CAS National Astronomical Observatories

Beijing, China

CAS National Astronomical Observatories

Beijing, China
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Wen Z.L.,CAS National Astronomical Observatories | Han J.L.,CAS National Astronomical Observatories | Liu F.S.,Shenyang Normal University
Astrophysical Journal, Supplement Series | Year: 2012

Using the photometric redshifts of galaxies from the Sloan Digital Sky Survey III (SDSS-III), we identify 132,684 clusters in the redshift range of 0.05 ≤ z < 0.8. Monte Carlo simulations show that the false detection rate is less than 6% for the whole sample. The completeness is more than 95% for clusters with a mass of M 200 > 1.0 × 10 14 M in the redshift range of 0.05 ≤ z < 0.42, while clusters of z > 0.42 are less complete and have a biased smaller richness than the real one due to incompleteness of member galaxies. We compare our sample with other cluster samples, and find that more than 90% of previously known rich clusters of 0.05 ≤ z < 0.42 are matched with clusters in our sample. Richer clusters tend to have more luminous brightest cluster galaxies (BCGs). Correlating with X-ray and the Planck data, we show that the cluster richness is closely related to the X-ray luminosity, temperature, and Sunyaev-Zel'dovich measurements. Comparison of the BCGs with the SDSS luminous red galaxy (LRG) sample shows that 25% of LRGs are BCGs of our clusters and 36% of LRGs are cluster member galaxies. In our cluster sample, 63% of BCGs of r petro < 19.5 satisfy the SDSS LRG selection criteria. © 2012 The American Astronomical Society. All rights reserved.

Ning Z.,CAS National Astronomical Observatories
Solar Physics | Year: 2014

Quasi-periodic oscillations in soft X-rays (SXR) are not well known due to the instrument limitations, especially the absence of imaging observations of SXR oscillations. We explore the quasi-periodic oscillations of SXR at 3 - 6 keV in a solar flare observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) on 26 December 2002. This was a B8.1 class event and showed three X-ray sources (S1, S2, and S3) at 3 - 6 keV and two sources (S1 and S2) at 12 - 25 keV. The light curves of the total fluxes display a two-minute oscillation at 3 - 6 keV, but not in the energy bands above 8 keV. To investigate imaging observations of the oscillations, we prepared CLEAN images at seven energy bands between 3 keV and 20 keV with an eight-second integration. The light curves of three sources were analyzed after integrating the flux of each source region. We used the Fourier method to decompose each source light curve into rapidly varying and slowly varying components. The rapidly varying components show seven individual peaks which are well fitted with a sine function. Then we used the wavelet method to analyze the periods in the rapidly varying component of each source. The results show that three sources display damped quasi-periodic oscillations with a similar two-minute period. The damped oscillations timescale varies between 2.5 to 6 minutes. Source S1 oscillates with the same phase as S3, but is almost in anti-phase with S2. Analyzing the flaring images in more detail, we found that these oscillation peaks are well consistent with the appearance of S3, which seems to split from or merge with S2 with a period of two minutes. The flare images with a high cadence of one second at 3 - 6 keV show that source S3 appears with a rapid period of 25 seconds. The two-minute oscillation shows the highest spectral power. Source S3 seems to shift its position along the flare loop with a mean speed of 130 km s-1, which is of the same order as the local sound speed. This connection between the oscillation peaks and emission enhancement appears to be an observational constraint on the emission mechanism at 3 - 6 keV. © 2013 Springer Science+Business Media Dordrecht.

Li L.,CAS National Astronomical Observatories
Nature Physics | Year: 2016

Magnetic reconnection is difficult to observe directly but coronal structures on the Sun often betray the magnetic field geometry and its evolution. Here we report the observation of magnetic reconnection between an erupting filament and its nearby coronal loops, resulting in changes in the filament connection. X-type structures form when the erupting filament encounters the loops. The filament becomes straight, and bright current sheets form at the interfaces. Plasmoids appear in these current sheets and propagate bi-directionally. The filament disconnects from the current sheets, which gradually disperse and disappear, then reconnects to the loops. This evolution suggests successive magnetic reconnection events predicted by theory but rarely detected with such clarity in observations. Our results confirm the three-dimensional magnetic reconnection theory and have implications for the evolution of dissipation regions and the release of magnetic energy for reconnection in many magnetized plasma systems. © 2016 Nature Publishing Group

Du Z.L.,CAS National Astronomical Observatories
Astrophysical Journal | Year: 2015

Some properties of the 11 yr solar cycle can be explained by the current solar dynamo models. However, some other features remain not well understood such as the asymmetry of the cycle, the double-peaked structure, and the "Waldmeier effect" that a stronger cycle tends to have less rise time and a shorter cycle length. We speculate that the solar cycle is governed by a bi-dynamo model forming two stochastic processes depicted by a bimodal Gaussian function with a time gap of about 2 yr, from which the above features can be reasonably explained. The first one describes the main properties of the cycle dominated by the current solar dynamo models, and the second one occurs either in the rising phase as a short weak explosive perturbation or in the declining phase as a long stochastic perturbation. The above function is the best one selected from several in terms of the Akaike information criterion. Through analyzing different distributions, one might speculate about the dominant physical process inside the convection zone. The secondary (main) process is found to be closely associated with complicated (simple) active ranges. In effect, the bi-dynamo model is a reduced form of a multi-dynamo model, which could occur from the base of the convection zone through its envelope and from low to high heliographic latitude, reflecting the active belts in the convection zone. These results are insensitive to the hemispheric asymmetry, smoothing filters, and distribution functions selected and are expected to be helpful in understanding the formation of solar and stellar cycles. © 2015. The American Astronomical Society. All rights reserved..

Ma J.,CAS National Astronomical Observatories
Astronomical Journal | Year: 2012

We present UBVRI photometry for 392 star clusters and candidates in the field of M33, which are selected from the most recent star cluster catalog. In this catalog, the authors listed star clusters' parameters such as cluster positions, magnitudes, colors in the UBVRIJHK s filters, and so on. However, a large fraction of objects in this catalog do not have previously published photometry. Photometry is performed using archival images from the Local Group Galaxies Survey, which covers 0.8deg 2 along the major axis of M33. Detailed comparisons show that, in general, our photometry is consistent with previous measurements. Positions (right ascension and declination) for some clusters are corrected here. Combined with previous literature, ours constitute a large sample of M33 star clusters. Based on this cluster sample, we present some statistical results: none of the youngest M33 clusters (10 7yr) have masses approaching 10 5 M ⊙; roughly half the star clusters are consistent with the 10 4-10 5 M ⊙mass models; the continuous distribution of star clusters along the model line indicates that M33 star clusters have been formed continuously from the epoch of the first star cluster formation until recent times; and there are 50 star clusters which are overlapped with the Galactic globular clusters on the color-color diagram, and these clusters are old globular cluster candidates in M33. © © 2012. The American Astronomical Society. All rights reserved.

Mao X.-C.,CAS National Astronomical Observatories
Astrophysical Journal | Year: 2012

The ability to subtract foreground contamination from low-frequency observations is crucial to reveal the underlying 21 cm signal. The traditional line-of-sight methods can deal with the removal of diffuse emission and unresolved point sources, but not bright point sources. In this paper, we introduce a foreground cleaning technique in Fourier space, which allows us to handle all such foregrounds simultaneously and thus sidestep any special treatments to bright point sources. Its feasibility is tested with a simulated data cube for the 21 CentiMeter Array experiment. This data cube includes more realistic models for the 21 cm signal, continuum foregrounds, detector noise, and frequency-dependent instrumental response. We find that a combination of two weighting schemes can be used to protect the frequency coherence of foregrounds: the uniform weighting in the uv plane and the inverse-variance weighting in the spectral fitting. The visibility spectrum is therefore well approximated by a quartic polynomial along the line of sight. With this method, we demonstrate that the huge foreground contamination can be cleaned out effectively with residuals on the order of ∼10 mK, while the spectrally smooth component of the cosmological signal is also removed, bringing about a systematic underestimate in the extracted power spectrum primarily on large scales.

Tan B.,CAS National Astronomical Observatories
Astrophysical Journal | Year: 2013

Solar small-scale microwave bursts (SMBs), including microwave dot, spike, and narrow-band type III bursts, are characterized by very short timescales, narrow frequency bandwidth, and very high brightness temperatures. Based on observations of the Chinese Solar Broadband Radio Spectrometer at Huairou with superhigh cadence and frequency resolution, this work presents an intensive investigation of SMBs in several flares that occurred in active region NOAA 10720 during 2005 January 14-21. Especially for long-duration flares, the SMBs occurred not only in the early rising and impulsive phase, but also in the flare decay phase and even after the end of the flare. These SMBs are strong bursts with inferred brightness temperatures of at least 8.18 × 10 11-1.92 × 1013 K, very short lifetimes of 5-18 ms, relative frequency bandwidths of 0.7%-3.5%, and superhigh frequency drifting rates. Together with their obviously different polarizations from background emission (the quiet Sun, and the underlying flaring broadband continuum), such SMBs should be individual, independent strong coherent bursts related to some non-thermal energy release and the production of energetic particles in a small-scale source region. These facts show the existence of small-scale strong non-thermal energy releasing activities after the flare maxima, which is meaningful for predicting space weather. Physical analysis indicates that a plasma mechanism may be the most favorable candidate for the formation of SMBs. From the plasma mechanism, the velocities and kinetic energy of fast electrons can be deduced and the region of electron acceleration can also be tracked. © 2013. The American Astronomical Society. All rights reserved.

Qiao E.,CAS National Astronomical Observatories | Liu B.F.,CAS National Astronomical Observatories
Astrophysical Journal | Year: 2013

Observations show that there is a positive correlation between the Eddington ratio λ and hard X-ray index Γ for λ ≳ 0.01, and there is an anti-correlation between λ and Γ for λ ≲ 0.01 in black hole X-ray binaries (with λ = L bol/L Edd). In this work, we theoretically investigate the correlation between Γ and λ within the framework of a disk-corona model. We improve the model by taking into account all cooling processes, including synchrotron and self-Compton radiations in the corona, Comptonization of the soft photons from the underlying accretion disk, and the bremsstrahlung radiations. Presuming that the coronal flow above the disk can reach up to the 0.1 Eddington rate at the outer region, we calculate the structure of the two-phase accretion flows and the emergent spectra for accretion rates from 0.003 to 0.1. We find that at accretion rates larger than ∼0.01 Eddington rate, a fraction of coronal gas condenses into the disk and an inner disk can be sustained by condensation. In this case, the X-ray emission is dominated by the scattering of the soft photon from the underlying disk in the corona. The emission from the inner disk and corona can produce the positive correlation between λ and Γ. While at accretion rates lower than ∼0.01 Eddington accretion rate, the inner disk vanishes completely by evaporation, and the accretion is dominated by advection-dominated accretion flows (ADAFs), in which the X-ray emission is produced by the Comptonization of the synchrotron and bremsstrahlung photons of ADAF itself. The emission from ADAFs can produce the anti-correlation between λ and Γ. We show that our model can roughly explain the observed evolution of Γ3-25 keV with L 0.5-25 keV/L Edd for the black hole X-ray transient H1743-322 in the decay of 2003 from the thermal-dominated state to low/hard state. © 2013. The American Astronomical Society. All rights reserved..

Zhang H.,CAS National Astronomical Observatories
Monthly Notices of the Royal Astronomical Society | Year: 2012

The transfer of magnetic chirality in solar active regions is related to the emerging magnetic flux ropes generated in the subatmosphere. This analysis has been presented based on the calculation of the injection of magnetic helicity at the solar surface. As the long-term evolution of the accumulated magnetic helicity is followed, it is found that the transfer of reversal magnetic helicity of active regions is a complex process, and is not monotonic with the same sign. It is found that the dominant contribution of helicity occurs mainly in the fast-developing stage of active regions. By considering the hemispheric trends of magnetic helicity, it is proposed that the reversal helicity in solar active regions has two possible causes: local generation in the convection zone, and trans-equatorial processes in the subatmosphere. In addition to the mirror-symmetrical reverse of twisted magnetic field in the convection zone, the possibility of a trans-equatorial process is discussed in this paper. © 2011 The Author Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Liu J.,CAS National Astronomical Observatories
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2016

X-ray fluorescent lines are unique features of the reflection spectrum of the torus when irradiated by the central active galactic nuclei (AGN). Their intrinsic line width can be used to probe the line-emitting region. Previous studies have focused on the Fe Ka line at 6.4 keV, which is the most prominent fluorescent line. These studies, however, are limited by the spectral resolution of currently available instruments, the best of which is ~1860 km s-1 afforded by the Chandra High-Energy Grating (HEG). The HEG spectral resolution is improved by a factor of 4 at 1.74 keV, where the Si Ka line is located.We measured the full width at half-maximum of the Si Ka line for Circinus, Mrk 3, and NGC 1068, which are 570 ± 240, 730 ± 320, and 320 ± 280 km s-1, respectively. They are 3-5 times smaller than those measured with the Fe Ka line previously. It shows that the intrinsic widths of the Fe Ka line are most likely to be overestimated. The measured widths of the Si Ka line put the line-emitting region outside the dust sublimation radius in these galaxies. It indicates that for Compton-thick AGN, the X-ray fluorescence material are likely to be the same as the dusty torus emitting in the infrared band. © 2016 The Author.

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