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X. H. Sun

Publications and source records attributed to X. H. Sun.

At least 37 records · Page 2Linked to original sources
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

Absolutely calibrated radio polarimetry of the inner Galaxy at 2.3 GHz and 4.8 GHz

We present high sensitivity and absolutely calibrated images of diffuse radio polarisation at a resolution of about 10 arcmin covering the range 10 degr <l<34 degr and |b|<5 degr at 2.3 GHz from the S-band Parkes All Sky Survey and at 4.8 GHz from the Sino-German 6 cm polarisation survey of the Galactic plane. Strong depolarisation near the Galactic plane is seen at 2.3 GHz, which correlates with strong Halpha emission. We ascribe the depolarisation to spatial Faraday rotation measure fluctuations of about 65 rad/m2 on scales smaller than 6-9 pc. We argue that most (about 90%) of the polarised emission seen at 4.8 GHz originates from a distance of 3-4 kpc in the Scutum arm and that the random magnetic field dominates the regular field there. A branch extending from the North Polar Spur towards lower latitudes can be identified from the polarisation image at 4.8 GHz but only partly from the polarised image at 2.3 GHz, implying the branch is at a distance larger than 2-3 kpc. We show that comparison of structure functions of complex polarised intensity with those of polarised intensity can indicate whether the observed polarised structures are intrinsic or caused by Faraday screens. The probability distribution function of gradients from the polarisation images at 2.3 GHz indicates the turbulence in the warm ionised medium is transonic.

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arXiv2013

Discovery of supernova remnants in the Sino-German 6cm polarization survey of the Galactic plane

The Sino-German 6cm polarization survey has mapped in total intensity and polarization intensity over an area of approximately 2200 square degrees in the Galactic disk. This survey provides an opportunity to search for Galactic supernova remnants (SNRs) that were previously unknown. We discovered the new SNRs G178.2-4.2 and G25.1-2.3 which have non-thermal spectra, using the 6cm data together with the observations with the Effelsberg telescope at 11 cm and 21 cm. Both G178.2-4.2 and G25.1-2.3 are faint and have an apparent diameter greater than 1deg. G178.2-4.2 shows a polarized shell. HI data suggest that G25.1-2.3 might have a distance of about 3 kpc. The 6cm survey data were also very important to identify two other new SNRs, G152.4-2.1 and G190.9-2.2.

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arXiv2012

Polarisation properties of Milky-Way-like galaxies

(Abridged) We study the polarisation properties, magnetic field strength, and synchrotron emission scale-height of Milky-Way-like galaxies in comparison with other spiral galaxies. We use our 3D-emission model of the Milky Way Galaxy for viewing the Milky Way from outside at various inclinations as spiral galaxies are observed. When seen edge-on the synchrotron emission from the Milky Way has an exponential scale-height of about 0.74 kpc, which is much smaller than the values obtained from previous models. We find that current analysis methods overestimate the scale-height of synchrotron emission of galaxies by about 10% at an inclination of 80 degree and about 40% at an inclination of 70 degree because of contamination from the disk. The observed RMs for face-on galaxies derived from high-frequency polarisation measurements approximate to the Faraday depths (FDs) when scaled by a factor of two. For edge-on galaxies, the observed RMs are indicative of the orientation of the large-scale magnetic field, but are not well related with the FDs. Assuming energy equipartition between the magnetic field and particles for the Milky Way results in an average magnetic-field strength, which is about two times larger than the intrinsic value for a K factor of 100. The number distribution of the integrated polarisation percentages of a large sample of unresolved Milky-Way-like galaxies peaks at about 4.2% at 4.8 GHz and at about 0.8% at 1.4GHz. Integrated polarisation angles rotated by 90 degree align very well with the position angles of the major axes, implying that unresolved galaxies do not have intrinsic RMs.

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arXiv2012

The sino-german 6cm polarization survey of the galactic plane: A summary

We have finished the 6cm polarization survey of the Galactic plane using the Urumqi 25m radio telescope. It covers 10deg<l<230deg in Galactic longitude and |b| <5deg in Galactic latitude. The new polarization maps not only reveal new properties of the diffuse magnetized interstellar medium, but also are very useful for studying individual objects such as Hii regions, which may act as Faraday screens with strong regular magnetic fields inside, and supernova remnants for their polarization properties and spectra. The high sensitivity of the survey enables us to discover two new SNRs G178.2-4.2 and G25.3-2.1 and a number of Hii regions.

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arXiv2011

A Sino-German 6cm polarization survey of the Galactic plane VII. Small supernova remnants

We study the spectral and polarization properties of supernova remnants (SNRs) based on our 6cm survey data. The observations were taken from the Sino-German 6cm polarization survey of the Galactic plane. By using the integrated flux densities at 6cm together with measurements at other wavelengths from the literature we derive the global spectra of 50 SNRs. In addition, we use the observations at 6cm to present the polarization images of 24 SNRs. We derived integrated flux densities at 6cm for 51 small SNRs with angular sizes less than 1 degree. Global radio spectral indices were obtained in all the cases except for Cas A. For SNRs G15.1-1.6, G16.2-2.7, G16.4-0.5, G17.4-2.3, G17.8-2.6, G20.4+0.1, G36.6+2.6, G43.9+1.6, G53.6-2.2, G55.7+3.4, G59.8+1.2, G68.6-1.2, and G113.0+0.2, the spectra have been significantly improved. From our analysis we argue that the object G16.8-1.1 is probably an HII region instead of a SNR. Cas A shows a secular decrease in total intensity, and we measured a flux density of 688+/-35 Jy at 6cm between 2004 and 2008. Polarized emission from 25 SNRs were detected. For G16.2-2.7, G69.7+1.0, G84.2-0.8 and G85.9-0.6, the polarized emission is detected for the first time confirming them as SNRs. High frequency observations of SNRs are rare but important to establish their spectra and trace them in polarization in particular towards the inner Galaxy where Faraday effects are important.

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arXiv2011

New 6cm and 11cm observations of the supernova remnant CTA 1

(Abridged) We conducted new 6cm and 11cm observations of CTA 1 using the Urumqi 25-m and Effelsberg 100-m telescopes. Data at other wavelengths were included to investigate the spectrum and polarisation properties. We obtained new total intensity and polarisation maps at 6cm and 11cm with angular resolutions of 9.5 arcmin and 4.4 arcmin, respectively. We derived a spectral index of alpha=-0.63+/-0.05 based on the integrated flux densities at 408 MHz, 1420 MHz, 2639 MHz, and 4800 MHz. The spectral index map calculated from data at the four frequencies shows a clear steepening of the spectrum from the strong shell emission towards the north-western breakout region with weak diffuse emission. The decrease of the spectral index is up to about 0.3. The RM map derived from polarisation data at 6cm and 11cm shows a sharp transition between positive RMs in the north-eastern and negative RMs in the south-western part of the SNR. We note a corresponding RM pattern of extragalactic sources and propose the existence of a large-diameter Faraday screen in front of CTA 1, which covers the north-eastern part of the SNR. The RM of the Faraday screen is estimated to be about +45 rad/m2. A RM structure function of CTA 1 indicates a very regular magnetic field within the Faraday screen, which is larger than about 2.7 microG in case of 500 pc distance.

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arXiv2011

A Sino-German $λ$6\ cm polarization survey of the Galactic plane VI. Discovery of supernova remnants G178.2-4.2 and G25.1-2.3

Supernova remnants (SNRs) were often discovered in radio surveys of the Galactic plane. Because of the surface-brightness limit of previous surveys, more faint or confused SNRs await discovery. The Sino-German $λ$6\ cm Galactic plane survey is a sensitive survey with the potential to detect new low surface-brightness SNRs. We want to identify new SNRs from the $λ$6\ cm survey map of the Galactic plane. We searched for new shell-like objects in the $λ$6\ cm survey maps, and studied their radio emission, polarization, and spectra using the $λ$6\ cm maps together with the $λ$11\ cm and $λ$21\ cm Effelsberg observations. Extended polarized objects with non-thermal spectra were identified as SNRs. We have discovered two new, large, faint SNRs, G178.2-4.2 and G25.1-2.3, both of which show shell structure. G178.2-4.2 has a size of 72 arcmin x 62 arcmin with strongly polarized emission being detected along its northern shell. The spectrum of G178.2-4.2 is non-thermal, with an integrated spectral index of $α= -0.48\pm0.13$. Its surface brightness is $Σ_{1 GHz} = 7.2 x 10^{-23}{Wm^{-2} Hz^{-1} sr^{-1}}$, which makes G178.2-4.2 the second faintest known Galactic SNR. G25.1-2.3 is revealed by its strong southern shell which has a size of 80 arcmin x 30\arcmin. It has a non-thermal radio spectrum with a spectral index of $α= -0.49\pm0.13$. Two new large shell-type SNRs have been detected at $λ$6\ cm in an area of 2200 deg^2 along the the Galactic plane. This demonstrates that more large and faint SNRs exist, but are very difficult to detect.

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arXiv2011

A Sino-German $λ$6\ cm polarization survey of the Galactic plane. V. Large supernova remnants

Observations of large supernova remnants (SNRs) at high frequencies are rare, but provide valuable information about their physical properties. The total intensity and polarization properties of 16 large SNRs in the Galactic plane were investigated based on observations of the Urumqi $λ$6\ cm polarization survey of the Galactic plane with an angular resolution of 9$\farcm$5. We extracted total intensity and linear polarization maps of large SNRs from the Urumqi $λ$6\ cm survey, obtained their integrated flux densities, and derived the radio spectra in context with previously published flux densities at various frequencies. In particular, Effelsberg $λ$11\ cm and $λ$21\ cm survey data were used for calculating integrated flux densities. The $λ$6\ cm polarization data also delineate the magnetic field structures of the SNRs. We present the first total intensity maps at $λ$6\ cm for SNRs G106.3+2.7, G114.3+0.3, G116.5+1.1, G166.0+4.3 (VRO 42.05.01), G205.5+0.5 (Monoceros Nebula) and G206.9+2.3 (PKS 0646+06) and the first polarization measurements at $λ$6\ cm for SNRs G82.2+5.3 (W63), G106.3+2.7, G114.3+0.3, G116.5+1.1, G166.0+4.3 (VRO 42.05.01), G205.5+0.5 (Monoceros Nebula) and G206.9+2.3 (PKS 0646+06). Most of the newly derived integrated radio spectra are consistent with previous results. The new flux densities obtained from the Urumqi $λ$6\ cm, Effelsberg $λ$11\ cm and $λ$21\ cm surveys are crucial to determine the spectra of SNR G65.1+0.6, G69.0+2.7 (CTB 80), G93.7-0.2 and G114.3+0.3. We find that G192.8$-$1.1 (PKS 0607+17) consists of background sources, \ion{H}{II} regions and the extended diffuse emission of thermal nature, and conclude that G192.8$-$1.1 is not a SNR.

astro-ph.GA↗
arXiv2011

A Sino-German 6cm polarization survey of the Galactic plane IV. The region from 60deg to 129deg longitude

Linear polarization of diffuse Galactic emission is a signature of magnetic fields in the interstellar medium of our Galaxy. Observations at high frequencies are less affected by Faraday depolarization than those at lower frequencies and are able to detect polarized emission from more distant Galactic regions. We attempt to perform a sensitive survey of the polarized emission from the Galactic disk at 6 cm wavelength. We made polarization observations of the Galactic plane using the Urumqi 25-m telescope at 6 cm covering the area of 60deg< l <129deg and |b|<5deg. Missing large-scale structures in polarization were restored by extrapolation of the WMAP polarization data. We present the 6cm total intensity and linear polarization maps of the surveyed region. We identify two new extended HII regions G98.3-1.6 and G119.6+0.4 in this region. Numerous polarized patches and depolarization structures are visible in the polarization maps. Depolarization along the periphery of a few HII complexes was detected and can be explained by a Faraday screen model. We discuss some prominent depolarization HII regions, which have regular magnetic fields of several $μ$G. Structure functions of U, Q and PI images of the entire 6cm survey region of 10deg< l < 230deg exhibit much larger fluctuation power towards the inner Galaxy, which suggests a higher turbulence in the arm regions of the inner Galaxy. The Sino-German 6cm survey reveals new properties of the diffuse magnetized interstellar medium. The survey is also very useful for studying individual objects such as HII regions, which act as Faraday screens and have high rotation measures and therefore strong regular magnetic fields inside the regions.

astro-ph.GA↗
arXiv2010

A Sino-German 6 cm polarization survey of the Galactic plane III. The region from 10 degree to 60 degree longitude

(Abridged) We have conducted a total intensity and polarization survey of the Galactic plane at 6 cm using the Urumqi 25 m telescope for the Galactic longitude range of 10 deg<l<60 deg and the Galactic latitude range of |b|<5 deg. Missing absolute zero levels of Stokes U and Q maps were restored by extrapolating the WMAP five-year K-band polarization data. For total intensities we recovered missing large-scale components by referring to the Effelsberg 11 cm survey. Total intensity and polarization maps are presented with an angular resolution of 9.5 arcmin and a sensitivity of 1 mK and 0.5 mK in total and polarized intensity, respectively. The 6 cm polarized emission in the Galactic plane originates within about 4 kpc distance, which increases for polarized emission out of the plane. The polarization map shows "patches", "canals" and "voids" with no correspondence in total intensity. We attribute the patches to turbulent magnetic field cells. Canals are caused by abrupt variation of polarization angles at the boundaries of patches rather than by foreground Faraday Screens. The superposition of foreground and Faraday Screen rotated background emission almost cancels polarized emission locally, so that polarization voids appear. By modelling the voids, we estimate the Faraday Screen's regular magnetic field along the line-of-sight to be larger than about 8 microG. We separated thermal (free-free) and non-thermal (synchrotron) emission according to their different spectral indices. The spectral index for the synchrotron emission was based on WMAP polarization data. The fraction of thermal emission at 6 cm is about 60% in the plane.

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arXiv2010

Do Ultrahigh Energy Cosmic Rays Come from Active Galactic Nuclei and Fermi $γ$-ray Sources?

We study possible correlations between ultrahigh energy cosmic rays (UHECRs), observed by Auger, AGASA and Yakutsk, and nearby active galactic nuclei (AGNs) and $Fermi$ sources. We consider the deflection effects by a Galactic magnetic field (GMF) model constrained by the most updated measurements. We found that the average deflection angles of UHECRs by the Galactic magnetic fields are less than $4^\circ$. A correlation between the Auger cosmic-ray events and nearby AGNs with a significance level of $\sim 4σ$ was found for the Auger UHECR data sets with or without deflection correction. No correlation was found between the AGASA/Yakutsk events with nearby AGNs. Marginal correlations between the Auger events and the $Fermi$ sources, and between AGASA events and $Fermi$ AGNs were found when the deflections calculated by the GMF model were considered. However, no correlation was found between the Yakutsk data and $Fermi$ sources. Some $Fermi$ sources are close to the arrival directions of UHECR events detected by Auger, AGASA, and Yakutsk, most of which are probably chance coincidence rather than objects producing UHECRs in the nearby Universe. Four $Fermi$ sources, NGC 4945, ESO 323-G77, NGC 6951, and Cen A, within 100~Mpc have UHECR events within $3.1^{\circ}$ from their positions, which could potentially be cosmic ray accelerators. However, the association can only be confirmed if more UHECRs are preferably detected in these directions.

astro-ph.HE↗
arXiv2010

A Sino-German 6 cm polarization survey of the Galactic plane II. The region from 129 degree to 230 degree longitude

Linearly polarized Galactic synchrotron emission provides valuable information about the properties of the Galactic magnetic field and the interstellar magneto-ionic medium, when Faraday rotation along the line of sight is properly taken into account. We aim to survey the Galactic plane at 6 cm including linear polarization. At such a short wavelength Faraday rotation effects are in general small and the Galactic magnetic field properties can be probed to larger distances than at long wavelengths. The Urumqi 25-m telescope is used for a sensitive 6 cm survey in total and polarized intensities. WMAP K-band (22.8 GHz) polarization data are used to restore the absolute zero-level of the Urumqi U and Q maps by extrapolation. Total intensity and polarization maps are presented for a Galactic plane region of 129 degree < l < 230 degree and |b| < 5 degree in the anti-centre with an angular resolution of 9'5 and an average sensitivity of 0.6 mK and 0.4 mK Tb in total and polarized intensity, respectively. We briefly discuss the properties of some extended Faraday Screens detected in the 6 cm polarization maps. The Sino-German 6 cm polarization survey provides new information about the properties of the magnetic ISM. The survey also adds valuable information for discrete Galactic objects and is in particular suited to detect extended Faraday Screens with large rotation measures hosting strong regular magnetic fields.

astro-ph.GA↗
arXiv2009

Simulated SKA maps from Galactic 3D-emission models

(Abridged) We present maps for various Galactic longitudes and latitudes at 1.4 GHz, which is the frequency where deep SKA surveys are proposed. The maps are about 1.5 deg in size and have an angular resolution of about 1.6 arcsec. We analyse the maps in terms of their probability density functions (PDFs) and structure functions. Total intensity emission is more smooth in the plane than at high latitudes due to the different contributions from the regular and random magnetic field. The high latitude fields show more extended polarized emission and RM structures than those in the plane, where patchy emission structures on very small scales dominate. The RM PDFs in the plane are close to Gaussians, but clearly deviate from that at high latitudes. The RM structure functions show smaller amplitudes and steeper slopes towards high latitudes. These results emerge from the fact that much more turbulent cells are passed through by the line-of-sights in the plane. Although the simulated random magnetic field components distribute in 3D, the magnetic field spectrum extracted from the structure functions of RMs conforms to 2D in the plane and approaches 3D at high latitudes. This is partly related to the outer scale of the turbulent magnetic field, but mainly to the different lengths of the line-of-sights.

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arXiv2008

The radio spectrum and magnetic field structure of SNR HB3

Evidence for a spectral flattening of the supernova remnant (SNR) HB3 (G132.7+1.3) was recently claimed in literature based on previously published total flux density data, and the flattening was further interpreted as the discovery of thermal bremsstrahlung emission in the shell of HB3. A spectral flattening has never been observed from any SNR before. Reliable observations of HB3 at frequencies above 3000 MHz are crucial to confirm such a spectral behaviour. We extracted 4800 MHz total intensity and polarisation data of HB3 from the Sino-German 6 cm polarisation survey of the Galactic plane made with the Urumqi 25 m telescope, and analysed the spectrum of HB3, together with Effelsberg data at 1408 MHz and 2675 MHz. We found an overall spectral index of HB3 of alpha=-0.61+-0.06 between 1408 MHz and 4800 MHz, similar to the index at lower frequencies. There is no spectral flattening at high frequencies. We detected strong polarised emission from HB3 at 4800 MHz. Our 4800 MHz data show a tangential field orientation in the HB3 shell.

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arXiv2007

Radio observational constraints on Galactic 3D-emission models

(Abridged) We constrain simulated all-sky maps in total intensity, linear polarization, and rotation measure (RM) by observations. We test a number of large-scale magnetic field configurations and take the properties of the warm interstellar medium into account. From a comparison of simulated and observed maps we are able to constrain the regular large-scale Galactic magnetic field in the disk and the halo of the Galaxy. The local regular field is 2 microG and the average random field is about 3 microG. The known local excess of synchrotron emission originating either from enhanced CR electrons or random magnetic fields is able to explain the observed high-latitude synchrotron emission. The thermal electron model (NE2001) in conjunction with a proper filling factor accounts for the observed optically thin thermal emission and low frequency absorption by optically thick emission. A coupling factor between thermal electrons and the random magnetic field component is proposed, which in addition to the small filling factor of thermal electrons increases small-scale RM fluctuations and thus accounts for the observed depolarization at 1.4 GHz. We conclude that an axisymmetric magnetic disk field configuration with reversals inside the solar circle fits available observations best. Out of the plane a strong toroidal magnetic field with different signs above and below the plane is needed to account for the observed high-latitude RMs. Our preferred 3D-model fits the observations better than other models over a wide frequency range.

astro-ph↗
arXiv2007

Polarization observations of SNR G156.2+5.7 at lambda 6cm

G156.2+5.7 is a large supernova remnant (SNR) first discovered in the ROSAT X-ray survey but radio observations are rare because of its low surface brightness and large size. Radio continuum and polarization images of the SNR G156.2+5.7 at lambda 6 cm are presented for the first time to be discussed in the context with available data to investigate its physical properties. We obtained an integrated flux density for G156.2+5.7 of S6cm= 2.5+-0.5Jy. The spectral index of the integrated emission is alpha=-0.48+-0.08 between lambda 74 cm and 6 cm excluding compact radio sources. We also obtained a spectral index map, which shows little variation between the shells and the central area of the SNR. This is consistent with the constant integrated flux density spectrum. Highly polarized radio emission has been detected from the SNR shell, but also from a central patch, which probably originates in the front part of the spherical SNR shell. We derived the distribution of rotation measure from polarization data at 11 cm and 6 cm and found RM gradients of opposite direction in the SNR shell. The SNR G156.2+5.7 is unusual by its exceptionally high X-ray brightness and very low surface-brightness in the radio range. The magnetic field is very well ordered along the shell periphery as expected for a compressed ambient magnetic field. A toroidal magnetic field component is indicated by the RM distribution. (abridged)

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