arXiv subjects

Qiang Yuan

Publications and source records attributed to Qiang Yuan.

At least 181 records · Page 10Linked to original sources

arXiv2014

Implications of the AMS-02 positron fraction in cosmic rays

The AMS-02 collaboration has just released its first result of the cosmic positron fraction $e^+/(e^-+e^+)$ with high precision up to $\sim 350$ GeV. The AMS-02 result shows the same trend with the previous PAMELA result, which requires extra electron/positron sources on top of the conventional cosmic ray background, either from astrophysical sources or from dark matter annihilation/decay. In this paper we try to figure out the nature of the extra sources by fitting to the AMS-02 $e^+/(e^-+e^+)$ data, as well as the electron and proton spectra by PAMELA and the $(e^-+e^+)$ spectrum by Fermi and HESS. We adopt the GALPROP package to calculate the propagation of the Galactic cosmic rays and the Markov Chain Monte Carlo sampler to do the fit. We find that the AMS-02 data have implied essential difference from the PAMELA data. There is {\rm tension} between the AMS-02 $e^+/(e^-+e^+)$ data and the Fermi/HESS $(e^-+e^+)$ spectrum, that the AMS-02 data requires less contribution from the extra sources than Fermi/HESS. Then we redo the fit without including the Fermi/HESS data. In this case both the pulsars and dark matter annihilation/decay can explain the AMS-02 data. The pulsar scenario has a soft inject spectrum with the power-law index $\sim 2$, while the dark matter scenario needs $τ^+τ^-$ final state with mass $\sim 600$ GeV and a boost factor $\sim 200$.

astro-ph.HE↗

arXiv2014

Reconcile the AMS-02 positron fraction and Fermi-LAT/HESS total $e^{\pm}$ spectra by the primary electron spectrum hardening

The recently reported positron fraction up to $\sim 350$ GeV by AMS-02 seems to have tension with the total electron/positron spectra detected by Fermi and HESS, for either pulsar or dark matter annihilation/decay scenario as the primary positron sources. In this work we will show that the tension will be removed by an adjustment of the primary electron spectrum. If the primary electron spectrum becomes harder above $\sim50$ GeV, similar as the cosmic ray nuclei spectrum, the AMS-02 positron fraction and Fermi/HESS data can be well fitted by both the pulsar and dark matter models. This result indicates that there should be a common origin of the cosmic ray nuclei and the primary electrons. Furthermore, this study also implies that the properties of the extra sources derived from the fitting to the AMS-02 data should depend on the form of background.

astro-ph.HE↗

arXiv2014

Multi-wavelength study of the MGRO J2019+37

MGRO J2019+37 within the Cygnus region is a bright and extended source revealed by Milagro at 12-35 TeV. This source is almost as bright as Crab Nebula in northern sky, while it is not confirmed by ARGO-YBJ around TeV. Up to now, no obvious counterpart at low energy wavelengths has been found. Hence, MGRO J2019+37 becomes mysterious and its VHE gamma-ray emission mechanism is attractive. In this paper, a brief summary of the multi-wavelength observations from Radio to gamma-ray is presented. All the available data from the XMM-Newton and INTEGRAL at X-ray, and the Fermi-LAT at gamma-ray bands were used to get constraint on its emission flux at low energy wavelengths. Then, its possible counterparts and the VHE emission mechanism are discussed.

astro-ph.HE↗

arXiv2013

Pulsar interpretation for the AMS-02 result

The AMS-02 collaboration has just published a high precision measurement of the cosmic positron fraction $e^+/(e^- + e^+)$, which rises with energy from $\sim 5$ GeV to $\sim 350$ GeV. The result indicates the existence of primary electron/positron sources to account for the positron excess. In this work, we investigate the possibility that the nearby mature pulsars are the primary positron sources. By fitting the data we find that the positrons from a single nearby pulsar, such as Geminga or Monogem, with the spectral index $α\sim 2$ can interpret the AMS-02 result. We also investigate the possibility that high energy positrons are generated by multiple known pulsars in the ATNF catalogue. Such a scenario can also fit the AMS-02 data well. Future precise measurements of fine structures in the positron spectrum would be a support to the pulsar scenario.

astro-ph.HE↗

arXiv2013

A generic method to constrain the dark matter model parameters from Fermi observations of dwarf spheroids

Observation of gamma-rays from dwarf galaxies is an effective way to search for particle dark matter. Using 4-year data of Fermi-LAT observations on a series of Milky Way satellites, we develop a general way to search for the signals from dark matter annihilation in such objects. Instead of giving prior information about the energy spectrum of dark matter annihilation, we bin the Fermi-LAT data into several energy bins and build a likelihood map in the "energy bin - flux" plane. The final likelihood of any spectrum can be easily derived through combining the likelihood of all the energy bins. It gives consistent result with that directly calculated using the Fermi Scientific Tool. This method is very efficient for the study of any specific dark matter models with gamma-rays. We use the new likelihood map with Fermi-LAT 4 year data to fit the parameter space in three representative dark matter models: i) toy dark matter model, ii) effective dark matter operators, and iii) supersymmetric neutralino dark matter.

astro-ph.HE↗

arXiv2013

PAMELA/Fermi-LAT electron cosmic ray spectrum at $\sim$100 GeV: implication for dark matter annihilation signal in accordance with the 130 GeV $γ$-ray line

Recently, a tentative 130 GeV $γ$-ray line signal was identified by quite a few groups. If correct it would constitute a ``smoking gun'' for dark matter annihilations. Interestingly, the spectra of the cosmic ray electrons detected by PAMELA and Fermi-LAT both show tiny wiggle-like structure at $\sim 100$ GeV, which might indicate a weak signal of the annihilation of $\sim 130$ GeV dark matter particles into electrons/positrons with a velocity-weighted cross section $\langleσv\rangle_{\rm χχ\rightarrow e^{+}e^{-}} \sim 4\times10^{-26}~{\rm cm^{3}~s^{-1}}$. The prospect of identifying such a potential weak dark-matter-annihilation electron and/or positron component by AMS-02, a mission to measure the high energy cosmic ray spectra with unprecedented accuracy, is investigated.

astro-ph.HE↗

arXiv2013

Emitting electrons spectra and acceleration processes in the jet of Mrk 421: from low state to giant flare state

We investigate the electron energy distributions (EEDs) and the acceleration processes in the jet of Mrk 421 through fitting the spectral energy distributions (SEDs) in different active states in the frame of a one-zone synchrotron self-Compton (SSC) model. After assuming two possible EEDs formed in different acceleration models: the shock accelerated power-law with exponential cut-off (PLC) EED and the stochastic turbulence accelerated log-parabolic (LP) EED, we fit the observed SEDs of Mrk 421 in both low and giant flare states by using the Markov Chain Monte Carlo (MCMC) method which constrains the model parameters in a more efficient way. Our calculating results indicate that (1) the PLC and LP models give comparably good fits for the SED in low state, but the variations of model parameters from low state to flaring can be reasonably explained only in the case of the PLC in low state; and (2) the LP model gives better fits compared to the PLC model for the SED in flare state, and the intra-day/night variability observed at GeV-TeV bands can be accommodated only in the LP model. The giant flare may be attributed to the stochastic turbulence re-acceleration of the shock accelerated electrons in low state. Therefore, we may conclude that shock acceleration is dominant in low state, while stochastic turbulence acceleration is dominant in flare state. Moreover, our result shows that the extrapolated TeV spectra from the best-fit SEDs from optical through GeV with the two EEDs are different. It should be considered in caution when such extrapolated TeV spectra are used to constrain extragalactic background light (EBL) models.

astro-ph.HE↗

arXiv2013

On the Galactic Center Being the Main Source of Galactic Cosmic Rays as Evidenced by Recent Cosmic Ray and Gamma Ray Observations

We revisit the idea that the Galactic center (GC) is the dominant source of Galactic cosmic rays (GCRs), based on a series of new observational evidence. A unified model is proposed to explain the new phenomena of GCRs and $γ$-rays simultaneously. The GCRs are thought to be accelerated during past activities of the GC. The pair production process of GCRs in the strong radiation field due to the GC activity is responsible for the knee structure of the cosmic ray spectra. A f raction of $e^+e^-$ produced by pair production interactions, can be reaccelerated in the induced bipolar jets and be transported into the halo, leaving the Fermi $γ$-ray bubbles and WMAP microwave haze as the remnant signal. Finally, the CRs diffuse in the bulge could further interact with the interstellar medium to produce low energy $e^+e^-$. After cooling down, these positrons may annihilate to produce the 511 keV line emission as discovered by INTEGRAL.

astro-ph.HE↗

arXiv2012

Measuring the extragalactic background light from very high energy gamma-ray observations of blazars

The extragalactic background light (EBL) contains important information about stellar and galaxy evolution. It leaves imprint on the very high energy $γ$-ray spectra from sources at cosmological distances due to the process of pair production. In this work we propose to {\em measure} the EBL directly by extracting the collective attenuation effects in a number of $γ$-ray sources at different redshifts. Using a Markov Chain Monte Carlo fitting method, the EBL intensities and the intrinsic spectral parameters of $γ$-ray sources are derived simultaneously. No prior shape of EBL is assumed in the fit. With this method, we can for the first time to derive the spectral shape of the EBL model-independently. Our result shows the expected features predicted by the present EBL models and thus support the understanding of the EBL origin.

astro-ph.HE↗

arXiv2012

Gamma-rays From Warm WIMP Dark Matter Annihilation

The weakly interacting massive particle (WIMP) often serves as a candidate for the cold dark matter, however when produced non-thermally it could behave like warm dark matter. In this paper we study the properties of the $γ$-ray emission from annihilation of WIMP dark matter in the halo of our own Milky-Way Galaxy with high resolution $N$-body simulations of a Milky-Way like dark matter halo, assuming different nature of WIMPs. Due to the large free-streaming length in the scenario of warm WIMPs, the substructure contend of the dark matter halo is significantly different from that of the cold WIMP counterpart, resulting in distinct predictions of the $γ$-ray signals from the dark matter annihilation. We illustrate these by comparing the predicted $γ$-ray signals from the warm WIMP annihilation to that of cold WIMPs. Pronounced differences from the subhalo skymap and statistical properties between two WIMP models are demonstrated. Due to the potentially enhanced cross section of the non-thermal production mechanism in warm WIMP scenario, the Galactic center might be prior for the indirect detection of warm WIMPs to dwarf galaxies, which might be different from the cold dark matter scenario. As a specific example we consider the non-thermally produced neutralino of supersymmetric model and discuss the detectability of warm WIMPs with Fermi $γ$-ray telescope.

astro-ph.HE↗

arXiv2012

Constraints on the dark matter annihilation scenario of Fermi 130 GeV $γ$-ray line emission by continuous gamma-rays, Milky Way halo, galaxy clusters and dwarf galaxies observations

It was recently reported that there may exist monochromatic $γ$-ray emission at $\sim 130$ GeV from the Galactic center in the Fermi Large Area Telescope data, which might be related with dark matter (DM) annihilation. In this work we carry out a comprehensive check of consistency of the results with the DM annihilation scenario, using the 3.7 yrs Fermi observation of the inner Galaxy, Galactic halo, clusters of galaxies and dwarf galaxies. The results found are as follows. 1) Very strong constraints on the DM annihilation into continuous $γ$-rays from the Galactic center are set, which are as stringent as the "natural" scale assuming thermal freeze-out of DM. Such limit sets strong constraint on the DM models to explain the line emission. 2) No line emission from the Galactic halo is found in the Fermi data, and the constraints on line emission is marginally consistent with the DM annihilation interpretation of the $\sim 130$ GeV line emission from the inner Galaxy. 3) No line emission from galaxy clusters and dwarf galaxies is detected, although possible concentration of photons from clusters in 120-140 GeV is revealed. The constraints from clusters and dwarf galaxies are weak and consistent with the DM annihilation scenario to explain the $\sim 130$ GeV line emission.

astro-ph.HE↗

arXiv2012

Unified model for the gamma-ray emission of supernova remnants

Shocks of supernova remnants (SNRs) are important (and perhaps the dominant) agents for production of the Galactic cosmic rays. Recent $γ$-ray observations of several SNRs have made this case more compelling. However, these broadband high-energy measurements also reveal a variety of spectral shape demanding more comprehensive modeling of emissions from SNRs. According to the locally observed fluxes of cosmic ray protons and electrons, the electron-to-proton number ratio is known to be about 1%. Assuming such a ratio is universal for all SNRs and identical spectral shape for all kinds of accelerated particles, we propose a unified model that ascribes the distinct $γ$-ray spectra of different SNRs to variations of the medium density and the spectral difference between cosmic ray electrons and protons observed at Earth to transport effects. For low density environments, the $γ$-ray emission is inverse-Compton dominated. For high density environments like systems of high-energy particles interacting with molecular clouds, the $γ$-ray emission is $π^0$-decay dominated. The model predicts a hadronic origin of $γ$-ray emission from very old remnants interacting mostly with molecular clouds and a leptonic origin for intermediate age remnants whose shocks propagate in a low density environment created by their progenitors via e.g., strong stellar winds. These results can be regarded as evidence in support of the SNR-origin of the Galactic cosmic rays.

astro-ph.HE↗

arXiv2012

Testing the 130 GeV gamma-ray line with high energy resolution detectors

Recently some hints of the existence of $γ$-ray line around 130 GeV are reported according to the analysis of Fermi-LAT data. If confirmed it would be the first direct evidence to show the existence of new physics beyond the standard model. Here we suggest that using the forthcoming high energy resolution $γ$-ray detectors, such as CALET and DAMPE, we may test whether it is real line structure or just the background effect. For DAMPE like detector with designed energy resolution $\sim1.5%$, a line significance will reach $11σ$ for the same statistics as Fermi-LAT. For about 1.4 yr survey observation, DAMPE may detect a $5σ$ signal of such a $γ$-ray line.

astro-ph.HE↗

arXiv2012

Statistical interpretation of the spatial distribution of current 130 GeV gamma-ray line signal within the dark matter annihilation scenario

Recently, several groups identified a tentative $γ$-ray line signal with energy $\sim 130$ GeV in the central Galaxy from the Fermi-LAT data. %The morphology study shows that the signal is consistent with dark matter %annihilation, but with an offset $\sim 220$ pc ($1.5^{\circ}$) of the %center from the Galactic center Sgr A$^{\star}$, Such a $γ-$ray line can be interpreted as the signal of dark matter annihilation. However, the offset $\sim 220$ pc ($1.5^{\circ}$) of the center of the most prominent signal region from the Galactic center Sgr A$^{\star}$ has been thought to challenge the dark matter annihilation interpretation. Considering the fact that such a 130 GeV $γ$-ray line signal consists of only $\sim14$ photons, we suggest that the "imperfect" consistency of these photons with the expected dark matter distribution is due to the limited statistics. The offset will be smaller as more signal photons have been collected in the near future. Our Monte Carlo simulation supports the above speculation.

astro-ph.CO↗

arXiv2012

Search for dark matter signals with Fermi-LAT observation of globular clusters NGC 6388 and M 15

The globular clusters are probably good targets for dark matter (DM) searches in $γ$-rays due to the possible adiabatic contraction of DM by baryons. In this work we analyse the three-year data collected by {\it Fermi} Large Area Telescope of globular clusters NGC 6388 and M 15 to search for possible DM signals. For NGC 6388 the detection of $γ$-ray emission was reported by {\it Fermi} collaboration, which is consistent with the emission of a population of millisecond pulsars. The spectral shape of NGC 6388 is also shown to be consistent with a DM contribution if assuming the annihilation final state is $b\bar{b}$. No significant $γ$-ray emission from M 15 is observed. We give the upper limits of DM contribution to $γ$-ray emission in both NGC 6388 and M 15, for annihilation final states $b\bar{b}$, $W^+W^-$, $μ^+μ^-$, $τ^+τ^-$ and monochromatic line. The constraints are stronger than that derived from observation of dwarf galaxies by {\it Fermi}.

astro-ph.HE↗

arXiv2012

Neutrino emission from dark matter annihilation/decay in light of cosmic $e^{\pm}$ and $\bar{p}$ data

A self-consistent global fitting method based on the Markov Chain Monte Carlo technique to study the dark matter (DM) property associated with the cosmic ray electron/positron excesses was developed in our previous work. In this work we further improve the previous study to include the hadronic branching ratio of DM annihilation/decay. The PAMELA $\bar{p}/p$ data are employed to constrain the hadronic branching ratio. We find that the 95% ($2σ$) upper limits of the quark branching ratio allowed by the PAMELA $\bar{p}/p$ data is $\sim 0.032$ for DM annihilation and $\sim 0.044$ for DM decay respectively. This result shows that the DM coupling to pure leptons is indeed favored by the current data. Based on the global fitting results, we further study the neutrino emission from DM in the Galactic center. Our predicted neutrino flux is some smaller than previous works since the constraint from $γ$-rays is involved. However, it is still capable to be detected by the forth-coming neutrino detector such as IceCube. The improved points of the present study compared with previous works include: 1) the DM parameters, both the particle physical ones and astrophysical ones, are derived in a global fitting way, 2) constraints from various species of data sets, including $γ$-rays and antiprotons are included, and 3) the expectation of neutrino emission is fully self-consistent.

astro-ph.CO↗

arXiv2012

CosRayMC: a global fitting method in studying the properties of the new sources of cosmic e$^{\pm}$ excesses

Recently PAMELA collaboration published the cosmic nuclei and electron spectra with high precision, together with the cosmic antiproton data updated, and the Fermi-LAT collaboration also updated the measurement of the total $e^+e^-$ spectrum to lower energies. In this paper we develop a Markov Chain Monte Carlo (MCMC) package {\it CosRayMC}, based on the GALPROP cosmic ray propagation model to study the implications of these new data. It is found that if only the background electrons and secondary positrons are considered, the fit is very bad with $χ_{\rm red}^2 \approx 3.68$. Taking into account the extra $e^+e^-$ sources of pulsars or dark matter annihilation we can give much better fit to these data, with the minimum $χ_{\rm red}^2 \approx 0.83$. This means the extra sources are necessary with a very high significance in order to fit the data. However, the data show little difference between pulsar and dark matter scenarios. Both the background and extra source parameters are well constrained with this MCMC method. Including the antiproton data, we further constrain the branching ratio of dark matter annihilation into quarks $B_q<0.5%$ at $2σ$ confidence level. The possible systematical uncertainties of the present study are discussed.

astro-ph.CO↗

arXiv2011

Constraints and tests of the OPERA superluminal neutrinos

The superluminal neutrinos detected by OPERA indicates Lorentz invariance violation (LIV) of the neutrino sector at the order of $10^{-5}$. We study the implications of the result in this work. We find that such a large LIV implied by OPERA data will make the neutrino production process $π\to μ+ ν_μ$ kinematically forbidden for neutrino energy greater than about 5 GeV. The OPERA detection of neutrinos at 40 GeV can constrain the LIV parameter to be smaller than $3\times 10^{-7}$. Furthermore the neutrino decay in the LIV framework will modify the neutrino spectrum greatly. The atmospheric neutrino spectrum measured by IceCube can constrain the LIV parameter to the level of $10^{-12}$. The future detection of astrophysical neutrinos of Galactic sources is expected to be able to give even stronger constraint on the LIV parameter of neutrinos.

hep-ph↗