Cosmic rays are as mischievous as the Monkey King / 宇宙射线像孙悟空一样恶作剧

A tribute to current Chinese science and technology and a wink to its literary classic The Journey to the West

Thanks to the Chinese satellite Wukong (aka Monkey King), also known as ...

The DArk Matter Particle Explorer (DAMPE), a high energy cosmic ray and γ-ray detector in space, has recently reported the new measurement of the total electron plus positron flux between 25 GeV and 4.6 TeV. A spectral softening at ∼0.9 TeV and a tentative peak at ∼1.4 TeV have been reported. We study the physical implications of the DAMPE data in this work... Both the astrophysical models and the exotic DM annihilation/decay scenarios are examined. Our findings are summarized as follows. 

• The spectral softening at ∼ 0.9 TeV suggests a cutoff (or break) of the background electron spectrum, which is expected to be due to either the discretness of cosmic rays (CR) source distributions in both space and time, or the maximum energies of electron acceleration at the sources. The DAMPE data enables a much improved determination of the cutoff energy of the background electron spectrum, which is about 3 TeV assuming an exponential form, compared with the pre-DAMPE data. 

• Both the annihilation and decay scenarios of the simplified DM models to account for the sub-TeV electron/positron excesses are severely constrained by the CMB and/or γ-ray observations. Additional tuning of such models, through e.g., velocity-dependent annihilation, is required to reconcile with those constraints. 

• The tentative peak at ∼ 1.4 TeV suggested by DAMPE implies that the sources should be close enough to the Earth (.0.3 kpc) and inject nearly monochromatic electrons into the Galaxy. We find that the cold and ultra-relativistic e⁺e⁻ wind from pulsars is a possible source of such a structure. Our analysis further shows that the pulsar should be middle-aged, relatively slowlyrotated, mildly magnetized, and isolate in a density cavity (e.g., the local bubble).

• An alternative explanation of the peak is the DM annihilation in a nearby clump or a local density enhanced region. The distance of the clump or size of the overdensity region needs to be .0.3 kpc. The required parameters of the DM clump or over-density are relatively extreme compared with that of numerical simulations, if the annihilation cross section is assumed to be 3×10⁻²⁶ cm³ s⁻¹ . Specifically, a DM clump as massive as 10⁷−10⁸ M☉ or a local density enhancement of 17 − 35 times of the canonical local density is required to fit the data if the annihilation product is a pair of e⁺e⁻ . Moderate enhancement of the annihilation cross section would be helpful to relax the tension between the model requirement and the N-body simulations of the CDM structure formation. The DM clump model or local density enhancement model is found to be consistent with the Fermi-LAT γ-ray observations. 

• The expected anisotropies from either the pulsar model or the DM clump model are consistent with the recent measurements by Fermi-LAT. Future observations by e.g., CTA, will be able to detect such anisotropies and test different models. 

DAMPE will keep on operating for a few more years. More precise measurements of the total e⁺+e⁻ spectrum extending to higher energies are available in the near future. Whether there are more structures in the high energy window, which can critically distinguish the pulsar model from the DM one, is particularly interesting. With more and more precise measurements, we expect to significantly improve our understandings of the origin of CR electrons.

The total e⁺+e⁻ fluxes (right) for a model with two nearby pulsars

Fluxes of the total e⁺+e⁻  , from the sum of the continuous background and the DM annihilation from a nearby clump. This panel is for DM annihilation into all flavor leptons with universal couplings. Three distances of the clump, as labelled in the plot, are considered.

Interpretations of the DAMPE electron data

Qiang Yuan, Lei Feng, Peng-Fei Yin, Yi-Zhong Fan, Xiao-Jun Bi, Ming-Yang Cui, Tie-Kuang Dong, Yi-Qing Guo, Kun Fang, Hong-Bo Hu, Xiaoyuan Huang, Shi-Jun Lei, Xiang Li, Su-Jie Lin, Hao Liu, Peng-Xiong Ma, Wen-Xi Peng, Rui Qiao, Zhao-Qiang Shen, Meng Su, Yi-Feng Wei, Zun-Lei Xu, Chuan Yue, Jing-Jing Zang, Cun Zhang, Xinmin Zhang, Ya-Peng Zhang, Yong-Jie Zhang, Yun-Long Zhang

(Submitted on 29 Nov 2017)

update 12/06/2017:

If the spectral feature comes from dark matter what can we learn from the former about the latter ?

We performed a model-independent analysis of particle dark matter explanations of the peak in the DAMPE electron spectrum and whether they can simultaneously satisfy constraints from other DM searches. We assumed that the signal originated from DM annihilation in a nearby subhalo with an enhanced density of DM. To account for the inevitable energy loss, we assumed a DM mass of about 1.5 TeV, which is slightly greater than the location of the observed peak. Rather than working in a specific UV-complete model, we investigated all renormalizable interactions between SM leptons, DM of spin 0 and 1/2, and mediators of spin 0 and 1... 

We found that 10 of 20 possible combinations of operators are helicity or velocity suppressed and cannot explain the DAMPE signal. Of the remaining combinations, PandaX strongly constrains the unsuppressed scattering cross sections in three models and LEP strongly constrains the mass of the mediator in the other 7. The remaining candidates are (1) a spin 0 mediator coupled to scalar DM, (2) a spin 0 mediator pseudoscalar coupled to fermionic DM, and (3) a spin 1 mediator vector coupled to Dirac DM. LEP constraints on four-fermion operators force the mediator mass to be heavy, ~2 TeV, in all of these scenarios.

Model-independent analysis of the DAMPE excess

Peter Athron, Csaba Balazs, Andrew Fowlie, Yang Zhang

(Submitted on 30 Nov 2017 (v1), last revised 5 Dec 2017 (this version, v2))

O cosmic rays! from where art thou? 

Nearby sources may contribute to cosmic-ray electron (CRE) structures at high energies. Recently, the first DAMPE results on the CRE flux hinted at a narrow excess at energy ~1.4 TeV. We show that in general a spectral structure with a narrow width appears in two scenarios: I) "Spectrum broadening" for the continuous sources with a delta-function-like injection spectrum. In this scenario, a finite width can develop after propagation through the Galaxy, which can reveal the distance of the source. Well-motivated sources include mini-spikes and subhalos formed by dark matter (DM) particles χs which annihilate directly into e+e- pairs. II) "Phase-space shrinking" for burst-like sources with a power-law-like injection spectrum. The spectrum after propagation can shrink at a cooling-related cutoff energy and form a sharp spectral peak. The peak can be more prominent due to the energy-dependent diffusion. In this scenario, the width of the excess constrains both the power index and the distance of the source. Possible such sources are pulsar wind nebulae (PWNe) and supernova remnants (SNRs). We analysis the DAMPE excess and find that the continuous DM sources should be fairly close within ~0.3 kpc, and the annihilation cross sections are close to the thermal value. For the burst-like source, the narrow width of the excess suggests that the injection spectrum must be hard with power index significantly less than two, the distance is within ~(3-4) kpc, and the age of the source is ~0.16 Myr. In both scenarios, large anisotropies in the CRE flux are predicted. We identify possible candidates of mini-spike (PWN) sources in the current Fermi-LAT 3FGL (ATNF) catalog. The diffuse gamma-rays from these sources can be well below the Galactic diffuse gamma-ray backgrounds and less constrained by the Ferm-LAT data, if they are located at the low Galactic latitude regions... 

The current experiments have entered the multi-TeV region where the CRE spectrum is unlikely to be smooth. We have proposed generic scenarios of the origins of the CRE structures and analysed the nature of sources responsible for the possible DAMPE excess. The predictions of these scenarios are highly testable in the near future with more accurate data.

Origins of sharp cosmic-ray electron structures and the DAMPE excess

Xian-Jun Huang, Yue-Liang Wu, Wei-Hong Zhang, Yu-Feng Zhou

(Submitted on 30 Nov 2017)