic potential. By using the information on the
age and space velocity of the XDINS RX J1856.5-3754, we computed backwards its
orbit in the Galactic potential and searched for its parental stellar cluster
by means of a closest approach criterion. We found a very likely association
with the Upper Scorpius OB association, for a neutron star age of 0.42+/-0.08
Myr, a radial velocity V_r^NS =67+/- 13$ km s^{-1}, and a present-time
parallactic distance d_\pi^NS = 123^{+11}_{-15} pc. Our result confirms that
the "true" neutron star age is much lower than the spin-down age (tau_{sd}=3.8
Myrs), and is in good agreement with the cooling age, as computed within
standard cooling scenarios. The mismatch between the spin-down and the
dynamical/cooling age would require either an anomalously large breaking index
(n~20) or a decaying magnetic field with initial value B_0 ~ 10^{14} G.
Unfortunately, owing to the uncertainty on the age of the Upper Scorpius OB
association and the masses of its members we cannot yet draw firm conclusions
on the estimated mass of the RX J1856.5-3754 progenitor.Comment: 6 pages, accepted for publication on Monthly Notices of the Royal
Astronomical Societ
Probing the Pulsar Wind Nebula of PSR B0355+54
We present XMM-Newton and Chandra X-ray observations of the middle-aged radio
pulsar PSR B0355+54. Our X-ray observations reveal emission not only from the
pulsar itself, but also from a compact diffuse component extending ~50'' in the
opposite direction to the pulsar's proper motion. There is also evidence for
the presence of fainter diffuse emission extending ~5' from the point source.
The compact diffuse feature is well-fitted with a power-law, the index of which
is consistent with the values found for other pulsar wind nebulae. The
morphology of the diffuse component is similar to the ram-pressure confined
pulsar wind nebulae detected for other sources. The X-ray emission from the
pulsar itself is described well by a thermal plus power-law fit, with the
thermal emission most likely originating in a hot polar cap.Comment: 9 pages (uses emulateapj.cls), 8 figures, 2 tables, accepted for
publication in Ap
Analysis of the Flux and Polarization Spectra of the Type Ia Supernova SN 2001el: Exploring the Geometry of the High-velocity Ejecta
SN 2001el is the first normal Type Ia supernova to show a strong, intrinsic
polarization signal. In addition, during the epochs prior to maximum light, the
CaII IR triplet absorption is seen distinctly and separately at both normal
photospheric velocities and at very high velocities. The high-velocity triplet
absorption is highly polarized, with a different polarization angle than the
rest of the spectrum. The unique observation allows us to construct a
relatively detailed picture of the layered geometrical structure of the
supernova ejecta: in our interpretation, the ejecta layers near the photosphere
(v \approx 10,000 km/s) obey a near axial symmetry, while a detached,
high-velocity structure (v \approx 18,000-25,000 km/s) with high CaII line
opacity deviates from the photospheric axisymmetry. By partially obscuring the
underlying photosphere, the high-velocity structure causes a more incomplete
cancellation of the polarization of the photospheric light, and so gives rise
to the polarization peak and rotated polarization angle of the high-velocity IR
triplet feature. In an effort to constrain the ejecta geometry, we develop a
technique for calculating 3-D synthetic polarization spectra and use it to
generate polarization profiles for several parameterized configurations. In
particular, we examine the case where the inner ejecta layers are ellipsoidal
and the outer, high-velocity structure is one of four possibilities: a
spherical shell, an ellipsoidal shell, a clumped shell, or a toroid. The
synthetic spectra rule out the spherical shell model, disfavor a toroid, and
find a best fit with the clumped shell. We show further that different
geometries can be more clearly discriminated if observations are obtained from
several different lines of sight.Comment: 14 pages (emulateapj5) plus 18 figures, accepted by The Astrophysical
Journa
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