arXiv subjects

J. L. Han

Publications and source records attributed to J. L. Han.

At least 109 records · Page 6Linked to original sources

arXiv2015

New measurement of the excited states in $^{11}$B via the elastic resonance scattering of $^{10}$Be + p

The elastic resonance scattering protons decayed from $^{11}$B to the ground state of $^{10}$Be were measured using the thick-target technique in inverse kinematics at the Heavy Ion Research Facility in Lanzhou (HIRFL). The obtained excitation functions were well described by a multichannel R-matrix procedure under the kinematics process assumption of resonant elastic scattering. The excitation energy of the resonant states ranges from 13.0 to 17.0 MeV, and their resonant parameters such as the resonant energy E$_{x}$, the spin-parity J$^π$, and the proton-decay partial width $Γ_p$ were determined from R-matrix fits to the data. Two of these states around E$_{x}$ = 14.55 MeV [J$^π$ = (3/2$^+$, 5/2$^+$), $Γ_p$ = 475 $\pm$ 80 keV] and E$_{x}$ = 14.74 MeV [J$^π$ = 3/2$^-$, $Γ_p$ = 830 $\pm$ 145 keV], and a probably populated state at E$_x$ = 16.18 MeV [J$^π$ =(1/2$^-$, 3/2$^-$), $Γ_p$ $<$ 60 keV], are respectively assigned to the well-known states in $^{11}$B at 14.34 MeV, 15.29 MeV, and 16.43 MeV. The isospin of these three states were previously determined to be T = 3/2, but discrepancies exist in widths and energies due to the current counting statistics and energy resolution. We have compared these states with previous measurements, and the observation of the possibly populated resonance is discussed.

nucl-ex↗

arXiv2015

The extremely long period X-ray source in a young supernova remnant: a Thorne-Zytkow Object descendant?

The origin of the 6.67 hr period X-ray source, 1E161348-5055, in the young supernova remnant RCW 103 is puzzling. We propose that it may be the descendant of a Thorne-Zytkow Object (TZO). A TZO may at its formation have a rapidly spinning neutron star as a core, and a slowly rotating envelope. We found that the core could be braked quickly to an extremely long spin period by the coupling between its magnetic field and the envelope, and that the envelope could be disrupted by some powerful bursts or exhausted via stellar wind. If the envelope is disrupted after the core has spun down, the core will become an extremely long-period compact object, with a slow proper motion speed, surrounded by a supernova-remnant-like shell. These features all agree with the observations of 1E161348-5055. TZOs are expected to have produced extraordinary high abundances of lithium and rapid proton process elements that would remain in the remnants and could be used to test this scenario.

astro-ph.HE↗

arXiv2014

On frequency dependence of pulsar linear polarization

Frequency dependence of pulsar linear polarization is investigated by simulations of emission and propagation processes. Linearly polarized waves are generated through curvature radiation by relativistic particles streaming along curved magnetic field lines, which have ordinary mode (O-mode) and extra-ordinary mode (X-mode) components. As emitted waves propagate outwards, two mode components are separated due to re- fraction of the O mode, and their polarization states are also modified. According to the radius to frequency mapping, low frequency emission is generated from higher magnetosphere, where significant rotation effect leads the X and O modes to be sepa- rated. Hence, the low frequency radiation has a large fraction of linear polarization. As the frequency increases, emission is generated from lower heights, where the rotation effect becomes weaker and the distribution regions of two modes are more overlapped. Hence, more significant depolarization appears for emission at higher frequencies. In addition, refraction effect of the O mode becomes serious in very deep magnetosphere, which bends the O mode emission towards outer parts of a pulsar beam and also causes the separation of mode distribution regions and hence the fractional linear polariza- tion increasing with frequency. If emission of different frequencies is generated from a region of the same height, serious O mode refraction can result in the decrease of both profile width and fractional linear polarization. The observed frequency dependence of linear polarization for some pulsars can be naturally explained within the scope of our scenario.

astro-ph.HE↗

arXiv2014

Dependence of the bright end of galaxy luminosity function on cluster dynamical state

Luminosity function of cluster galaxies provides a fundamental constraint on galaxy evolution in cluster environments. By using the bright member galaxies of a large sample of rich clusters identified from Sloan Digital Sky Survey, we obtain the bright end of composite luminosity functions of cluster galaxies, and study their dependence on cluster dynamical state. After a redshift-evolution correction of absolute magnitude, the luminosity function of member galaxies can be well fitted by a Schechter function when the brightest cluster galaxies (BCGs) are excluded. The absolute magnitudes of BCGs follow a Gaussian function with a characteristic width of about 0.36 mag. We find that the luminosity function of galaxies in more relaxed clusters has a fainter characteristic absolute magnitude (M_{\ast}), and these clusters have fewer bright non-BCG member galaxies but a brighter BCG. Our results suggest the co-evolution of galaxy population with cluster dynamical state and somewhat support the hierarchical formation scenario of the BCGs.

astro-ph.GA↗

arXiv2014

The observed spiral structure of the Milky Way

The spiral structure of the Milky Way is not yet well determined. The keys to understanding this structure are to increase the number of reliable spiral tracers and to determine their distances as accurately as possible. HII regions, giant molecular clouds (GMCs), and 6.7-GHz methanol masers are closely related to high mass star formation, and hence they are excellent spiral tracers. We update the catalogs of Galactic HII regions, GMCs, and 6.7-GHz methanol masers, and then outline the spiral structure of the Milky Way. We collected data for more than 2500 known HII regions, 1300 GMCs, and 900 6.7-GHz methanol masers. If the photometric or trigonometric distance was not yet available, we determined the kinematic distance using a Galaxy rotation curve with the current IAU standard, $R_0$ = 8.5 kpc and $Θ_0$ = 220 km s$^{-1}$, and the most recent updated values of $R_0$ = 8.3 kpc and $Θ_0$ = 239 km s$^{-1}$, after we modified the velocities of tracers with the adopted solar motions. With the weight factors based on the excitation parameters of HII regions or the masses of GMCs, we get the distributions of these spiral tracers. The distribution of tracers shows at least four segments of arms in the first Galactic quadrant, and three segments in the fourth quadrant. The Perseus Arm and the Local Arm are also delineated by many bright HII regions. The arm segments traced by massive star forming regions and GMCs are able to match the HI arms in the outer Galaxy. We found that the models of three-arm and four-arm logarithmic spirals are able to connect most spiral tracers. A model of polynomial-logarithmic spirals is also proposed, which not only delineates the tracer distribution, but also matches the observed tangential directions.

astro-ph.GA↗

arXiv2014

A compiled catalog of rotation measures of radio point sources

We compiled a catalog of Faraday rotation measures (RMs) for 4553 extragalactic radio point sources ublished in literature. These RMs were derived from multi-frequency polarization observations. The RM data are compared to those in the NRAO VLA Sky Survey (NVSS) RM catalog. We reveal a systematic uncertainty of about $10.0 \pm 1.5$\,rad~m$^{-2}$ in the NVSS RM catalog. The Galactic foreground RM is calculated through a weighted averaging method by using the compiled RM catalog together with the NVSS RM catalog, with careful consideration of uncertainties in the RM data. The data from the catalog and the interface for the Galactic foreground RM calculations are publicly available on the webpage: http://zmtt.bao.ac.cn/RM/.

astro-ph.GA↗

arXiv2014

An efficient method to identify galaxy clusters by using SuperCOSMOS, 2MASS and WISE data

The survey data of Wide-field Infrared Survey Explorer (WISE) provide an opportunity for the identification of galaxy clusters. We present an efficient method for detecting galaxy clusters by combining the WISE data with SuperCOSMOS and 2MASS data. After performing star-galaxy separation, we calculate the number of companion galaxies around the galaxies with photometric redshifts previously estimated by the SuperCOSMOS, 2MASS and WISE data. A scaled richness is then defined to identify clusters. From a sky area of 275 deg^2 coincident with Sloan Digital Sky Survey Stripe 82 region, we identify 302 clusters in the redshift range of 0.1<z<0.35, 247 (82%) of which are previously known SDSS clusters. The results indicate that our method is efficient for identifying galaxy clusters by using the all sky data of the SuperCOSMOS, 2MASS and WISE.

astro-ph.GA↗

arXiv2014

Discovery of a new supernova remnant G150.3+4.5

Large-scale radio continuum surveys have good potential for discovering new Galactic supernova remnants (SNRs). Surveys of the Galactic plane are often limited in the Galactic latitude of |b| ~ 5 degree. SNRs at high latitudes, such as the Cygnus Loop or CTA~1, cannot be detected by surveys in such limited latitudes. Using the available Urumqi 6 cm Galactic plane survey data, together with the maps from the extended ongoing 6 cm medium latitude survey, we wish to discover new SNRs in a large sky area. We searched for shell-like structures and calculated radio spectra using the Urumqi 6 cm, Effelsberg 11 cm, and 21 cm survey data. Radio polarized emission and evidence in other wavelengths are also examined for the characteristics of SNRs. We discover an enclosed oval-shaped object G150.3+4.5 in the 6 cm survey map. It is about 2.5 degree wide and 3 degree high. Parts of the shell structures can be identified well in the 11 cm, 21 cm, and 73.5 cm observations. The Effelsberg 21 cm total intensity image resembles most of the structures of G150.3+4.5 seen at 6 cm, but the loop is not closed in the northwest. High resolution images at 21 cm and 73.5 cm from the Canadian Galactic Plane Survey confirm the extended emission from the eastern and western shells of G150.3+4.5. We calculated the radio continuum spectral indices of the eastern and western shells, which are $β\sim -2.4$ and $β\sim -2.7$ between 6 cm and 21 cm, respectively. The shell-like structures and their non-thermal nature strongly suggest that G150.3+4.5 is a shell-type SNR. For other objects in the field of view, G151.4+3.0 and G151.2+2.6, we confirm that the shell-like structure G151.4+3.0 very likely has a SNR origin, while the circular-shaped G151.2+2.6 is an HII region with a flat radio spectrum, associated with optical filamentary structure, H$α$, and infrared emission.

astro-ph.GA↗

arXiv2014

Radio polarization observations of large supernova remnants at 6cm

We have observed 79 supernova remnants (SNRs) with the Urumqi 25m telescope at 6cm during the Sino-German 6cm polarization survey of the Galactic plane. We measured flux densities of SNRs at 6cm, some of which are the first ever measured or the measurements at the highest frequency, so that we can determine or improve spectra of SNRs. Our observations have ruled out spectral breaks or spectral flattening that were suggested for a few SNRs, and confirmed the spectral break of S147. By combining our 6cm maps with 11cm and 21cm maps from the Effelsberg 100 m telescope, we calculated the spectral index maps of several large SNRs. For many remnants we obtained for the first time polarization images, which show the intrinsic magnetic field structures at 6cm. We disapproved three objects as being SNRs, OA184, G192.8-1.1 and G16.8-1.1, which show a thermal spectrum and no polarization. We have discovered two large supernova remnants, G178.2-4.2 and G25.1-2.3., in the survey maps.

astro-ph.GA↗

arXiv2014

Redshift evolution of extragalactic rotation measures

We obtained rotation measures of 2642 quasars by cross-identification of the most updated quasar catalog and rotation measure catalog. After discounting the foreground Galactic Faraday rotation of the Milky Way, we get the residual rotation measure (RRM) of these quasars. We carefully discarded the effects from measurement and systematical uncertainties of RRMs as well as large RRMs from outliers, and get marginal evidence for the redshift evolution of real dispersion of RRMs which steady increases to 10 rad m$^{-2}$ from $z=0$ to $z\sim1$ and is saturated around the value at higher redshifts. The ionized clouds in the form of galaxy, galaxy clusters or cosmological filaments could produce the observed RRM evolutions with different dispersion width. However current data sets can not constrain the contributions from galaxy halos and cosmic webs. Future RM measurements for a large sample of quasars with high precision are desired to disentangle these different contributions.

astro-ph.CO↗

arXiv2014

Polarized Curvature Radiation in Pulsar Magnetosphere

The propagation of polarized emission in pulsar magnetosphere is investigated in this paper. The polarized waves are generated through curvature radiation from the relativistic particles streaming along curved magnetic field lines and co-rotating with the pulsar magnetosphere. Within the 1/° emission cone, the waves can be divided into two natural wave mode components, the ordinary (O) mode and the extraord nary (X) mode, with comparable intensities. Both components propagate separately in magnetosphere, and are aligned within the cone by adiabatic walking. The refraction of O-mode makes the two components separated and incoherent. The detectable emission at a given height and a given rotation phase consists of incoherent X-mode and O-mode components coming from discrete emission regions. For four particle-density models in the form of uniformity, cone, core and patches, we calculate the intensities for each mode numerically within the entire pulsar beam. If the co-rotation of relativistic particles with magnetosphere is not considered, the intensity distributions for the X-mode and O-mode components are quite similar within the pulsar beam, which causes serious depolarization. However, if the co-rotation of relativistic particles is considered, the intensity distributions of the two modes are very different, and the net polarization of out-coming emission should be significant. Our numerical results are compared with observations, and can naturally explain the orthogonal polarization modes of some pulsars. Strong linear polarizations of some parts of pulsar profile can be reproduced by curvature radiation and subsequent propagation effect.

astro-ph.HE↗

arXiv2014

Braking PSR J1734-3333 with a possible fall-back disk

The very small braking index of PSR J1734-3333, $n=0.9\pm0.2$, challenges the current theories of braking mechanisms in pulsars. We present a possible interpretation that this pulsar is surrounded by a fall-back disk and braked by it. A modified braking torque is proposed based on the competition between the magnetic energy density of a pulsar and the kinetic energy density of a fall-back disk. With this torque, a self-similar disk can fit all the observed parameters of PSR J1734-3333 with natural initial parameters. In this regime, the star will evolve to the region having anomalous X-ray pulsars and soft gamma repeaters in the $P-\dot{P}$ diagram in about 20000 years and stay there for a very long time. The mass of the disk around PSR J1734-3333 in our model is about $10M_{\oplus}$, similar to the observed mass of the disk around AXP 4U 0142+61.

astro-ph.HE↗

arXiv2013

A Sino-German 6cm polarisation survey of the Galactic plane - VIII. Small-diameter sources

Information of small-diameter sources is extracted from the Sino-German 6cm polarisation survey of the Galactic plane carried out with the Urumqi 25-m telescope. We performed two-dimensional elliptical Gaussian fits to the 6cm maps to obtain a list of sources with total-intensity and polarised flux densities. The source list contains 3832 sources with a fitted diameter smaller than 16 arcmin and a peak flux density exceeding 30 mJy, so about 5 times the rms noise, of the total-intensity data. The cumulative source count indicates completeness for flux densities exceeding about 60 mJy. We identify 125 linearly polarised sources at 6cm with a peak polarisation flux density greater than 10 mJy, so about 3 times the rms noise, of the polarised-intensity data. Despite lacking compact steep spectrum sources, the 6cm catalogue lists about 20 percent more sources than the Effelsberg 21cm source catalogue at the same angular resolution and for the same area. Most of the faint 6cm sources must have a flat spectrum and are either HII regions or extragalactic. When compared with the Green Bank 6cm (GB6) catalogue, we obtain higher flux densities for a number of extended sources with complex structures. Polarised 6cm sources density are uniformly distributed in Galactic latitude. Their number density decreases towards the inner Galaxy. More than 80 percent of the polarised sources are most likely extragalactic. With a few exceptions, the sources have a higher percentage polarisation at 6cm than at 21cm. Depolarisation seems to occur mostly within the sources with a minor contribution from the Galactic foreground emission.

astro-ph.GA↗

arXiv2013

A new method to analyse pulsar nulling phenomenon

Pulsar nulling is a phenomenon of sudden cessation of pulse emission for a number of periods. The nulling fraction was often used to characterize the phenomenon. We propose a new method to analyse pulsar nulling phenomenon, by involving two key parameters, the nulling degree, $χ$, which is defined as the angle in a rectangular coordinates for the numbers of nulling periods and bursting periods, and the nulling scale, $ N $, which is defined as the effective length of the consecutive nulling periods and bursting periods. The nulling degree $χ$ can be calculated by $\tan χ= N_{\rm nulling} / N_{\rm bursting} $ and the mean is related to the nulling fraction, while the nulling scale, $ N $, is also a newly defined fundamental parameter which indicates how often the nulling occurs. We determined the distributions of $χ$ and $ N $ for 10 pulsars by using the data in literature. We found that the nulling degree $χ$ indicates the relative length of nulling to that of bursting, and the nulling scale $ N $ is found to be related to the derivative of rotation frequency and hence the loss rate of rotational energy of pulsars. Their deviations reflect the randomness of the nulling process.

astro-ph.HE↗

arXiv2013

Discovery of eight lensing clusters of galaxies

Clusters of galaxies have a huge mass which can act as gravitational lenses. Galaxies behind clusters can be distorted to form arcs in images by the lenses. Herein a search was done for giant lensed arcs by galaxy clusters using the SDSS data. By visually inspecting SDSS images of newly identified clusters in the SDSS DR8 and Stripe 82 data, we discover 8 strong lensing clusters together with additional 3 probable and 6 possible cases. The lensed arcs show bluer colors than the member galaxies of clusters. The masses and optical luminosities of galaxy clusters interior to the arcs are calculated, and the mass-to-light ratios are found to be in the range of a few tens of M_Solar/L_Solar, consistent with the distribution of previously known lensing clusters.

astro-ph.CO↗

arXiv2013

Substructure and dynamical state of 2092 rich clusters of galaxies derived from photometric data

Dynamical state of galaxy clusters is closely related to their observational properties in X-ray, optical and radio wavelengths. We develop a method to diagnose the substructure and dynamical state of galaxy clusters by using photometric data of Sloan Digital Sky Survey (SDSS). To trace mass distribution, the brightness distribution of member galaxies is smoothed by using a Gaussian kernel with a weight of their optical luminosities. After deriving the asymmetry, the ridge flatness and the normalized deviation of the smoothed optical map, we define a relaxation parameter, Gamma, to quantify dynamical state of clusters. This method is applied to a test sample of 98 clusters of 0.05<z\lesssim0.42 collected from literature with known dynamical states and can recognize dynamical state for relaxed (Gamma\ge0) and unrelaxed (Gamma<0) clusters with a success rate of 94%. We then calculate relaxation parameters of 2092 rich clusters previously identified from the SDSS, of which 28% clusters are dynamically relaxed with Gamma\ge0. We find that the dominance and absolute magnitude of the brightest cluster galaxy closely correlate with dynamical state of clusters. The emission power of radio halos is quantitatively related to cluster dynamical state, beside the known dependence on the X-ray luminosity.

astro-ph.CO↗

arXiv2013

Thermal and non-thermal emission in the Cygnus X region

Radio continuum observations detect non-thermal synchrotron and thermal bremsstrahlung radiation. Separation of the two different emission components is crucial to study the properties of diffuse interstellar medium. The Cygnus X region is one of the most complex areas in the radio sky which contains a number of massive stars and HII regions on the diffuse thermal and non-thermal background. More supernova remnants are expected to be discovered. We aim to develop a method which can properly separate the non-thermal and thermal radio continuum emission and apply it to the Cygnus X region. The result can be used to study the properties of different emission components and search for new supernova remnants in the complex. Multi-frequency radio continuum data from large-scale surveys are used to develop a new component separation method. Spectral analysis is done pixel by pixel for the non-thermal synchrotron emission with a realistic spectral index distribution and a fixed spectral index of beta = -2.1 for the thermal bremsstrahlung emission. With the new method, we separate the non-thermal and thermal components of the Cygnus X region at an angular resolution of 9.5arcmin. The thermal emission component is found to comprise 75% of the total continuum emission at 6cm. Thermal diffuse emission, rather than the discrete HII regions, is found to be the major contributor to the entire thermal budget. A smooth non-thermal emission background of 100 mK Tb is found. We successfully make the large-extent known supernova remnants and the HII regions embedded in the complex standing out, but no new large SNRs brighter than Sigma_1GHz = 3.7 x 10^-21 W m^-2 Hz^-1 sr^-1 are found.

astro-ph.GA↗

arXiv2013

Beam Patterns of the Five-hundred-metre Aperture Spherical Telescope: Optimisation

The Five-hundred-metre Aperture Spherical Telescope (FAST) uses adaptive spherical panels to achieve a huge collecting area for radio waves. In this paper, we try to explore the optimal parameters for the curvature radius of spherical panels and the focal distance by comparison of the calculated beam patterns. We show that to get the best beam shape and maximum gain, the optimal curvature radius of panels is around 300 m, and a small shift in the focal distance of a few cm is needed. The aperture efficiency can be improved by ~10% at 3 GHz by this small shift. We also try to optimise the panel positioning for the best beam, and find that panel shifts of a few mm can improve the beam pattern by a similar extent. Our results indicate that accurate control of the feed and panel positions to the mm level is very crucial for the stability of FAST's observational performance.

astro-ph.IM↗