654 research outputs found
QZ Serpentis: A Dwarf Nova with a 2-Hour Orbital Period and an Anomalously Hot, Bright Secondary Star
We present spectroscopy and time-series photometry of the dwarf nova QZ Ser.
The spectrum shows a rich absorption line spectrum of type K4 +- 2. K-type
secondary stars are generally seen in dwarf novae with orbital periods P-orb
around 6 h, but in QZ Ser the absorption radial velocities show an obvious
modulation (semi-amplitude 207(5) km/s) at P-orb = 119.752(2) min, much shorter
than typical for such a relatively warm and prominent secondary spectrum. The
H-alpha emission-line velocity is modulated at the same period and roughly
opposite phase. Time-series photometry shows flickering superposed on a
modulation with two humps per orbit, consistent with ellipsoidal variation of
the secondary's light. QZ Ser is a second example of a relatively short-period
dwarf nova with a surprisingly warm secondary. Model calculations suggest that
the secondary is strongly enhanced in helium, and had already undergone
significant nuclear evolution when mass transfer began. Several sodium
absorption features in the secondary spectrum are unusually strong, which may
indicate that the present-day surface was the site of CNO-cycle hydrogen
burning in the past.Comment: 11 pages, 3 postscript figures, 1 jpeg greyscale figure. Accepted for
publication in PAS
Evidence for an oscillation of the magnetic axis of the white dwarf in the polar DP Leonis
From 1979 to 2001, the magnetic axis of the white dwarf in the polar DP Leo
slowly rotated by 50 deg in azimuth, possibly indicating a small asynchronism
between the rotational and orbital periods of the magnetic white dwarf. We have
obtained phase-resolved orbital light curves between 2009 and 2013, which show
that this trend has not continued in recent years. Our data are consistent with
the theoretically predicted oscillation of the magnetic axis of the white dwarf
about an equilibrium orientation, which is defined by the competition between
the accretion torque and the magnetostatic interaction of the primary and
secondary star. Our data indicate an oscillation period of ~60 yr, an amplitude
of about 25 deg, and an equilibrium orientation leading the connecting line of
the two stars by about 7 deg.Comment: Accepted by A&
A long-term optical and X-ray ephemeris of the polar EK Ursae Majoris
We searched for long-term period changes in the polar EK UMa using new
optical data and archival X-ray/EUV data. An optical ephemeris was derived from
data taken remotely with the MONET/N telescope and compared with the X-ray
ephemeris based on Einstein, Rosat, and EUVE data. A three-parameter fit to the
combined data sets yields the epoch, the period, and the phase offset between
the optical minima and the X-ray absorption dips. An added quadratic term is
insignificant and sets a limit to the period change. The derived linear
ephemeris is valid over 30 years and the common optical and X-ray period is
P=0.0795440225(24) days. There is no evidence of long-term O-C variations or a
period change over the past 17 years Delta P = -0.14+-0.50 ms. We suggest that
the observed period is the orbital period and that the system is tightly
synchronized. The limit on Delta P and the phase constancy of the bright part
of the light curve indicate that O-C variations of the type seen in the polars
DP Leo and HU Aqr or the pre-CV NN Ser do not seem to occur in EK UMa. The
X-ray dips lag the optical minima by 9.5+-0.7 deg in azimuth, providing some
insight into the accretion geometry.Comment: 4 pages, 2 Postscript figures, accepted for publication in Astronomy
& Astrophysic
Physical Parameters for the Afterglows of GRB 980703, 990123, 990510, and 991216 Determined from Modeling of Multi-Frequency Data
We model the radio, optical, and X-ray emission for the afterglows of GRB
980703, 990123, 990510, and 991216, within the framework of relativistic jets,
to determine their physical parameters. The models that yield acceptable fits
to the data have jet energies mostly between 10^{50} to 10^{51} erg and initial
opening angles between 1 deg and 4 deg. The external medium density is
uncertain by at least one order of magnitude in each case, being around
10^{-3}/cm^3 for GRB 980703 and 990123, ~0.1/cm^3 for GRB 990510, and ~3/cm^3
for GRB 991216. If the jets are uniform (i.e. there are no angular gradients of
the energy per solid angle) then the 20 keV -- 1 MeV radiative efficiency
during the GRB phase must have been at least 2-3% for GRB 990510, 20% for GRB
990123, and 30% for GRB 991216.Comment: accepted for publication by the ApJ, vol. 554. 11 pages, color
figures. Last figures replaced with probability distributions of model
parameter
A Systematic Analysis of Supernova Light in Gamma-Ray Burst Afterglows
We systematically reanalyzed all Gamma-Ray Burst (GRB) afterglow data
published through the end of 2002, in an attempt to detect the predicted
supernova light component and to gain statistical insight on its
phenomenological properties. We fit the observed photometric light curves as
the sum of an afterglow, an underlying host galaxy, and a supernova component.
The latter is modeled using published multi-color light curves of SN 1998bw as
a template. The total sample of afterglows with established redshifts contains
21 bursts (GRB 970228 - GRB 021211). For nine of these GRBs a weak supernova
excess (scaled to SN 1998bw) was found, what makes this to one of the first
samples of high-z core collapse supernovae. Among this sample are all bursts
with redshifts less than ~0.7. These results strongly support the notion that
in fact all afterglows of long-duration GRBs contain light from an associated
supernova. A statistics of the physical parameters of these GRB-supernovae
shows that SN 1998bw was at the bright end of its class, while it was not
special with respect to its light curve shape. Finally, we have searched for a
potential correlation of the supernova luminosities with the properties of the
corresponding bursts and optical afterglows, but we have not found such a
relation.Comment: 25 pages, 7 figures, accepted by ApJ; revised, shortened and updated
compared to version 1; Title slightly changed; all figures showing individual
afterglow light curves removed, as advised by the referee; conclusions
unchange
X-Shooting EF Eridani: further evidence for a massive white dwarf and a sub-stellar secondary
High resolution spectral observations of the polar EF Eridani obtained in a
low state with X-Shooter revealed narrow emission lines from the irradiated
secondary. The lines were most prominent in the near-IR Ca2-triplet, the more
intensive Halpha line had additional emission likely originating from an
accretion stream. The lines with a radial velocity amplitude, K2' = 385 +- 4
kmps, serve as tracer of the otherwise unseen companion. The mass function
implies a massive white dwarf with Mwd > 0.65 Msun at 3sigma confidence, and a
short distance to the binary, d ~ 111 pc (<145pc at 3sigma confidence). The
spectral energy distribution from the UV to the IR together with the high mass
ratio gives further strong evidence of EF Eri being a post period-minimum
object with M2 < 0.06 MsunComment: Astron, Astrophys., in pres
The high-field polar RX J1007.5-2017
We report optical and X-ray observations of the high-field polar
RXJ1007.5-2017 performed between 1990 and 2012. It has an orbital period of
208.60 min determined from the ellipsoidal modulation of the secondary star in
an extended low state. The spectral flux of the dM3- secondary star yields a
distance of 790+-105 pc. At low accretion levels, \RX{} exhibits pronounced
cyclotron emission lines. The second and third harmonic fall in the optical
regime and yield a field strength in the accretion spot of 94 MG. The source is
highly variable on a year-to-year basis and was encountered at visual
magnitudes between V \sim 20 and V \sim 16. In the intermediate state of 1992
and 2000, the soft X-ray luminosity exceeds the sum of the luminosities of the
cyclotron source, the hard X-ray source, and the accretion stream by an order
of magnitude. An X-ray high state, corresponding to the brightest optical
level, has apparently not been observed so far.Comment: To be published in A&
Afterglow Light Curves and Broken Power Laws: A Statistical Study
In gamma-ray burst research it is quite common to fit the afterglow light
curves with a broken power law to interpret the data. We apply this method to a
computer simulated population of afterglows and find systematic differences
between the known model parameters of the population and the ones derived from
the power law fits. In general, the slope of the electron energy distribution
is overestimated from the pre-break light curve slope while being
underestimated from the post-break slope. We also find that the jet opening
angle derived from the fits is overestimated in narrow jets and underestimated
in wider ones. Results from fitting afterglow light curves with broken power
laws must therefore be interpreted with caution since the uncertainties in the
derived parameters might be larger than estimated from the fit. This may have
implications for Hubble diagrams constructed using gamma-ray burst data.Comment: 4 pages, 5 figures, accepted for publication in ApJ Letter
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