arXiv subjects

Rui-zhi Yang

Publications and source records attributed to Rui-zhi Yang.

31 records · Page 2Linked to original sources

arXiv2016

On the energy distribution of relativistic electrons in the young supernova remnant G1.9+0.3

The broad-band X-ray observations of the youngest known galactic supernova remnant, G1.9+0.3, provide unique information about the particle acceleration at the early stages of evolution of supernova remnants. Based on the publicly available X-ray data obtained with the Chandra and NuSTAR satellites over two decades in energy, we derived the energy distribution of relativistic electrons under the assumption that detected X-rays are of entirely synchrotron origin. The acceleration of electrons was found to be an order of magnitude slower than the maximum rate provided by the shock acceleration in the nominal Bohm diffusion regime. We discuss the implications of this result in the context of contribution of SNRs to the Galactic Cosmic Rays at PeV energies.

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arXiv2016

Detection of diffuse gamma-ray emission near the young massive cluster NGC 3603

We report the Fermi Large Area Telescope's detection of extended gamma-ray emission towards the direction of the young massive star cluster NGC 3603. The emission shows a hard spectrum with a photon index of 2.3 from 1 GeV to 250 GeV. The large size and high luminosity of this structure make it unlikely a pulsar wind nebular. On the other hand the spatial correlation with the ionised gas indicate a hadronic origin. The total cosmic ray (CR) protons energy are estimated to be of the order $10^{50} ~\rm erg$ assuming the gamma-ray are produced in the interaction of CRs with ambient gas . The environment and spectral features show significant similarity with the Cygnus cocoon. It reveals that the young star clusters may be a gamma-ray source population and they can potentially accelerate a significant fraction of the Galactic cosmic rays.

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arXiv2016

Giant lobes of Centaurus A as seen in radio and gamma-ray images obtained with the Fermi-LAT and Planck satellites

The γ-ray data of Fermi-LAT on the giant lobes of Centaurus A are analysed together with the high frequency radio data obtained with the Planck satellite. The large γ-ray photon statistics, accumulated during seven years of observations, and the recently updated Fermi collaboration software tools allow substantial extension of the detected γ-ray emission towards higher energy, up to 30 GeV, and lower energy, down to 60 MeV. Moreover, the new γ-ray data allow us to explore the spatial features of γ-ray emission of the lobes. For the north lobe, we confirm, with higher statistical significance, our earlier finding on the extension of γ-ray emission beyond the radio image. Moreover, the new analysis reveals significant spatial variation of γ-ray spectra from both lobes. On the other hand, the Planck observations at microwave frequencies contain important information on spectra of synchrotron emission in the cutoff region, and thus allow model-independent derivation of the strength of the magnetic field and the distribution of relativistic electrons based on the combined γ-ray and radio data. The interpretation of multiwavelength spectral energy distributions (SEDs) of the lobes within a pure leptonic model requires strong enhancement of the magnetic field at the edge of the south lobe. Alternatively, a more complex, leptonic-hadronic model of the γ-ray emission, postulating a non-negligible contribution of the π^0 -decay component at highest energies, can explain the γ-ray data with a rather homogeneous distribution of the magnetic field over the giant lobes.

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arXiv2016

Investigation of cosmic ray penetration with wavelet cross-correlation analysis

Aims. We use Fermi and Planck data to calculate the cross correlation between gamma ray signal and gas distribution in different scales in giant molecular clouds (GMC). Then we investigate the cosmic rays (CRs) penetration in GMCs with these informations. Methods.We use the wavelet technique to decompose both the gamma ray and dust opacity maps in different scales, then we calculate the wavelet cross correlation functions in these scales. We also define wavelet response as an analog to the impulsive response in Fourier transform and calculate that in different scales down to Fermi angular resolution. Results. The gamma ray maps above 2 GeV show strong correlation with the dust opacity maps, the correlation coefficient is larger than 0.9 above a scale of 0.4 degree.The derived wavelet response is uniform in different scales. Conclusions. We argue that the CR above 10 GeV can penetrate the giant molecular cloud freely and the CRs distributions in the same energy range are uniform down to parsec scale.

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arXiv2016

On the GeV excess in the diffuse γ-ray emission towards the Galactic Center

The Fermi-LAT γ-ray data have been used to study the morphological and spectral features of the so-called GeV excess - a diffuse radiation component recently discovered towards the Galactic centre. We used the likelihood method to analyze Fermi-LAT data. Our study does confirm the existence of such an extra component in the diffuse γ-ray emission at GeV energies. Based on a detailed morphological analysis, a spatial template that fits the data best was generated and adopted. Using this template, the energy distribution of γ-rays was derived in the 0.3-30 GeV energy interval. The spectrum appeared to have less distinct ('bump'-like) structure than previous reported. We argue that the morphology of this radiation component has a bipolar rather than a spherically symmetric structure as has been assumed a priori in previous studies. This finding excludes the associations of the GeV excess with Dark Matter. We briefly discuss the radiation mechanisms and possible source populations that could be responsible for this new component of diffuse gamma radiation.

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arXiv2016

Radial distribution of the diffuse gamma-ray emissivity in the galactic disk

The Fermi-LAT data accumulated over 7 years of γ-ray observations, together with the high resolution gas (CO & HI) and the dust opacity maps, are used to study the emissivity of γ-rays induced by interactions of cosmic rays (CRs) with the interstellar medium. Based on the dust opacity templates, the γ-ray emissivity was measured for 36 segments of the Galactic plane. Furthermore, the γ-ray emissivity was evaluated in six Galactocentric rings. Both the absolute emissivity and the energy spectra of γ-rays derived in the interval 0.2-100 GeV show significant variations along the galactic plane. The density of CRs, derived under the assumption that γ-rays are predominately produced in CR interactions with the interstellar gas, is characterised by a strong radial dependence. In the inner Galaxy the CR density substantially exceeds the density in the outer parts of the Galaxy: by a factor of a few at 10 GeV, and by more than an order of magnitude at 1 TeV. Remarkably, the energy distribution of CRs appears to be substantially harder than the energy spectrum obtained from direct measurements of local CRs. At the same time, the flux and the energy spectrum of multi-GeV protons derived from γ-ray data in the outskirts of the Galaxy is quite close to the measurements of local CRs.

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arXiv2015

Fermi-LAT observations of the Sagittarius B complex

Aims. We use 5 years of Fermi-LAT data towards the Galactic-centre giant molecular cloud complex, Sagittarius B, to test questions of how well-mixed the Galactic component of cosmic rays are and what the level of the cosmic-ray sea in different parts of the Galaxy is. Methods. We use dust-opacity maps from the Planck satellite to obtain independent methods for background subtraction and an estimate for the mass of the region. We then present high-quality spectra of gamma-ray emission from 0.3 to 30 GeV and obtain an estimate of the cosmic-ray spectrum from the region. Results. We obtain an estimate of the mass of the region of $1.5 \pm 0.2 \times 10^7~\rm M_{\odot}$ using the Planck data, which agrees well with molecular-line-derived estimates for the same region. We find the the gamma-ray flux from this region is fitted well with a cosmic-ray spectrum that is the same as is observed locally, with evidence of a small over-density at intermediate (1-10 GeV) energies. Conclusions. We conclude that the gamma-ray and cosmic-ray spectrum in the region can be well-fitted using a local cosmic-ray spectrum.

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arXiv2014

Probing Cosmic Rays in Nearby Giant Molecular Clouds with the Fermi Large Area Telescope

We report the results of our study of the energy spectra and absolute fluxes of cosmic rays (CRs) in the Local Galaxy based on a five-year $γ$-ray observation with the Fermi Large Area Telescope (LAT) of eight nearby giant molecular clouds (GMCs) belonging to the Gould Belt. The $γ$-ray signals obtained with high statistical significance allow the determination of $γ$-ray spectra above 300~MeV with adequate precision for extraction of the energy distributions of CRs in these clouds. Remarkably, both the derived spectral indices and the absolute fluxes of CR protons in the energy interval $10 - 100~ \rm \ GeV$ agree with the recent direct measurements of local CRs by the PAMELA experiment. This is strong evidence of a quite homogeneous distribution of CRs, at least within several hundred parsecs of the Local Galaxy. Combined with the well established energy-dependent time of escape of CRs from the Galaxy, $τ(E) \propto E^{-δ}$ with $δ\approx 0.5-0.6$, the measured spectrum implies a CR spectral index of the (acceleration) source of $\approx$E$^{-2.3}$. At low energies, the spectra of $γ$ rays appear to vary from one cloud to another. This implies spatial variations of the energy spectra of CRs below 10~GeV, which at such low energies could be explained naturally by both the impact of the propagation effects and the contribution of CR locally accelerated inside the clouds.

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arXiv2014

Fermi Large Area Telescope observations of the supernova remnant HESS J1731-347

Context: HESS J1731-347 has been identified as one of the few TeV-bright shell-type supernova remnants (SNRs). These remnants are dominated by nonthermal emission, and the nature of TeV emission has been continuously debated for nearly a decade. Aims: We carry out the detailed modeling of the radio to gamma-ray spectrum of HESS J1731-347 to constrain the magnetic field and energetic particles sources, which we compare with those of the other TeV-bright shell-type SNRs explored before. Methods: Four years of data from Fermi Large Area Telescope (LAT) observations for regions around this remnant are analyzed, leading to no detection correlated with the source discovered in the TeV band. The Markov Chain Monte Carlo method is used to constrain parameters of one-zone models for the overall emission spectrum. Results: Based on the 99.9% upper limits of fluxes in the GeV range, one-zone hadronic models with an energetic proton spectral slope greater than 1.8 can be ruled out, which favors a leptonic origin for the gamma-ray emission, making this remnant a sibling of the brightest TeV SNR RX J1713.7-3946, the Vela Junior SNR RX J0852.0-4622, and RCW 86. The best-fit leptonic model has an electron spectral slope of 1.8 and a magnetic field of about 30 muG, which is at least a factor of 2 higher than those of RX J1713.7-3946 and RX J0852.0-4622, posing a challenge to the distance estimate and/or the energy equipartition between energetic electrons and the magnetic field of this source. A measurement of the shock speed will address this challenge and has implications on the magnetic field evolution and electron acceleration driven by shocks of SNRs.

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arXiv2014

The Fermi Bubbles Revisited

We analyze 60 months of all sky data from the Fermi-LAT. The Fermi Bubble structures discovered previously are clearly revealed by our analysis. With more data and, consequently, better statistics we can now divide each bubble into constant longitude slices to investigate their gross $γ$-ray spectral morphology. While the detailed spectral behaviour of each slice derived within our analysis is somewhat dependent on the assumed background model, we find, robustly, a relative deficit of the flux at low energies (i.e., hardening) towards the top of the South Bubble. In neither Bubble does the spectrum soften with longitude. The morphology of the Fermi Bubbles is also revealed to be energy dependent: at high energies they are more extended. We conclude from the gamma-ray spectrum at high latitudes that a low energy break in the parent cosmic ray population is required in both leptonic and hadronic models. We briefly discuss possible leptonic and hadronic interpretation of this phenomenology.

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arXiv2013

Deep Observation of the Giant Radio Lobes of Centaurus A with the Fermi Large Area Telescope

The detection of high energy (HE) γ-ray emission up to about 3 GeV from the giant lobes of the radio galaxy Centaurus A has been recently reported by the Fermi-LAT Collaboration based on ten months of all-sky survey observations. A data set more than three times larger is used here to study the morphology and photon spectrum of the lobes with higher statistics. The larger data set results in the detection of HE γ-ray emission (up to about 6 GeV) from the lobes with a significance of more than 10 and 20 σ for the North and the South lobe, respectively. Based on a detailed spatial analysis and comparison with the associated radio lobes, we report evidence for a substantial extension of the HE γ-ray emission beyond the WMAP radio image in the case of the Northern lobe of Cen A. We reconstruct the spectral energy distribution (SED) of the lobes using radio (WMAP) and Fermi-LAT data from the same integration region. The implications are discussed in the context of hadronic and leptonic scenarios.

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arXiv2012

An extended source of GeV gamma rays coincident with the supernova remnant HB 21

We analyze 3.5 years of public Fermi/LAT data around the position of the supernova remnant HB 21, where four point-like sources from the 2nd Fermi/LAT catalog are located. We determine that the gamma-ray source is produced by a single extended source. We model the observed morphology as a uniform circle. The spectral energy distribution is best described by a curved power law, with a maximum at 413+-11 MeV. We divide the circle into three regions defined by previously identified shocked molecular clouds, and find that one of these regions has a softer spectrum. The >3 GeV gamma-ray emission of the soft spectrum region is bow-shaped and coincident with the supernova remnant shell seen at radio wavelengths. We suggest that the gamma-ray emission from HB 21 can be understood as a combination of emission from shocked/illuminated molecular clouds, one of them coincident with the supernova remnant shell itself.

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arXiv2012

Constraining Asymmetric Bosonic Non-interacting Dark Matter with Neutron Stars

The Hawking evaporation of small black holes formed by the collapse of dark matter at the center of neutron stars plays a key role in loosing the constraint on the mass of asymmetric bosonic non-interacting dark matter particles. Different from previous works we show that such a kind of dark matter is viable in the mass range from 3.3 GeV to ~ 10 TeV, which covers the most attractive regions, including the preferred asymmetric dark matter mass ~ 5.7 GeV as well as the 5-15 GeV range favored by DAMA and CoGeNT.

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