281 research outputs found
Quiescent photometric modulations of two low-inclination cataclysmic variables KZGem and TWVir
The quiescent periodic photometric modulations of two low-inclination
cataclysmic variables observed in Kepler K2 Campaigns 0 and 1, KZ Gem and TW
Vir, are investigated. A phase-correcting method was successfully used to
detect the orbital modulations of KZ Gem and TW Vir and improve their orbital
periods. The light curve morphologies of both CVs were further analyzed by
defining flux ratios and creating colormaps. KZ Gem shows ellipsoidal
modulations with an orbital period of 0.22242(1) day, twice the period listed
in the updated RK catalogue (Edition 7.24). With this newly determined period,
KZ Gem is no longer a CV in the period gap, but a long-period CV. A part of the
quiescent light curve of TW Vir that had the highest stability was used to
deduce its improved orbital period of 0.182682(3) day. The flat patterns shown
in the colormaps of the flux ratios for KZ Gem demonstrate the stability of
their orbital modulations, while TW Vir show variable orbital modulations
during the K2 datasets. In TW Vir, the single versus double-peaked nature of
the quiescent orbital variations before and after superoutburst may be related
to the effect of the superoutburst on the accretion disk.Comment: 10 pages, 12 figures, accepted by A&
Resolving Gamma-Ray Burst 000301C with a Gravitational Microlens
The afterglow of the Gamma-Ray Burst (GRB) 000301C exhibited achromatic,
short time-scale variability that is difficult to reconcile with the standard
relativistic shock model. We interpret the observed light curves as a
microlensing event superimposed on power-law flux decays typical of afterglows.
In general, a relativistic GRB shock appears on the sky as a thin ring
expanding at a superluminal speed. Initially the ring is small relative to its
angular separation from the lens and so its flux is magnified by a constant
factor. As the ring grows and sweeps across the lens its magnification reaches
a maximum. Subsequently, the flux gradually recovers its unlensed value. This
behavior involves only three free parameters in its simplest formulation and
was predicted theoretically by Loeb & Perna (1998). Fitting the available
R-band photometric data of GRB 000301C to a simple model of the microlensing
event and a broken power-law for the afterglow, we find reasonable values for
all the parameters and a reduced chi^2/DOF parameter of 1.48 compared with 2.99
for the broken power-law fit alone. The peak magnification of ~2 occurred 3.8
days after the burst. The entire optical-IR data imply a width of the GRB ring
of order 10% of its radius, similar to theoretical expectations. The angular
resolution provided by microlensing is better than a micro-arcsecond. We infer
a mass of approximately 0.5 M_Sun for a lens located half way to the source at
z_s=2.04. A galaxy 2'' from GRB 000301C might be the host of the stellar lens,
but current data provides only an upper-limit on its surface brightness at the
GRB position.Comment: to appear in the ApJ Letters, 13 pages, 3 figures (one additional
figure included); all data used for the fits available at
ftp://cfa-ftp.harvard.edu/pub/kstanek/GRB000301C/ and through WWW at
http://cfa-www.harvard.edu/cfa/oir/Research/GRB
The mass of the neutron star in the binary millisecond pulsar PSR J1012+5307
We have measured the radial velocity variation of the white dwarf secondary in the binary system containing the millisecond pulsar PSR J1012 + 5307. Combined with the orbital parameters of the radio pulsar, we infer a mass ratio q (=M-1/M-2) = 10.5 +/- 0.5 OUT optical spectroscopy has also allowed us to determine the mass of the white dwarf companion by fitting the spectrum to a grid of DA model atmospheres: we estimate M-2 = 0.16 +/- 0.02 M., and hence the mass of the neutron star is 1.64 +/- 0.22 M., where the error is dominated by that of M-2. The orbital inclination is 52 +/- 4 deg. For an initial neutron star mass of similar to 1.4 M., only a few tenths of a solar mass at most has been successfully accreted over the lifetime of the progenitor low-mass X-ray binary. If the initial mass of the secondary was similar to 1 M., our result suggests that the mass transfer may have been non-conservative
- …