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1.
By performing Monte-Carlo calculations of the spin-down of young pulsars surrounded by a supernova fallback disk, we present
possible constraints on the propeller torques exerted by the disks on neutron stars.
This work was supported by the National Natural Science Foundation of China. 相似文献
2.
Within the framework of the partially screened inner acceleration region the relationship between the X-ray luminosity and
the circulational periodicity of drifting subpulses is derived. This relationship is quite well satisfied in pulsars for which
an appropriate radio and X-ray measurements exist. A special case of PSR B0943+10 is presented and discussed. The problem
of formation of a partially screened inner acceleration region for all pulsars with drifting subpulses is also considered.
It is argued that an efficient inner acceleration region just above the polar cap can be formed in a very strong and curved
non-dipolar surface magnetic field.
We acknowledge the support of the Polish State Committee for scientific research under Grant P03D 029 26. G.M. was partially
supported by Georgian NSF grant ST06/4-096. 相似文献
3.
Ye Lu Wei Wang Yong-Heng ZhaoNational Astronomical Observatories Chinese Academy of Sciences Beijing 《中国天文和天体物理学报》2003,3(6):543-550
We examine the spectra of the persistent emission from anomalous X-ray pulsars (AXPs) and their variati on with the spin-down rate Ω. Based onan accretion-powered model, the influences of both the magnetic field and the mass accretion rate on the spectral properties of AXPs are addressed. We then investigate the relation between the spectral property of AXPs and mass accretion rate M. The result shows that there exists a linear correlation between the photon index and the mass accretion rate: the spectral hardness increases with increasing M. A possible emission mechanism for the explanation of the spectral properties of AXPs is also discussed. 相似文献
4.
5.
6.
Mark Wardle 《Astrophysics and Space Science》2007,311(1-3):35-45
Magnetic fields likely play a key role in the dynamics and evolution of protoplanetary disks. They have the potential to efficiently
transport angular momentum by MHD turbulence or via the magnetocentrifugal acceleration of outflows from the disk surface.
Magnetically-driven mixing has implications for disk chemistry and evolution of the grain population, and the effective viscous
response of the disk determines whether planets migrate inwards or outwards. However, the weak ionisation of protoplanetary
disks means that magnetic fields may not be able to effectively couple to the matter. I examine the magnetic diffusivity in
a minimum solar nebula model and present calculations of the ionisation equilibrium and magnetic diffusivity as a function
of height from the disk midplane at radii of 1 and 5 AU. Dust grains tend to suppress magnetic coupling by soaking up electrons
and ions from the gas phase and reducing the conductivity of the gas by many orders of magnitude. However, once grains have
grown to a few microns in size their effect starts to wane and magnetic fields can begin to couple to the gas even at the
disk midplane. Because ions are generally decoupled from the magnetic field by neutral collisions while electrons are not,
the Hall effect tends to dominate the diffusion of the magnetic field when it is able to partially couple to the gas, except
at the disk surfaces where the low density of neutrals permits the ions to remain attached to the field lines. For a standard
population of 0.1 μm grains the active surface layers have a combined column Σactive≈2 g cm−2 at 1 AU; by the time grains have aggregated to 3 μm, Σactive≈80 g cm−2. Ionisation in the active layers is dominated by stellar X-rays. In the absence of grains, X-rays maintain magnetic coupling
to 10% of the disk material at 1 AU (i.e. Σactive≈150 g cm−2). At 5 AU the Σactive≈Σtotal once grains have aggregated to 1 μm in size. 相似文献
7.
We report the discovery of a decay in the superorbital period of the binary X-ray pulsar LMC X-4. Combining archival data
and published long term X-ray light curves, we have found a decay in the third period in this system (P ∼ 30.3 day, P ∼ −2 × 10−5 s s−1). Along with this result, a comparison of the superorbital intensity variations in LMC X-4, Her X-1 and SMC X-1 is also presented. 相似文献
8.
R. K. Manchanda 《Journal of Astrophysics and Astronomy》2001,22(1):9-20
The black hole candidate Cyg X-1 was observed in ultra low state on march 30, 1997 using Large Area Scintillation counter
Experiment (LASE) in the hard X-ray energy region of 20–180 keV. During the 30 minute exposure a combined signal of 68 sigma
was obtained, however, the measured flux at 50 keV was lower by a factor of 2 than the minimum flux reported so far. Using
the recent orbital ephemeris of the source, our snap-shot observations were made at ϕ5.6 = 0.915, which corresponds to the binary minimum revealed by the ASM light curves. The daily average data from the BATSE
detectors give the source intensity level to be higher by a factor of 5. Very low flux values measured in the present experiment
suggest that the hard X-ray source may have been partially occulted by the primary companion during its transit near the X-ray
minimum. 相似文献
9.
We employ the supernova fallback disk model to simulate the spin evolution of isolated young neutron stars(NSs). We consider the submergence of the NS magnetic fields during the supercritical accretion stage and its succeeding reemergence. It is shown that the evolution of the spin periods and the magnetic fields in this model is able to account for the relatively weak magnetic fields of central compact objects and the measured braking indices of young pulsars. For a range of initial parameters, evolutionary links can be established among various kinds of NS sub-populations including magnetars, central compact objects and young pulsars. Thus, the diversity of young NSs could be unified in the framework of the supernova fallback accretion model. 相似文献
10.
Khee-Gan Lee Kinwah Wu Steven V. Fuerst Graziella Branduardi-Raymonl Oliver Crowley 《中国天文和天体物理学报》2009,9(4)
We have calculated the relativistic reflection component of the X-ray spectra of accretion disks in active galactic nuclei (AGN). Our calculations have shown that the spectra can be significantly modified by the motion of the accretion flow, and the gravity and rotation of the central black hole. The absorption edges in the spectra suffer severe en- ergy shifts and smearing, and the degree of distortion depends on the system parameters, in particular, the inner radius of the accretion disk and the disk viewing inclination angles. The effects are significant. Fluorescent X-ray emission lines from the inner accretion disk could be a powerful diagnostic of space-time distortion and dynamical relativistic effects near the event horizons of accreting black holes. However, improper treatment of the re- flection component in fitting the X-ray continuum could give rise to spurious line-like features. These features mimic the true fluorescent emission lines and may mask their relativistic signatures. Fully relativistic models for reflection continua together with the emission lines are needed in order to extract black-hole parameters from the AGN X-ray spectra. 相似文献
11.
S. Naik P. C. Agrawal B. Paul A. R. Rao S. Seetha K. Kasturirangan 《Journal of Astrophysics and Astronomy》2000,21(1-2):29-38
The outburst of X-ray transient source XTE J2012+381 was detected by the RXTE All-Sky Monitor on 1998 May 24th. Following
the outburst, X-ray observations of the source were made in the 2–18 keV energy band with the Pointed Proportional Counters
of the Indian X-ray Astronomy Experiment (IXAE) on-board the Indian satellite IRS-P3 during 1998 June 2nd–10th. The X-ray
flux of the source in the main outburst decreased exponentially during the period of observation. No large amplitude short-term
variability in the intensity is detected from the source. The power density spectrum obtained from the timing analysis of
the data shows no indication of any quasi-periodic oscillations in 0.002–0.5 Hz band. The hardness ratio i.e. the ratio of
counts in 6–18 keV to 2–6 keV band, indicates that the X-ray spectrum is soft with spectral index >2. From the similarities
of the X-ray properties with those of other black hole transients, we conclude that the X-ray transient XTE J2012+381 is likely
to be a black hole. 相似文献
12.
Y. J. Du R. X. Xu G. J. Qiao J. L. Han 《Monthly notices of the Royal Astronomical Society》2009,399(3):1587-1596
Pulsars have been recognized to be normal neutron stars, but sometimes have been argued to be quark stars. Submillisecond pulsars, if detected, would play an essential and important role in distinguishing quark stars from neutron stars. We focus on the formation of such submillisecond pulsars in this paper. A new approach to the formation of a submillisecond pulsar (quark star) by means of the accretion-induced collapse (AIC) of a white dwarf is investigated. Under this AIC process, we found that: (i) almost all newborn quark stars could have an initial spin period of ∼0.1 ms; (ii) nascent quark stars (even with a low mass) have a sufficiently high spin-down luminosity and satisfy the conditions for pair production and sparking process and appear as submillisecond radio pulsars; (iii) in most cases, the times of newborn quark stars in the phase with spin period <1 (or <0.5) ms are long enough for the stars to be detected.
As a comparison, an accretion spin-up process (for both neutron and quark stars) is also investigated. It is found that quark stars formed through the AIC process can have shorter periods (≤0.5 ms), whereas the periods of neutron stars formed in accretion spin-up processes must be longer than 0.5 ms. Thus, if a pulsar with a period shorter than 0.5 ms is identified in the future, it could be a quark star. 相似文献
As a comparison, an accretion spin-up process (for both neutron and quark stars) is also investigated. It is found that quark stars formed through the AIC process can have shorter periods (≤0.5 ms), whereas the periods of neutron stars formed in accretion spin-up processes must be longer than 0.5 ms. Thus, if a pulsar with a period shorter than 0.5 ms is identified in the future, it could be a quark star. 相似文献
13.
Over the last 40 years, X-ray astronomy missions have revealed long-term, superorbital periods in a variety of X-ray binaries. These modulations can provide significant constraints on the physical properties of accretion discs. Some of these modulations are Her X-1-like and are interpreted as irradiation-driven, tilted, precessing accretion discs. Others show more complex light curves, with the period changing on timescales >1000 d, and are interpreted in terms of the Ogilvie and Dubus [Ogilvie, G.I., Dubus, G., 2001, MNRAS 320, 485 (OD01)] disc stability criteria. We suggest a categorisation of superorbital periods into six different types, based on their observed characteristics. 相似文献
14.
R. K. Manchanda 《Journal of Astrophysics and Astronomy》2000,21(1-2):39-52
We report the spectral measurement of GRS 1915+105 in the hard X-ray energy band of 20–140keV. The observations were made
on March 30th, 1997 during a quiescent phase of the source. We discuss the mechanism of emission of hard X-ray photons and
the evolution of the spectrum by comparing the data with earlier measurements and an axiomatic model for the X-ray source. 相似文献
15.
In this paper we review the possibilities for
magnetohydrodynamic processes to handle the angular momentum transport
in accretion disks. Traditionally the angular momentum transport has
been considered to be the result of turbulent viscosity in the disk,
although the Keplerian flow in accretion disks is linearly stable towards
hydrodynamic perturbations. It is on the other hand linearly unstable
to some magnetohydrodynamic (MHD) instabilities.
The most important instabilities are the Parker and Balbus-Hawley
instabilities that are related to the magnetic buoyancy and the shear
flow, respectively. We discuss these instabilities not only in the
traditional MHD framework, but also in the context of slender flux
tubes, that reduce the complexity of the problem while keeping most of
the stability properties of the complete problem. In the non-linear
regime the instabilities produce turbulence. Recent numerical
simulations describe the generation of magnetic fields by a dynamo in
the resulting turbulent flow. Eventually such a dynamo may generate a
global magnetic field in the disk. The relation of the MHD-turbulence
to observations of accretion disks is still obscure. It is commonly
believed that magnetic fields can be highly efficient in transporting
the angular momentum, but emission lines, short-time scale variability
and non-thermal radiation, which a stellar astronomer would take as
signs of magnetic variability, are more commonly observed during periods
of low accretion rates.
Received October 12, 1995 / Accepted November 16, 1995 相似文献
16.
Sergei Popov David Blaschke Hovik Grigorian Mikhail Prokhorov 《Astrophysics and Space Science》2007,308(1-4):381-385
We discuss three topics related to the neutron star (NS) mass spectrum. At first we discuss the possibility to form low-mass
(M≲1M
⊙) objects. In our opinion this and suggest this is possible only due to fragmentation of rapidly rotating proto-NSs. Such
low-mass NSs should have very high spatial velocities which could allow identification. A critical assessment of this scenario
is given. However, the mechanism has its own problems, and so formation of such objects is not very probable. Secondly, we
discuss mass growth due to accretion for NSs in close binary systems. With the help of numerical population synthesis calculations
we derive the mass spectrum of massive (M>1.8M
⊙) NSs. Finally, we discuss the role of the mass spectrum in population studies of young cooling NSs. We formulate a kind of
mass constraint which can be helpful, in our opinion, in discussing different competitive models of the thermal evolution of NSs.
S.B.P. wants to thank the Organizers for support and hospitality. The work of S.B.P. was supported by the RFBR grant 06-02-16025
and by the “Dynasty” Foundation (Russia). The work of M.E.P.—by the RFBR grant 04-02-16720 and that of H.G. by DFG grant 436
ARM 17/4/05. 相似文献
17.
Sandro Mereghetti 《Astronomy and Astrophysics Review》2008,15(4):225-287
Two classes of X-ray pulsars, the anomalous X-ray pulsars and the soft gamma-ray repeaters, have been recognized in the last
decade as the most promising candidates for being magnetars: isolated neutron stars powered by magnetic energy. I review the
observational properties of these objects, focussing on the most recent results, and their interpretation in the magnetar
model. Alternative explanations, in particular those based on accretion from residual disks, are also considered. The possible
relations between these sources and other classes of neutron stars and astrophysical objects are also discussed. 相似文献
18.
Elena M. Rossi Philip J. Armitage Tiziana Di Matteo 《Astrophysics and Space Science》2007,311(1-3):185-190
We discuss the physics of the power source for gamma-ray bursts (GRBs). There is a great variety of stellar systems proposed
as progenitors of long and short GRBs, but any current model for the engine ultimately involves the formation of a hyper-accreting
disk around a newly-formed black hole of a few solar masses. The study of such disks can shed light onto the burst composition
and energy content. We present preliminary results from disk vertical structure calculations. These include neutrino transport,
a height-dependent determination of the nuclear composition, and a simplified treatment of turbulent mixing. We find that
vertical mixing is rapid enough to largely erase compositional gradients, and as a consequence the upper layers of the disk
reflect the neutron-rich composition of the midplane close to the black hole. We review the implications of this for the nuclear
composition of outflows in GRBs. Our models suggest an increasing role for pairs in the upper regions of the disk, and we
speculate that a pair-driven wind could be a significant source of cooling.
Elena M. Rossi is a Chandra Fellow 相似文献
19.
Gloria Sala Jochen Greiner Eugenio Bottacini Frank Haberl 《Astrophysics and Space Science》2007,309(1-4):315-319
The new black hole candidate XTE J1817-330, discovered on 26 January 2006 with RXTE, was observed with XMM-Newton and INTEGRAL
in February and March 2006, respectively. The X-ray spectrum is dominated by the thermal emission of the accretion disk in
the soft band, with a low absorption column density (N
H=1.77(±0.01)×1021 cm−2) and a maximum disk temperature kT
max=0.68(±0.01) keV, plus a power law component, with the photon index decreasing from 2.66±0.02 to 1.98±0.07 between the two
observations. Several interstellar absorption lines are detected in the X-ray spectrum, corresponding to O I, O II, O III,
O VII and Fe XXIV. We constrain the distance to the system to be in the range 1–5 kpc.
相似文献
20.
D. K. Klochkov N. I. Shakura K. A. Postnov R. Staubert J. Wilms N. A. Ketsaris 《Astronomy Letters》2006,32(12):804-815
We analyze and interpret the RXTE/ASM X-ray light curves for the close binary system Her X-1/HZ Her obtained from February 1996 to September 2004. Some of the features found previously in the averaged X-ray light curves are confirmed by the new RXTE/ASM data. In particular, the anomalous dips and post-eclipse recoveries in two successive orbits in the short-on state are clearly distinguishable and are stable features of the X-ray light curves. We argue that to account for these features, the tilt of the accretion disk to the orbital plane must be assumed to change with phase of the 35-day period. We present a numerical model that can reproduce the observed features of the light curves. 相似文献