337 research outputs found
Simultaneous X-ray and optical observations of thermonuclear bursts in the LMXB EXO 0748-676
We report the detection of a large number of optical bursts in the Low Mass
X-ray Binary (LMXB) EXO 0748-676 simultaneous with the thermonuclear X-ray
bursts. The X-ray and the optical bursts are detected in a long observation of
this source with the XMM-Newton observatory. This has increased the number of
thermonuclear X-ray bursts in the LMXBs with simultaneous optical detection by
several factors. The optical bursts are found to have a linear rise followed by
a slow, somewhat exponential decay. Most of the optical bursts have longer rise
and decay timescale compared to the corresponding X-ray bursts. We have
determined the X-ray and optical excess photon counts in the bursts that allow
us to look at the optical to X-ray burst fluence ratio for each burst and the
ratio as a function of the X-ray burst intensity and as a function of the
orbital phase. The delay between the onset of the X-ray bursts and the onset of
the optical bursts have also been measured and is found to have an average
value of 3.25 seconds. We do not find any convincing evidence of orbital phase
dependence of the following parameters: X-ray to optical delay, rise time of
the optical bursts, and optical to X-ray burst intensity ratio as would be
expected if the optical bursts were produced by reprocessing from the surface
of the secondary star that is facing the compact star. On the other hand, if
the optical bursts are produced by reprocessing of the X-rays in the accretion
disk, the onset of the bursts is not expected to have a sharp, linear shape as
is observed in a few of the bursts in EXO 0748-676. We emphasise the fact that
simultaneous optical observations of the X-ray bursts in multiple wavelength
bands will enable further detailed investigations of the reprocessing
phenomena, including any non-linear effect of the X-ray irradiation.Comment: 12 pares, 5 figures (Submitted to the Bulletin of the Astronomical
Society of India
An Untriggered Search for Optical Bursts
We present an untriggered search for optical bursts with the ROTSE-I
telephoto array. Observations were taken which monitor an effective 256 square
degree field continuously over 125 hours to m_{ROTSE}=15.7. The uniquely large
field, moderate limiting magnitude and fast cadence of 10 minutes permits
transient searches in a new region of sensitivity. Our search reveals no
candidate events. To quantify this result, we simulate potential optical bursts
with peak magnitude, m_{p}, at t=10 s, which fade as f=(\frac{t}{t_{0}})
^{\alpha_{t}}, where \alpha_t < 0. Simple estimates based on observational
evidence indicate that a search of this sensitivity begins to probe the
possible region occupied by GRB orphan afterglows. Our observing protocol and
image sensitivity result in a broad region of high detection efficiency for
light curves to the bright and slowly varying side of a boundary running from
[\alpha_{t},m_{p}]=[-2.0,6.0] to [-0.3,13.2]. Within this region, the
integrated rate of brief optical bursts is less than 1.1\times 10^{-8} {\rm
s}^{-1} {\rm deg}^{-2}. At 22 times the observed GRB rate from BATSE,
this suggests a limit on \frac{\theta_{opt}}{\theta_{\gamma}}\lesssim 5 where
\theta_{opt} and \theta_{\gamma} are the optical and gamma-ray collimation
angles, respectively. Several effects might explain the absence of optical
bursts, and a search of the kind described here but more sensitive by about 4
magnitudes should offer a more definitive probe.Comment: 8 pages, 6 figures, 1 tabl
CSI 2264: Simultaneous optical and X-ray variability in pre-Main Sequence stars. I: Time resolved X-ray spectral analysis during optical dips and accretion bursts in stars with disks
Pre-main sequence stars are variable sources. In stars with disks, this
variability is related to the morphology of the inner circumstellar region
(<0.1 AU) and that of the photosphere and corona, all impossible to be
spatially resolved with present day techniques. This has been the main
motivation for the Coordinated Synoptic Investigation of NGC 2264. In this
paper, we focus on the stars with disks. We analyze the X-ray spectral
properties extracted during optical bursts and dips in order to unveil the
nature of these phenomena. We analyze simultaneous CoRoT and Chandra/ACIS-I
observations to search for coherent optical and X-ray flux variability in stars
with disks. Then, stars are analyzed in two different samples. In stars with
variable extinction, we look for a simultaneous increase of optical extinction
and X-ray absorption during the optical dips; in stars with accretion bursts,
we search for soft X-ray emission and increasing X-ray absorption during the
bursts. Results. We find evidence for coherent optical and X-ray flux
variability among the stars with variable extinction. In 9/24 stars with
optical dips, we observe a simultaneous increase of X-ray absorption and
optical extinction. In seven dips, it is possible to calculate the NH/AV ratio
in order to infer the composition of the obscuring material. In 5/20 stars with
optical accretion bursts, we observe increasing soft X-ray emission during the
bursts that we associate to the emission of accreting gas. It is not surprising
that these properties are not observed in all the stars with dips and bursts,
since favorable geometric configurations are required. The observed variable
absorption during the dips is mainly due to dust-free material in accretion
streams. In stars with accretion bursts, we observe on average a larger soft
X-ray spectral component not observed in non accreting stars.Comment: Accepted for publication by Astronomy & Astrophysic
Long-Term Optical Observations Of Two Lmxbs: Uw Crb (=Ms 1603+260) And V1408 Aql (=4U 1957+115)
We present new optical photometry of two low-mass X-ray binary stars, UW CrB (MS 1603+260) and V1408 Aql (4U 1957+115). UW CrB is an eclipsing binary and we refine its eclipse ephemeris and measure an upper limit to the rate of change of its orbital period, vertical bar P vertical bar < 4.2 x 10(-11) (unitless). The light curve of UW CrB shows optical counterparts of type I X-ray bursts. We tabulate the times, orbital phases, and fluences of 33 bursts and show that the optical flux in the bursts comes primarily from the accretion disk, not from the secondary star. The new observations are consistent with a model in which the accretion disk in UW CrB is asymmetric and precesses in the prograde direction with a period of similar to 5.5 days. The light curve of V1408 Aql has a low-amplitude modulation at its 9.33 hr orbital period. The modulation remained a nearly pure sine curve in the new data as it was in 1984 and 2008, but its mean amplitude was lower, 18% against 23% in the earlier data. A model in which the orbital modulation is caused by the varying aspect of the heated face of the secondary star continues to give an excellent fit to the light curve. We derive a much improved orbital ephemeris for the system.NSF 0958783Astronom
XTE J2123-058: A New Neutron Star X-Ray Transient
We report on optical and RXTE observations of a new high-latitude bursting
X-ray transient, XTE J2123-058. We identified the optical counterpart, measured
the 5.9573 hr orbital period and constrained the binary inclination and the
source distance. The distance lower limit indicates that the source is at least
2.6 kpc from the Galactic plane, which is unusual for an LMXB. RXTE
observations were made between June and August 1998 during the first detected
outburst from this source. We describe correlations between X-ray timing and
spectral properties and discuss the possibility that the propeller mechanism
turned on during the decay of the outburst. During one of the RXTE
observations, we detect a pair of high frequency QPOs at 847.1 +/- 5.5 Hz and
1102 +/- 13 Hz simultaneously. According to the beat frequency model, the QPO
separation implies a neutron star spin period near 3.9 ms. As the X-ray flux
decreases at the end of the outburst, the amplitude of the optical modulation
increases significantly. This behavior can be explained if the size of the
accretion disk decreases during the decay of the outburst.Comment: 24 pages, 9 figures, accepted by Ap
The X-ray quiescence of Swift J195509.6+261406 (GRB 070610): an optical bursting X-ray binary?
We report on a 63ks Chandra observation of the X-ray transient Swift
J195509.6+261406 discovered as the afterglow of what was first believed to be a
long duration Gamma-Ray Burst (GRB 070610). The outburst of this source was
characterized by unique optical flares on timescales of second or less,
morphologically similar to the short X-ray bursts usually observed from
magnetars. Our Chandra observation was performed ~2 years after the discovery
of the optical and X-ray flaring activity of this source, catching it in its
quiescent state. We derive stringent upper limits on the quiescent emission of
Swif J195509.6+261406 which argues against the possibility of this object being
a typical magnetar. Our limits show that the most viable interpretation on the
nature of this peculiar bursting source, is a binary system hosting a black
hole or a neutron star with a low mass companion star (< 0.12 M_{\odot}), and
with an orbital period smaller than a few hours.Comment: 5 pages, 3 figures, accepted for publication in ApJ Letter
Pair Production Constraints on Superluminal Neutrinos Revisited
We revisit the pair creation constraint on superluminal neutrinos considered
by Cohen and Glashow in order to clarify which types of superluminal models are
constrained. We show that a model in which the superluminal neutrino is
effectively light-like can evade the Cohen-Glashow constraint.Comment: 5 pages, typo fixed, footnote on sterile neutrinos added, refs. adde
Prompt Optical Observations of Gamma-ray Bursts
The Robotic Optical Transient Search Experiment (ROTSE) seeks to measure
simultaneous and early afterglow optical emission from gamma-ray bursts (GRBs).
A search for optical counterparts to six GRBs with localization errors of 1
square degree or better produced no detections. The earliest limiting
sensitivity is m(ROTSE) > 13.1 at 10.85 seconds (5 second exposure) after the
gamma-ray rise, and the best limit is m(ROTSE) > 16.0 at 62 minutes (897 second
exposure). These are the most stringent limits obtained for GRB optical
counterpart brightness in the first hour after the burst. Consideration of the
gamma-ray fluence and peak flux for these bursts and for GRB990123 indicates
that there is not a strong positive correlation between optical flux and
gamma-ray emission.Comment: 4 pages, 3 figures, submitted to ApJ Letter
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