177 research outputs found
X-ray Variability and Evidence for Pulsations from the Unique Radio Pulsar/X-ray Binary Transition Object FIRST J102347.6+003841
We report on observations of the unusual neutron-star binary system FIRST
J102347.6+003841 carried out using the XMM-Newton satellite. This system
consists of a radio millisecond pulsar in an 0.198-day orbit with a ~0.2
solar-mass Roche-lobe-filling companion, and appears to have had an accretion
disk in 2001. We observe a hard power-law spectrum (\Gamma = 1.26(4)) with a
possible thermal component, and orbital variability in X-ray flux and possibly
hardness of the X-rays. We also detect probable pulsations at the pulsar period
(single-trial significance ~4.5 sigma from an 11(2)% modulation), which would
make this the first system in which both orbital and rotational X-ray
pulsations are detected. We interpret the emission as a combination of X-rays
from the pulsar itself and from a shock where material overflowing the
companion meets the pulsar wind. The similarity of this X-ray emission to that
seen from other millisecond pulsar binary systems, in particular 47 Tuc W (PSR
J0024-7204W) and PSR J1740-5340, suggests that they may also undergo disk
episodes similar to that seen in J1023 in 2001.Comment: 14 pages, 5 figures, 1 table; accepted to Ap
The Dwarf Novae of Shortest Period
We present observations of the dwarf novae GW Lib, V844 Her, and DI UMa.
Radial velocities of H-alph yield orbital periods of 0.05332 +- 0.00002 d (=
76.78 m) for GW Lib and and 0.054643 +- 0.000007 d (= 78.69 m) for V844 Her.
Recently, the orbital period of DI UMa was found to be only 0.054564 +-
0.000002 d (= 78.57 m) by Fried et al. (1999), so these are the three shortest
orbital periods among dwarf novae with normal-abundance secondaries.
GW Lib has attracted attention as a cataclysmic binary showing apparent ZZ
Ceti-type pulsations of the white dwarf primary. Its spectrum shows sharp
Balmer emission flanked by strong, broad Balmer absorption, indicating a
dominant contribution by white-dwarf light. Analysis of the Balmer absorption
profiles is complicated by the unknown residual accretion luminosity and lack
of coverage of the high Balmer lines. Our best-fit model atmospheres are
marginally hotter than the ZZ Ceti instability strip, in rough agreement with
recent ultraviolet results from HST. The spectrum and outburst behavior of GW
Lib make it a near twin of WZ Sge, and we estimate it to have a quiescent V
absolute magnitude 12. Comparison with archival data reveals proper motion of
65 +- 12 mas/yr.
The mean spectrum of V844 Her is typical of SU UMa dwarf novae. We detected
superhumps in the 1997 May superoutburst with superhump period = 0.05597 +-
0.00005 d. The spectrum of DI UMa appears normal for a dwarf nova near minimum
light.
These three dwarf novae have nearly identical short periods but completely
dissimilar outburst characteristics. We discuss possible implications.Comment: Accepted for publication in Publications of the Astronomical Society
of the Pacific; 16 pages, 6 figure
A photometric and spectroscopic study of the cataclysmic variable SX Leonis Minoris in quiescence and superoutburst
We present CCD imaging, CCD photometry on long and short timescales, and time-resolved spectroscopy of SX LMi, a new SU Ursae Majoris type dwarf nova. The quiescent optical spectrum shows broad double-peaked Balmer, He I, and He II emission lines, similar to other quiescent dwarf novae. Absorption lines from a late-type secondary are not detected. Time-resolved spectra obtained in quiescence reveal radial velocity variations of the Balmer emission lines on a period of 0.06717 +/- 0.00011 days, or 96.72 +/- 0.16 minutes, with only a slight possibility of a daily cycle-count error. Optical photometry obtained between 1987 and 1991 shows flickering with a peak-to-peak amplitude of 0.18 mag. The binary orbital period can sometimes be seen in the photometric record. Long-term photometric monitoring for a three-year period between 1992 October and 1995 June shows seven well-defined outbursts and marginally detects a few others. The outburst interval varies between 34 and 64 days. During the 1994 December outburst, optical photometric observations show that SX LMi exhibited superhumps with a period of 0.06893 +/- 0.00012 days, which is 2.6 percent +/- 0.2 percent longer than the orbital period, as expected for a normal SU UMa star at this period. Spectra obtained during superoutburst show dramatic variations in the emission-line profiles on timescales of 10 minutes. Profile fits indicate that underlying absorption contributes to the shape of the
Balmer emission-line profiles during superoutburst as in other dwarf novae in outburst or superoutburst.
Direct images in good seeing show a ~D19 mag companion star from SX LMi
IGR J19552+0044: A new asynchronous short period polar: "Filling the gap between intermediate and ordinary polars"
Based on XMM--Newton X-ray observations IGR J19552+0044 appears to be either
a pre-polar or an asynchronous polar. We conducted follow-up optical
observations to identify the sources and periods of variability precisely and
to classify this X-ray source correctly. Extensive multicolor photometric and
medium- to high-resolution spectroscopy observations were performed and period
search codes were applied to sort out the complex variability of the object. We
found firm evidence of discording spectroscopic (81.29+/-0.01m) and photometric
(83.599+/-0.002m) periods that we ascribe to the white dwarf (WD)\ spin period
and binary orbital period, respectively. This confirms that IGR J19552+0044 is
an asynchronous polar. Wavelength-dependent variability and its continuously
changing shape point at a cyclotron emission from a magnetic WD with a
relatively low magnetic field below 20 MG.
The difference between the WD spin period and the binary orbital period
proves that IGR J19552+0044 is a polar with the largest known degree of
asynchronism (0.97 or 3%).Comment: 9 pages, 10 figures, A&A accepte
SDSS J162520.29+120308.7 â a new SU Ursae Majoris star in the period gap
We report results of an extensive world-wide observing campaign devoted to the recently discovered dwarf nova
SDSS J162520.29+120308.7 (SDSS J1625). The data were obtained during the July 2010 eruption of the star and in August and
September 2010 when the object was in quiescence. During the July 2010 superoutburst, SDSS J1625 clearly displayed superhumps
with a mean period of Psh = 0.095942(17) days (138.16 ± 0.02 min) and a maximum amplitude reaching almost 0.4 mag. The superhump
period was not stable, decreasing very rapidly at a rate of ËP = â1.63(14) Ă 10â3 at the beginning of the superoutburst and
increasing at a rate of ËP = 2.81(20) Ă 10â4 in the middle phase. At the end of the superoutburst, it stabilized around the value of
Psh = 0.09531(5) day.
During the first twelve hours of the superoutburst, a low-amplitude double wave modulation was observed whose properties are
almost identical to early superhumps observed in WZ Sge stars. The period of early superhumps, the period of modulations observed
temporarily in quiescence, and the period derived from radial velocity variations are the same within measurement errors, allowing
us to estimate the most probable orbital period of the binary to be Porb = 0.09111(15) days (131.20 ± 0.22 min). This value clearly
indicates that SDSS J1625 is another dwarf nova in the period gap. Knowledge of the orbital and superhump periods allows us to
estimate the mass ratio of the system to be q â 0.25. This high value poses serious problems for both the thermal and tidal instability
(TTI) model describing the behaviour of dwarf novae and for some models explaining the origin of early superhumps
The WEBT Campaign on the Blazar 3C279 in 2006
The quasar 3C279 was the target of an extensive multiwavelength monitoring
campaign from January through April 2006, including an optical-IR-radio
monitoring campaign by the Whole Earth Blazar Telescope (WEBT) collaboration.
In this paper we focus on the results of the WEBT campaign. The source
exhibited substantial variability of optical flux and spectral shape, with a
characteristic time scale of a few days. The variability patterns throughout
the optical BVRI bands were very closely correlated with each other. In
intriguing contrast to other (in particular, BL Lac type) blazars, we find a
lag of shorter- behind longer-wavelength variability throughout the RVB ranges,
with a time delay increasing with increasing frequency. Spectral hardening
during flares appears delayed with respect to a rising optical flux. This, in
combination with the very steep IR-optical continuum spectral index of ~ 1.5 -
2.0, may indicate a highly oblique magnetic field configuration near the base
of the jet. An alternative explanation through a slow (time scale of several
days) acceleration mechanism would require an unusually low magnetic field of <
0.2 G, about an order of magnitude lower than inferred from previous analyses
of simultaneous SEDs of 3C279 and other FSRQs with similar properties.Comment: Accepted for publication in Ap
DE Canum Venaticorum : a bright, eclipsing red dwarfâwhite dwarf binary
Context. Close white dwarfâred dwarf binaries must have gone through a common-envelope phase during their evolution. DE CVn is a detached white dwarfâred dwarf binary with a relatively short (âŒ8.7 h) orbital period. Its brightness and the presence of eclipses makes this system ideal for a more detailed study.
Aims. From a study of photometric and spectroscopic observations of DE CVn we derive the system parameters that we discuss in the framework of common-envelope evolution.
Methods. Photometric observations of the eclipses are used to determine an accurate ephemeris. From a model fit to an average lowresolution spectrum of DE CVn, we constrain the temperature of the white dwarf and the spectral type of the red dwarf. The eclipse light curve is analysed and combined with the radial velocity curve of the red dwarf determined from time-resolved spectroscopy to derive constraints on the inclination and the masses of the components in the system.
Results. The derived ephemeris is HJDmin = 2 452 784.5533(1) + 0.3641394(2) Ă E. The red dwarf in DE CVn has a spectral type of M3V and the white dwarf has an effective temperature of 8 000 K. The inclination of the system is 86+3⊠â2 and the mass and radius of the red dwarf are 0.41 ± 0.06 M and 0.37+0.06 â0.007 R, respectively, and the mass and radius of the white dwarf are 0.51+0.06
â0.02 M and 0.0136+0.0008 â0.0002 R, respectively.
Conclusions. We found that the white dwarf has a hydrogen-rich atmosphere (DA-type). Given that DE CVn has experienced a common-envelope phase, we can reconstruct its evolution and we find that the progenitor of the white dwarf was a relatively lowmass star (M †1.6 M). The current age of this system is 3.3â7.3 Ă 109 years, while it will take longer than the Hubble time for DE CVn to evolve into a semi-detached system
AKARI and BLAST Observations of the Cassiopeia A Supernova Remnant and Surrounding Interstellar Medium
We use new large area far infrared maps ranging from 65 - 500 microns
obtained with the AKARI and the Balloon-borne Large Aperture Submillimeter
Telescope (BLAST) missions to characterize the dust emission toward the
Cassiopeia A supernova remnant (SNR). Using the AKARI high resolution data we
find a new "tepid" dust grain population at a temperature of ~35K and with an
estimated mass of 0.06 solar masses. This component is confined to the central
area of the SNR and may represent newly-formed dust in the unshocked supernova
ejecta. While the mass of tepid dust that we measure is insufficient by itself
to account for the dust observed at high redshift, it does constitute an
additional dust population to contribute to those previously reported. We fit
our maps at 65, 90, 140, 250, 350, and 500 microns to obtain maps of the column
density and temperature of "cold" dust (near 16 K) distributed throughout the
region. The large column density of cold dust associated with clouds seen in
molecular emission extends continuously from the surrounding interstellar
medium to project on the SNR, where the foreground component of the clouds is
also detectable through optical, X-ray, and molecular extinction. At the
resolution available here, there is no morphological signature to isolate any
cold dust associated only with the SNR from this confusing interstellar
emission. Our fit also recovers the previously detected "hot" dust in the
remnant, with characteristic temperature 100 K.Comment: Accepted for publication in the Astrophysical Journal. Maps and
related data are available at http://blastexperiment.info
GRB 991216 Joins the Jet Set: Discovery and Monitoring of its Optical Afterglow
The optical light curve of the energetic gamma-ray burst GRB 991216 is
consistent with jet-like behavior in which a power-law decay steepens from
t**(-1.22 +/- 0.04) at early times to t**(-1.53 +/- 0.05) in a gradual
transition at around 2 d. The derivation of the late-time decay slope takes
into account the constant contribution of a host or intervening galaxy which
was measured 110 d after the event at R = 24.56 +/- 0.14, although the light
curve deviates from a single power law whether or not a constant term is
included. The early-time spectral energy distribution of the afterglow can be
described as F_nu ~ nu**(-0.74 +/- 0.05) or flatter between optical and X-ray,
which, together with the slow initial decay, is characteristic of standard
adiabatic evolution in a uniformly dense medium. Assuming that a reported
absorption-line redshift of 1.02 is correct, the apparent isotropic energy of
6.7 x 10**53 erg is reduced by a factor of ~ 200 in the jet model, and the
initial half-opening angle is ~ 6 deg. GRB 991216 is the third good example of
a jet-like afterglow (following GRB 990123 and GRB 990510), supporting a trend
in which the apparently most energetic gamma-ray events have the narrowest
collimation and a uniform ISM environment. This, plus the absence of evidence
for supernovae associated with jet-like afterglows, suggests that these events
may originate from a progenitor in which angular momentum plays an important
role but a massive stellar envelope or wind does not, e.g., the coalescence of
a compact binary.Comment: 19 pages, accepted by The Astrophysical Journa
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