2001, Vol.1, No.5 (October 20, 2001)

Contents      


LETTERS


The X-ray Background from the Warm Gas of the Galactic Halo
Yan-Jie Xue  ................................................................. p.383
Abstract | | PDF | PS (*.gz)  | PS (*.zip)

A Possible Origin of 6.7 GHz Linear Methanol Maser Sources
Ye Xu  ....................................................................... p.389
Abstract | | PDF | PS (*.gz)  | PS (*.zip)
RESEARCH PAPERS


A Method of Obtaining the Pitch Angle of Spiral Arms and
the Inclination of Galactic Discs

Jun Ma  ...................................................................... p.395
Abstract | | PDF | PS (*.gz)  | PS (*.zip)

Radio Observations of Sgr~B2
Xin-Jie Mao and Jiang-Tao Su ................................................  p.406
Abstract | | PDF | PS (*.gz) | PS (*.zip)

Hardening of Thermal Photons Through Inverse Compton Scattering in
Strong Magnetic Fields
Yi Xu and Hong-Hua Xu ......................................................   p.411
Abstract | | PDF | PS (*.gz)  | PS (*.zip)

Scintillation Dynamic Spectra and Transverse Velocities of Seven Pulsars
Na Wang, Xin-Ji Wu, R. N. Manchester, Jin Zhang, Aili Yusup and Hong-Bo Zhang  ............................................................................   p.421
Abstract | | PDF | PS (*.gz)  | PS (*.zip)

On the Light Curves of GRB Afterglows
Ji-Rong Mao and Jian-Cheng Wang   ...........................................   p.433
Abstract | | PDF | PS (*.gz)  | PS (*.zip)

Radio Observations at 232 MHz and Multifrequency Spectral Studies of 
the SNR HB21

Xi-Zhen Zhang, L. A. Higgs, T. L. Landecker, Xin-Ji Wu and Shan-Jie Qian  ...  p.443
Abstract | | PDF | PS (*.gz)  | PS (*.zip)

On the Lower Energy Cutoff of Nonthermal Electrons in Solar Flares
Wei-Qun Gan, You-Ping Li and Jin Chang .....................................  p.453
Abstract | | PDF | PS (*.gz)  | PS (*.zip)

Physical Parameters of a Flare Derived from Multi-line [2mm] 2D Spectroscopy
Ying Liu and Ming-De Ding  ..................................................  p.460
Abstract | | PDF | PS (*.gz)  | PS (*.zip)

Some Observed Results of Solar Radio Spectrometer at [2mm] 4.5--7.5 GHz
Fu-Ying Xu, Qi-Jun Yao, Xuan Meng and Hong-Ao Wu  ...........................  p.469
Abstract | | PDF | PS (*.gz)  | PS (*.zip)

Anisotropic Beam Model for the Spectral Observations of Radio Burst
Fine Structures on 1998 April 15

V. G. Ledenev, Yi-Hua Yan and Qi-Jun Fu .................................... p.475 
Abstract | | PDF | PS (*.gz)  | PS (*.zip)


Abstract


The X-ray Background from the Warm Gas of the Galactic Halo


Yan-Jie Xue

E-mail: jie@class2.bao.ac.cn

Abstract

Within the framework of the hierarchical scenario of galaxy formation, spiral galaxies like our own Galaxy are still growing at present. This opens a possibility that one might be able to see X-ray galactic halos from gravitationally heated gas with temperatures of $\sim$ 106 K as a result of bremsstrahlung. An interesting issue is whether the X-ray background produced by the warm gas in the halo of our Galaxy is detectable. We present a simple estimate of the strength and spectrum of the X-ray background from the Galactic halo and compare with the recent findings of a spatially variable soft X-ray component seen towards the north Galactic polar cap by Kuntz et al. (2001). It is shown that a good agreement, regardless of cosmological models, can be achieved if the gas fraction is as low as $\sim$ 0.01. This requirement seems to be consistent with the extrapolated result from a number of independent observational and theoretical constraints established for groups and clusters of galaxies. In particular, the expected soft X-ray background from the warm gas of the Galactic halo is comparable to, or even exceeds that produced by the warm-hot gas in massive groups, and it may constitute the major source of contamination in the search
for missing baryons through the detection of their soft X-ray emission, unless we can work out a way to properly remove the X-ray background (e.g., from anisotropy) from the halo of our Galaxy.

Key words: cosmology: diffuse radiation --- Galaxy: formation --- Galaxy: halo --- X-rays: galaxies

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A Possible Origin of 6.7 GHz Linear Methanol Maser Sources

Ye Xu

E-mail: xy@center.shao.ac.cn

Abstract

We present a model which explains the observed multi-linear spatial structures with different velocity gradients of the 50–61A+ methanol line at 6.7 GHz. We consider this and other hypotheses and conclude that the methanol maser emission probably comes from some thin rings variously inclined to the line of sight.

Key words: masers --- stars: circumstellar matter --- stars: formation

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A Method of Obtaining the Pitch Angle of Spiral Arms and the Inclination of Galactic Discs

Jun Ma

E-mail: majun@vega.bac.pku.edu.cn

Abstract

We investigate the mathematical form, the symmetry of spiral structure and the projected images of galactic discs. The measured pitch angles of spiral arms and inclination angles of galactic discs for 60 spiral galaxies are presented. The global spiral structure is emphasized in the study. It is found that, except for small-scale distortions, the spiral arms of those galaxies that were classified as AC 12 in the arm classification system of Elmegreen & Elmegreen, can be represented by the logarithmic spiral form.

Key words: galaxies: spiral galaxies --- galaxies: spiral arm --- disc: inclination

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Radio Observations of Sgr~B2

Xin-Jie Mao and Jiang-Tao Su

E-mail: maoxj@bnu.edu.cn 

Abstract

The 13CO (J=1-0) map of the molecular cloud Sgr B2 reveals that the mass center of the molecular cloud nucleus does not coincide with that of compact HII regions which are likely to be the outcome of a shock on the cloud. We find evidence of cloud contraction probably resulting from cloud-cloud collision at subsonic speed.

Key words: ISM: clouds --- evolution --- ISM: lines and bands

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Hardening of Thermal Photons Through Inverse Compton Scattering in
Strong Magnetic Fields

Yi Xu and Hong-Hua Xu

E-mail: xuhh@mail.sjtu.edu.cn

Abstract

A new spectrum function is obtained by use of the Compton scattering cross section in the laboratory frame derived earlier. This spectrum function, besides some modifications in the coefficients of the resonant term, contains also a non-resonant term which is inversely proportional to the square of the magnetic field. Based on this spectrum function, the hardening of thermal photons through inverse Compton scattering by relativistic electron beams on the surface of a strongly magnetized neutron star is investigated. Two new features are found. First, there is a maximum scattered photon energy for a given resonant scattering, beyond which resonance disappears. This maximum depends on the electron energy and the magnetic field, but is independent of the incident photon energy. Second, beyond each resonant scattering, there is a high-energy tail, resulting from non-resonant scattering. It is also found that all the tails have a common upper limit which is the highest scattered photon energy for the given incident photon and electron energies. These two new features are absent in the Monte Carlo simulations and therefore, may have physical implications for $\gamma$-ray emissions.

Key words: stars: neutron star --- magnetic fields --- scattering --- gamma rays

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Scintillation Dynamic Spectra and Transverse Velocities of Seven Pulsars

Na Wang, Xin-Ji Wu, R. N. Manchester, Jin Zhang, Aili Yusup and Hong-Bo Zhang

E-mail: wangna@ms.xjb.ac.cn

Abstract

Using a pulsar timing system developed at Urumqi Astronomical Observatory's 25-m telescope, we observed scintillation dynamic spectra for seven pulsars at the relatively high frequency of 1540 MHz over a wide frequency band of 320 MHz. Averaging observations at different epochs, we obtain time scales and decorrelation bandwidths for diffractive scintillation and show that these imply a power-law index for the electron density fluctuation close to 4.0. Assuming this value and that the scattering disk is approximately midway between the pulsar and the earth, we compute transverse velocities for the seven pulsars. These values are generally in good agreement with the proper motion velocities.

Key words: pulsars: general -- ISM: general

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On the Light Curves of GRB Afterglows

Ji-Rong Mao and Jian-Cheng Wang

E-mail: mjr@cosmos.ynao.ac.cn

Abstract

We present gamma-ray burst afterglow light curves in X-ray, optical and radio bands for various distributions of accelerated electrons behind the shock. The effects of lateral expansion of the jet and of winds in typical Wolf-Rayet star on the evolution are discussed. The light curves in the radiative case decline more rapidly than those in the adiabatic case. Under the combined effect of jet expansion and wind environment, the light curves have the greatest deviation from those of the standard model. All these results refer to the relativistic phase.

Key words: gamma-rays: burst --- hydrodynamics --- radiation mechanisms: nonthermal

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Radio Observations at 232 MHz and Multifrequency Spectral Studies of the SNR HB21

Xi-Zhen Zhang, L. A. Higgs, T. L. Landecker, Xin-Ji Wu and Shan-Jie Qian

E-mail: zxz@ns.bao.ac.cn

Abstract

Radio observational results at 232 MHz and multifrequency studies of the supernova remnant (SNR) HB21 are presented. Its integrated flux density at 232 MHz is about 390 $\pm$30 Jy. Both the integrated spectral index and the spatial variations of spectral index of the remnant were calculated by combining the new map at 232 MHz with previously published maps made at 408, 1420, 2695, and 4750 MHz. The SNR has an integrated spectral index of about $\alpha$ = -0.43 ( $S{\nu} \propto \nu^{\alpha}$) between 232 and 4750 MHz. In general the spectral index varies from -0.5 in southeast and west regions of the remnant to --0.3 in the central region and near the northwest edge. The new data of 232 MHz reveal that there is interaction between the remnant and the surrounding gas along the east edge of the remnant which causes the spectrum flattening at low frequency, while the very good agreement between the structure of X-ray emission and the central flat spectrum area suggests that the existence of thermal emission is the reason of spectrum flattening in the area.

Key words: ISM: supernova remnants -- ISM: structure

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On the Lower Energy Cutoff of Nonthermal Electrons in Solar Flares

Wei-Qun Gan, You-Ping Li and Jin Chang

E-mail: wqgan@pmo.ac.cn

Abstract

A quantitative method to determine the lower energy cutoff ( Ec) of power-law electron beams is established. We apply this method to the 54 hard X-ray events observed with BATSE/Compton Gamma Ray Observatory (CGRO). The results show that about 75% of the observed broken double power-law spectra of hard X-rays can be explained by a lower energy cutoff in the power-law electron beams. The values of Ec, varying among the flares, are all greater than the usually accepted 20 keV! On average, Ec) is about 69 keV. So high a lower energy cutoff of nonthermal electrons implies that nonthermal electrons might not be as important in powering solar flares as was previously thought. Further significance of this finding is discussed.

Key words: Sun: hard X-rays -- Sun: gamma-rays -- Sun: flares -- Sun: particles

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Physical Parameters of a Flare Derived from Multi-line [2mm] 2D Spectroscopy

Ying Liu and Ming-De Ding

E-mail: dmd@nju.edu.cn

Abstract

Time series of 2D spectra of H$\alpha$ and $\lambda$8542 for a flare of 1999 December 22 are obtained and analyzed with a new fitting technique. The method we proposed can simultaneously yield the four parameters: the line source function, the optical thickness at line center, the line-of-sight velocity and the Doppler width. We present the spatial distributions of the physical parameters and their temporal evolutions determined from the 2D spectra. Our results are consistent with the general picture predicted by the flare dynamic models.

Key words: Sun: flares -- Sun: chromosphere

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Some Observed Results of Solar Radio Spectrometer at [2mm] 4.5--7.5 GHz

Fu-Ying Xu, Qi-Jun Yao, Xuan Meng and Hong-Ao Wu

E-mail: wuhongao@sina.com

Abstract

A new instrument of broadband solar radio spectrometer working at waveband 4.5–7.5 GHz was developed at Purple Mountain Observatory for Solar Maximum 23. Some new results of spectral observation have been obtained since August 1999. Two typical type III$\mu$ bursts with rich fine structures are presented and some interesting features discussed.

Key words: Sun: flares --- Sun: radio radiatio

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Anisotropic Beam Model for the Spectral Observations of Radio Burst Fine Structures on 1998 April 15

V. G. Ledenev, Yi-Hua Yan and Qi-Jun Fu

E-mail: yyh@bao.ac.cn

Abstract

A fine structure consisting of three almost equidistant frequency bands was observed in the high frequency part of a solar burst on 1998 April 15 by the spectrometer of Beijing Astronomical Observatory in the range 2.6–3.8 GHz. A model for this event based on beam-anisotropic instability in the solar corona is presented. Longitudinal plasma waves are excited at cyclotron resonance and then transformed into radio emission at their second harmonic. The model is in accordance with the observations if we suppose a magnetic field strength in the region of emission generation of about 200 G.

Key words: Sun: radio radiation --- Sun: activity --- Sun: corona

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ChJAA, 2001, Vol.1, No.5

Chinese Journal of Astronomy and Astrophysics: Online Edition

http://www.chjaa.org 

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