482 research outputs found
Differences between the Two Anomalous X-Ray Pulsars: Variations in the Spin Down Rate of 1E 1048.1-5937 and An Extended Interval of Quiet Spin Down in 1E 2259+586
We analysed the RXTE archival data of 1E 1048.1-5937 covering a time span of
more than one year. The spin down rate of this source decreases by 30 percent
during the observation. We could not resolve the X-ray flux variations because
of contamination by Eta Carinae. We find that the level of pulse frequency
fluctuations of 1E 1048.1-5937 is consistent with typical noise levels of
accretion powered pulsars. Recent RXTE observations of 1E 2259+586 have shown a
constant spin down with a very low upper limit on timing noise. We used the
RXTE archival X-ray observations of 1E 2259+586 to show that the intrinsic
X-ray luminosity times series is also stable, with an rms fractional variation
of less than 15 percent. The source could have been in a quiet phase of
accretion with a constant X-ray luminosity and spin down rate.Comment: MNRAS in pres
Bumpy Spin-Down of Anomalous X-Ray Pulsars: The Link with Magnetars
The two anomalous X-ray pulsars (AXPs) with well-sampled timing histories, 1E
1048.1-5937 and 1E 2259+586, are known to spin down irregularly, with `bumps'
superimposed on an overall linear trend. Here we show that if AXPs are
non-accreting magnetars, i.e. isolated neutron stars with surface magnetic
fields B_0 > 10^{10} T, then they spin down electromagnetically in exactly the
manner observed, due to an effect called `radiative precession'. Internal
hydromagnetic stresses deform the star, creating a fractional difference
epsilon=(I_3-I_1)/I_1 ~ 10^{-8} between the principal moments of inertia I_1
and I_3; the resulting Eulerian precession couples to an oscillating component
of the electromagnetic torque associated with the near-zone radiation fields,
and the star executes an anharmonic wobble with period tau_pr ~ 2 pi / epsilon
Omega(t) ~ 10 yr, where Omega(t) is the rotation frequency as a function of
time t. We solve Euler's equations for a biaxial magnet rotating in vacuo; show
that the computed Omega(t) matches the measured timing histories of 1E
1048.1-5937 and 1E 2259+586; predict Omega(t) for the next 20 years for both
objects; predict a statistical relation between and tau_pr, to be
tested as the population of known AXPs grows; and hypothesize that radiative
precession will be observed in future X-ray timing of soft gamma-ray repeaters
(SGRs).Comment: 9 pages, 2 figures, to be published in The Astrophysical Journal
Letter
ROTSE observations of the young cluster IC 348
CCD observations of stars in the young cluster IC 348 were obtained from 2004
August to 2005 January with a 0.45 m ROTSEIIId robotic reflecting telescope at
the Turkish National Observatory site, Bakirlitepe, Turkey. The timing analysis
of selected stars whose X-Ray counterpart were detected by Chandra X-Ray
Observatory were studied. The time series of stars were searched for rotational
periodicity by using different period search methods. 35 stars were found to be
periodic with periods ranging from 0.74 to 32.3 days. Eighteen of the 35
periodic stars were new detections. Four of the new detections were CTTSs and
the others were WTTSs and G type (or unknown spectral class) stars. In this
study, we confirmed the stability of rotation periods of TTauri stars. The
periods obtained by Cohen et al. and us were different by 1%. We also confirmed
the 3.24 h pulsation period of H254 which is a delta Scuti type star as noted
by Ripepi et al. but the other periods detected by them were not found. We
examined correlation between X-ray luminosity and rotational period of our
sample of TTSs. There is a decline in the rotational period with X-ray
luminosity for late type TTSs.Comment: 15 pages, 14 figures, accepted for publication in Astronomical
Journa
Monitoring the changes of Lake Uluabat Ramsar site and its surroundings in the 1985-2021 period using RS and GIS methods
Ramsar sites are important ecosystems that are protected by international status, have great value in terms of biodiversity, and constitute a resource in terms of economic, cultural, scientific and recreational aspects. In this study, the change of Lake Uluabat Ramsar Site and its surroundings, between the years 1985-2021 has been observed. For this, Remote Sensing (RS) and Geographic Information Systems (GIS) methods were used. Vegetation change in the lake and its surroundings in 1985, 2000, 2015 and 2021 with Normalized Difference Vegetation Index (NDVI), and changes in water surfaces with the water indices Normalized Difference Water Index (NDWI) and Modified Normalized Difference Water Index (mNDWI) were analyzed by using Landsat multi-band satellite images (Landsat 5 TM, Landsat 7 ETM and Landsat 8 OLI/TIRS) as RS data. The resulting changes were monitored and the success of the indices in determining these areas and the relations of the indices with each other were questioned by Accuracy index, Kappa coefficent, and Correlation analyses. The results show 36-year long-term changes and reveal a 13.06% shrinkage of Uluabat Lake wetland and surrounding water areas with the highest kappa coefficients for mNDWI as 0.83, 0.90, 0.93, 0.97, respectively, over the years studied
Cylindrically Symmetric Vacuum Solutions in Higher Dimensional Brans-Dicke Theory
Higher dimensional, static, cylindrically symmetric vacuum solutions with and
without a cosmological constant in the Brans-Dicke theory are presented. We
show that, for a negative cosmological constant and for specific values of the
parameters, a particular subclass of these solutions include higher dimensional
topological black hole-type solutions with a flat horizon topology. We briefly
extend our discussion to stationary vacuum and vacuum solutions.Comment: V3: Published Versio
Spin-down rate of 1E 2259+586 from RXTE observation
We present new X-ray observations of the X-ray pulsar 1E 2259+586, obtained
during March 1997, with the Rossi X-Ray Timing Explorer (RXTE). We have
measured the pulse frequency derivative Hz s from pulse arrival times obtained in a sequence of 5
observations spread over one month. This is consistent with the long
term spin-down trend. We also found that the observed X-ray luminosity is
consistent with that measured at quiescent X-ray flux levels by previous
missions. Our observations imply that 1E 2259+586 was spinning down steadily
without exhibiting any stochastic torque noise fluctuations during the month
covered by our observations.Comment: 4 pages, Latex (l-aa), Accepted for publication in Astronomy and
Astrophysic
Broad-band X-ray measurements of the black hole candidate XTE J1908+094
XTE J1908+094 is an X-ray transient that went into outburst in February 2002.
After two months it reached a 2-250 keV peak flux of 1 to 2 X 10-8 erg/s/cm2.
Circumstantial evidence points to an accreting galactic black hole as the
origin of the the X-radiation: pulsations nor thermonuclear flashes were
detected that would identify a neutron star and the spectrum was unusually hard
for a neutron star at the outburst onset. We report on BeppoSAX and RXTE All
Sky Monitor observations of the broad-band spectrum of XTE J1908+094. The
spectrum is consistent with a model consisting of a Comptonization component by
a ~40 keV plasma (between 2 and 60 keV this component can be approximated by a
power law with a photon index of 1.9 to 2.1), a multicolor accretion disk
blackbody component with a temperature just below 1 keV and a broad emission
line at about 6 keV. The spectrum is heavily absorbed by cold interstellar
matter with an equivalent hydrogen column density of 2.5 X 10+22 cm-2, which
makes it difficult to study the black body component in detail. The black body
component exhibits strong evolution about 6 weeks into the outburst. Two weeks
later this is followed by a swift decay of the power law component. The
broadness of the 6 keV feature may be due to relativistic broadening or Compton
scattering of a narrow Fe-K line.Comment: Accepted for publication in Astronomy & Astrophysic
Accretion disk reversal and the spin-up/spin-down of accreting pulsars
We numerically investigate the hydrodynamics of accretion disk reversal and
relate our findings to the observed spin-rate changes in the accreting X-ray
pulsar GX~1+4. In this system, which accretes from a slow wind, the accretion
disk contains two dynamically distinct regions. In the inner part viscous
forces are dominant and disk evolution occurs on a viscous timescale. In the
outer part dynamical mixing of material with opposite angular momentum is more
important, and the externally imposed angular momentum reversal timescale
governs the flow. In this outer region the disk is split into concentric rings
of material with opposite senses of rotation that do not mix completely but
instead remain distinct, with a clear gap between them. We thus predict that
torque reversals resulting from accretion disk reversals will be accompanied by
minima in accretion luminosity.Comment: 13 pages, 7 figures, accepted for publication in Ap
Recent X-ray measurements of the accretion-powered pulsar 4U 1907+09
X-ray observations of the accreting X-ray pulsar 4U~1907+09, obtained during
February 1996 with the Proportional Counter Array on the Rossi X-ray Timing
Experiment (RXTE), have enabled the first measurement of the intrinsic pulse
period Ppulse since 1984: Ppulse=440.341[+0.012,-0.017] s. 4U 1907+09 is in a
binary system with a blue supergiant. The orbital parameters were solved and
this enabled the correction for orbital delay effects of a measurement of
Ppulse obtained in 1990 with Ginga. Thus, three spin down rates could be
extracted from four pulse periods obtained in 1983, 1984, 1990, and 1996. These
are within 8% equal to a value of dPpulse/dt=+0.225 s/yr. This suggest that the
pulsar is perhaps in a monotonous spin down mode since its discovery in 1983.
Furthermore, the RXTE observations show transient ~18 s oscillations during a
flare that lasted about 1 hour. The oscillations may be interpreted as
Keplerian motion of an accretion disk near the magnetospheric radius. This, and
the notion that the co-rotation radius is much larger than any conceivable
value for the magnetospheric radius (because of the long spin period), renders
it unlikely that this pulsar spins near equilibrium like is suspected for other
slowing accreting X-ray pulsars. We suggest as an alternative that perhaps the
frequent occurrence of a retrograde transient accretion disk may be
consistently slowing the pulsar down. Further observations of flares can
provide more evidence of this.Comment: 26 pages, 11 figures, to be published in Astrophysical Journal part I
on March 20, 199
- …