共查询到20条相似文献，搜索用时 313 毫秒
1.
Ian Heywood Katherine M. Blundell Hans-Rainer Klöckner Anthony J. Beasley 《Monthly notices of the Royal Astronomical Society》2009,392(2):855-867
We present simulated images of Supernova 1993J at 8.4 GHz using Very Long Baseline Interferometry (VLBI) techniques. A spherically symmetric source model is convolved with realistic u v -plane distributions, together with standard imaging procedures, to assess the extent of instrumental effects on the recovered brightness distribution. In order to facilitate direct comparisons between the simulations and published VLBI images of SN 1993J, the observed u v -coverage is determined from actual VLBI observations made in the years following its discovery.
The underlying source model only exhibits radial variation in its density profile, with no azimuthal dependence and, even though this model is morphologically simple, the simulated VLBI observations qualitatively reproduce many of the azimuthal features of the reported VLBI observations, such as appearance and evolution of complex azimuthal structure and apparent rotation of the shell. We demonstrate that such features are inexorably coupled to the u v -plane sampling.
The brightness contrast between the peaks and the surrounding shell material are not as prominent in the simulations (which of course assume no antenna- or baseline-based amplitude or phase errors, meaning no self-calibration procedures will have incorporated any such features in models). It is conclusive that incomplete u v -plane sampling has a drastic effect on the final images for observations of this nature. Difference imaging reveals residual emission up to the 8σ level. Extreme care should be taken when using interferometric observations to directly infer the structure of objects such as supernovae. 相似文献
The underlying source model only exhibits radial variation in its density profile, with no azimuthal dependence and, even though this model is morphologically simple, the simulated VLBI observations qualitatively reproduce many of the azimuthal features of the reported VLBI observations, such as appearance and evolution of complex azimuthal structure and apparent rotation of the shell. We demonstrate that such features are inexorably coupled to the u v -plane sampling.
The brightness contrast between the peaks and the surrounding shell material are not as prominent in the simulations (which of course assume no antenna- or baseline-based amplitude or phase errors, meaning no self-calibration procedures will have incorporated any such features in models). It is conclusive that incomplete u v -plane sampling has a drastic effect on the final images for observations of this nature. Difference imaging reveals residual emission up to the 8σ level. Extreme care should be taken when using interferometric observations to directly infer the structure of objects such as supernovae. 相似文献
2.
J. A. Pons J. F. Pérez-Azorín J. A. Miralles G. Miniutti 《Astrophysics and Space Science》2007,308(1-4):247-257
We propose a self–consistent model to explain all observational properties reported so far on the isolated neutron star (INS)
RX J0720-3125 with the aim of giving a step forward towards our understanding of INSs. For a given magnetic field structure,
which is mostly confined to the crust and outer layers, we obtain theoretical models and spectra which account for the broadband
spectral energy distribution (including the apparent optical excess), the X-ray pulsations, and for the spectral feature seen
in the soft X-ray spectrum of RX J0720-3125 around 0.3 keV. By fitting our models to existing archival X-ray data from 6 different
XMM–Newton observations and available optical data, we show that the observed properties are fully consistent with a normal neutron star, with a proper radius of about 12 km, a temperature at the magnetic pole of about 100 eV, and a magnetic field
strength of 2–3×1013 G. Moreover, we are able to reproduce the observed long–term spectral evolution in terms of free precession which induces
changes in the orientation angles of about 40 degrees with a periodicity of 7 years. In addition to the evidence of internal
toroidal components, we also find strong evidence of non–dipolar magnetic fields, since all spectral properties are better
reproduced with models with strong quadrupolar components.
相似文献
3.
Previous VLA observations of the triple radio continuum source in Serpens showed that it has very unusual and extraordinary characteristics. While this source is associated with a star forming region, its outer components exhibit a combination of thermal and nonthermal spectra and large proper motions, Furthermore, the NW lobe has knotty and extended emission connecting the central source with the bright outer knot. Here, we present results of new VLA radio continuum high-angular resolution observations of this Radio Jet. Combining these observations with those obtained previously, we find that: a) one of the knots along the main body of the radio jet (knot G) exhibits proper motions similar to those observed in the outer NW and SE components, and the time variable knot A; b) the outer knots are moving away from the central source in slightly different directions; and c) the orientation of the central source seems to change with time. These results are consistent with a central precessing source that undergoes periodic ejection of material. 相似文献
4.
5.
利用文[1]中权重5的大量高精度测光数据,用文[2]中的周期分析法,得出BWVul的主要脉动周期P_1=0.21010425d。另有变幅仅为前者十分之一的二倍频P_2=0.10052300d及四倍频P_4=0.05026096d。文[3]求出的变幅更小的P_3、P_5、P_6、P_7、P_8等倍频可信度很低。故多重周期不是引起P_1长期变化及限制脉动变幅的主因。 相似文献
6.
D. C. Gabuzda P. YU. Kochenov R. I. Kollgaard T. V. Cawthorne 《Monthly notices of the Royal Astronomical Society》2000,315(2):229-240
Total intensity and polarization λ =6 cm Very Large Array (VLA) and global very long baseline interferometry (VLBI) images of the quasar 0917+624 and the BL Lacertae object 0954+658 (both at epoch 1991.43) are analysed. Integrated measurements using the VLA during the VLBI observations indicated that, although there were no substantial total intensity variations, there were significant polarization variations for both sources during the 24-h VLBI experiment. The VLBI data were divided into 2–3 h segments in order to try to identify corresponding rapid variability in the VLBI structure. This analysis revealed intraday variability (IDV) in the VLBI core of 0917+624: both the polarized flux and the polarization position angle varied substantially on time-scales of ∼5–10 h. There is evidence that the VLA polarization variations for 0954+658 occurred in an inner VLBI jet component, where the polarized flux varied by ∼30–40 per cent on time-scales of ∼2 h. 0917+624 and 0954+658 were observed together with 0716+714, an object that also displayed IDV in the polarized flux density measured during our experiment (analysed in a separate paper). These three sources were targeted for the VLBI observations since they had been previously identified as intraday variables, but we had no way of knowing whether they would vary during our observations. The fact that all three exhibited IDV in polarization (but not in total intensity) during our experiment suggests that polarization IDV occurs frequently in at least some IDV sources. 相似文献
7.
It has been suggested that the 35 day clock mechanism of Her X-1 is caused by the precession of the neutron star. We made calculations for this model. Comparing our calculated results with observations we find:
- 1. (1) The theoretical light curve over the the 35 day cycle does not agree with the observed curve by Jones and Forman (1976).
2. (2) The change of pulse width and intensity with the 35 day phase is large in the model, seemingly disagreeing with the observations (e.g., Gruber et al. 1980).
3. (3) Parmer et al. (1985) reported that they did not see the expected 35 day variation in X-ray intensity expected in the model.
Pravdo et al. (1977) suggested that the observed X-ray pulses were formed by Thomson scattering of symmetric radiating beams generated near the surface of the neutron star. Our analysis based on the theory of Thomson scattering in strong magnetic fields shows that this model agrees well with the observations. 相似文献
8.
We study the steady-state structure of an accretion disc with a corona surrounding a central, rotating, magnetized star. We
assume that the magneto-rotational instability is the dominant mechanism of angular momentum transport inside the disc and
is responsible for producing magnetic tubes above the disc. In our model, a fraction of the dissipated energy inside the disc
is transported to the corona via these magnetic tubes. This energy exchange from the disc to the corona which depends on the
disc physical properties is modified because of the magnetic interaction between the stellar magnetic field and the accretion
disc. According to our fully analytical solutions for such a system, the existence of a corona not only increases the surface
density but reduces the temperature of the accretion disc. Also, the presence of a corona enhances the ratio of gas pressure
to the total pressure. Our solutions show that when the strength of the magnetic field of the central neutron star is large
or the star is rotating fast enough, profiles of the physical variables of the disc significantly modify due to the existence
of a corona. 相似文献
9.
The Ursa Major group is a nearby stellar supercluster which, while not gravitationally bound, is defined by co-moving members. DD UMa is a δ Scuti star whose membership in the Ursa Major group is unclear.The objective of this study is to confirm the membership of DD UMa in the Ursa Major group, as well as perform a detailed spectral analysis of the star. Since DD UMa is a low-amplitude δ Scuti star, we performed a frequency analysis. We determined fundamental parameters, chemical abundances, and derive a mass and age for the star.For this study we observed DD UMa at the Okayama Astrophysical Observatory with the high-resolution spectrograph HIDES, between the 27th of February and the 4th March, 2009. Additional observations were extracted from the ELODIE archive in order to expand our abundance analysis. Group membership of DD UMa was assessed by examining the velocity of the star in Galactic coordinates. Pulsational frequencies were determined by examining line profile variability in the HIDES spectra. Stellar fundamental parameters and chemical abundances were derived by fitting synthetic spectra to both the HIDES and ELODIE observations.DD UMa is found to be a member of the extended stream of the Ursa Major group, based on the space motion of the star. This is supported by the chemical abundances of the star being consistent with those of Ursa Major group members. The star is found to be chemically solar, with Teff = 7450 ± 150 K and logg = 3.98 ± 0.2. We found pulsational frequencies of 9.4 and 15.0 c/d. While these frequencies are insufficient to perform an asteroseismic study, DD UMa is a good bright star candidate for future study by the BRITE-constellation. 相似文献
10.
We report on the progress of world array VLBI observations of the BL Lac object 1803+784 performed at λ=18 cm. A 16-station VLBI array, plus the phased VLA, and the 7-station MERLIN array simultaneously observed the source for 11 hours. These observations provide images of the transition region between the misaligned milliarcsecond- and arcsecond-jet. We show the MERLIN map and a combined full-array map at medium resolution. In addition we describe the present status of short-cm VLBI observations of this source. 相似文献
11.
The search of roAp stars at Mt. Dushak-Erekdag Observatory was started in 1992 using the 0.8 m Odessa telescope equipped with
a two-star high-speed photometer. We have observed more than a dozen stars so far and discovered HD 99563 as roAp star while
BD+8087 is suspected to have rapid oscillations. Negative results of our observations for the search of rapid oscillations
in four stars in NGC 752 are also discussed. 相似文献
12.
We present some results from our submillimeter single-dish and aperture synthesis imaging surveys of protoplanetary disks
using the JCMT, CSO, and Submillimeter Array (SMA) on Mauna Kea, Hawaii. Employing a simple disk model, we simultaneously
fit the spectral energy distributions and spatially resolved submillimeter continuum emission from our SMA survey to constrain
disk structure properties, including surface density profiles and sizes. The typical disk structure we infer is consistent
with a fiducial accretion disk model with a viscosity parameter α≈0.01. Combined with a large, multiwavelength single-dish survey of similar disks, we show how these observations provide
evidence for significant grain growth and rapid evolution in the outer regions of disks, perhaps due to an internal photoevaporation
process. In addition, we discuss SMA observations of the disks in the Orion Trapezium (proplyds) in the context of disk evolution
in a more extreme environment. 相似文献
13.
《Chinese Astronomy and Astrophysics》2019,43(4):539-548
Machine learning has achieved great success in many areas today. The lifting algorithm has a strong ability to adapt to various scenarios with a high accuracy, and has played a great role in many fields. But in astronomy, the application of lifting algorithms is still rare. In response to the low classification accuracy of the dark star/galaxy source set in the Sloan Digital Sky Survey (SDSS), a new research result of machine learning, eXtreme Gradient Boosting (XGBoost), has been introduced. The complete photometric data set is obtained from the SDSS-DR7, and divided into a bright source set and a dark source set according to the star magnitude. Firstly, the ten-fold cross-validation method is used for the bright source set and the dark source set respectively, and the XGBoost algorithm is used to establish the star/galaxy classification model. Then, the grid search and other methods are used to adjust the XGBoost parameters. Finally, based on the galaxy classification accuracy and other indicators, the classification results are analyzed, by comparing with the models of function tree (FT), Adaptive boosting (Adaboost), Random Forest (RF), Gradient Boosting Decision Tree (GBDT), Stacked Denoising AutoEncoders (SDAE), and Deep Belief Nets (DBN). The experimental results show that, the XGBoost improves the classification accuracy of galaxies in the dark source classification by nearly 10% as compared to the function tree algorithm, and improves the classification accuracy of sources with the darkest magnitudes in the dark source set by nearly 5% as compared to the function tree algorithm. Compared with other traditional machine learning algorithms and deep neural networks, the XGBoost also has different degrees of improvement. 相似文献
14.
P. M. Phillips M. A. Norbury L. V. E. Koopmans I. W. A. Browne N. J. Jackson P. N. Wilkinson A. D. Biggs R. D. Blandford A. G. de Bruyn C. D. Fassnacht P. Helbig S. Mao D. R. Marlow S. T. Myers T. J. Pearson A. C. S. Readhead D. Rusin E. Xanthopoulos 《Monthly notices of the Royal Astronomical Society》2000,319(2):L7-L11
High-resolution MERLIN observations of a newly discovered four-image gravitational lens system, B0128+437, are presented. The system was found after a careful re-analysis of the entire CLASS data set. The MERLIN observations resolve four components in a characteristic quadruple-image configuration; the maximum image separation is 542 mas and the total flux density is 48 mJy at 5 GHz. A best-fitting lens model with a singular isothermal ellipsoid results in large errors in the image positions. A significantly improved fit is obtained after the addition of a shear component, suggesting that the lensing system is more complex and may consist of multiple deflectors. The integrated radio spectrum of the background source indicates that it is a gigahertz peaked spectrum source. It may therefore be possible to resolve structure within the radio images with deep VLBI observations and thus to constrain the lensing mass distribution better. 相似文献
15.
Stephen Skinner Manuel Güdel Kevin Briggs Stanislav Melnikov Marc Audard 《Astrophysics and Space Science》2006,304(1-4):165-167
We present new results from recent X-ray observations of the accreting pre-main sequence stars FU Orionis and T Tauri. XMM-Newton observations of the close binary system FU Ori reveal an unusual X-ray spectrum consisting of a cool moderately-absorbed component and a hot component viewed through much higher absorption. The two components thus originate in physically distinct regions. The double absorption spectrum is qualitatively different than observed in typical coronal sources and may signal either non-coronal emission or separate unresolved X-ray contributions from more than one star in the system. High-resolution Chandra imaging of the T Tau triple system shows that its X-ray emission is dominated by the optically-revealed northern component T Tau N. X-ray spectra of T Tau obtained with XMM can be acceptably fitted with a moderately absorbed two-temperature thermal plasma model. Its spectral properties are similar to those seen in coronal X-ray sources. 相似文献
16.
D. C. Gabuzda P. YU. Kochenov T. V. Cawthorne R. I. Kollgaard 《Monthly notices of the Royal Astronomical Society》2000,313(3):627-636
Rapid ('intraday') cm-wavelength variations in both total and polarized flux density have been observed in a number of strong extragalactic radio sources. It is difficult to explain these variations purely as propagation effects, but if they are intrinsic to the sources, implausibly high brightness temperatures are required. We discuss here rapid polarization variability during our λ =6 cm global very long-baseline interferometry (VLBI) observations of the active galactic nucleus 0716+714. Measurements made with the Very Large Array (VLA) during the VLBI observations indicate a ≃50° swing in the position angle χ of the VLA core polarization in 12 hours. Corresponding variations were observed only for short VLBI baselines, so that they could not have occurred in the VLBI core (the only feature detected in our VLBI polarization map). The fact that the variations appear both in the VLA data and in the VLBI data for short baselines makes it difficult to explain them as instrumental effects. This leads us to conclude that the rapid variations occurred outside the area covered by our VLBI map; we estimate that the variability occurred in some compact feature roughly 25 milliarcseconds from the nucleus. It is clear that compact structures on a wide range of scales must be taken into account in studies of intraday variability in AGN. 相似文献
17.
Huib Jan van Langevelde Philip J. Diamond Wouter Vlemmings Alain Baudry Harm J. Habing Richard T. Schilizzi 《New Astronomy Reviews》1999,43(8-10)
The OH 1667 maser in the circumstellar shell around the Mira variable U Her has been observed with the VLBA at 6 epochs, spread over 4 years. By using straightforward phase referencing techniques the stellar proper motion can be measured. Preliminary analysis indicates that the parallactic motion is also detected. An important question is whether the maser spots can be assumed to be fixed with respect to the star. The observations show both evidence supporting and contradicting the idea that one maser spot is the amplified stellar image. 相似文献
18.
J. N. McCallum S. P. Ellingsen J. E. J. Lovell C. J. Phillips J. E. Reynolds 《Monthly notices of the Royal Astronomical Society》2009,392(4):1339-1362
We have used the Long Baseline Array to observe the Circinus Galaxy H2 O megamasers on two epochs separated by 49 d. The masers were observed to be highly variable, consistent with previous studies. We observe rapid variations in the linewidth and central velocity of the maser's spectral profile. The characteristics of these spectral variations suggest that the masers have an anisotropic structure, with axial ratios ≲5, and display velocity gradients in their microarcsecond structure. We also find a strong anticorrelation between the linewidth and the modulation index of the maser features and a relationship between the modulation index and characteristic time-scale of the variations. These various phenomena can all be simply explained through a model where the rapid variations in the masers are produced by quenched diffractive scintillation in the presence of modest velocity gradients in the maser emission.
The very long baseline interferometry (VLBI) images are consistent with previous high-resolution observations and show maser emission arising both in a circumnuclear accretion disc, and in a low-velocity outflow. We find no systematic differences in the variability properties of the disc and outflow masers. The VLBI images reveal that there is considerable spectral blending in single-dish spectra of the Circinus megamasers.
In addition to the VLBI imaging we also used these observations to look for a time delay in the intensity variations observed at the different telescopes in the array, as expected for scintillation-induced variability. No time delay was detected, which allows us to place a lower limit on the velocity of the interstellar medium during these observations 22 km s−1 for the second epoch. We conclude that any future attempts to measure a time delay in this source should utilize intercontinental baselines to increase the chances of success. 相似文献
The very long baseline interferometry (VLBI) images are consistent with previous high-resolution observations and show maser emission arising both in a circumnuclear accretion disc, and in a low-velocity outflow. We find no systematic differences in the variability properties of the disc and outflow masers. The VLBI images reveal that there is considerable spectral blending in single-dish spectra of the Circinus megamasers.
In addition to the VLBI imaging we also used these observations to look for a time delay in the intensity variations observed at the different telescopes in the array, as expected for scintillation-induced variability. No time delay was detected, which allows us to place a lower limit on the velocity of the interstellar medium during these observations 22 km s
19.
M. Chernyakova A. Neronov F. Aharonian Y. Uchiyama T. Takahashi 《Monthly notices of the Royal Astronomical Society》2009,397(4):2123-2132
PSR B1259−63 is a 48-ms radio pulsar in a highly eccentric 3.4-yr orbit with a Be star SS 2883. Unpulsed γ-ray, X-ray and radio emission components are observed from the binary system. It is likely that the collision of the pulsar wind with the anisotropic wind of the Be star plays a crucial role in the generation of the observed non-thermal emission. The 2007 periastron passage was observed in unprecedented details with Suzaku , Swift , XMM–Newton and Chandra missions. We present here the results of this campaign and compare them with previous observations. With these data we are able, for the first time, to study the details of the spectral evolution of the source over a 2-month period of the passage of the pulsar close to the Be star. New data confirm the pre-periastron spectral hardening, with the photon index reaching a value smaller than 1.5, observed during a local flux minimum. If the observed X-ray emission is due to the inverse Compton (IC) losses of the 10-MeV electrons, then such a hard spectrum can be a result of Coulomb losses, or can be related to the existence of the low-energy cut-off in the electron spectrum. Alternatively, if the X-ray emission is a synchrotron emission of very high-energy electrons, the observed hard spectrum can be explained if the high-energy electrons are cooled by IC emission in Klein–Nishina regime. Unfortunately, the lack of simultaneous data in the TeV energy band prevents us from making a definite conclusion on the nature of the observed spectral hardening and, therefore, on the origin of the X-ray emission. 相似文献
20.
We constrain the properties of the spotted regions on the photosphere of the active late-type star DX Leonis by comparing the observed amplitudes of light and color variations with synthetic amplitudes obtained by means of Dorren's spot model and computed for a grid of values of spot temperatures, areas and latitudes. 相似文献