分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The delayed GeV$-$TeV cascade emission from extragalactic TeV$-$PeV sources are regarded as an ideal probe of the inter-galactic magnetic fields. Recently, LHAASO has detected $\sim 10$ TeV emission of the extraordinary powerful GRB 221009A within $\sim 2000$ s after the burst. Here we report the detection of a $\sim 400$ GeV photon, without accompanying prominent low-energy emission, by Fermi-LAT in the direction of GRB 221009A with the time delay of 0.4 days. Such a hard spectrum can be generated from electromagnetic cascades initiated by early primary $\sim 10$ TeV photons in the intergalactic space. An inter-galactic magnetic field strength of $B_{\rm IGMF}\sim 4 \times 10^{-17}$ G, comparable to limits from TeV blazars, can naturally account for the arrival time of the $\sim 400$ GeV photon as well as the HAWC non-detection. Such a strength will be stringently tested by the upcoming CTA.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The delayed GeV$-$TeV cascade emission from extragalactic TeV$-$PeV sources are regarded as an ideal probe of the inter-galactic magnetic fields. Recently, LHAASO has detected $\sim 10$ TeV emission of the extraordinary powerful GRB 221009A within $\sim 2000$ s after the burst. Here we report the detection of a $\sim 400$ GeV photon, without accompanying prominent low-energy emission, by Fermi-LAT in the direction of GRB 221009A with the time delay of 0.4 days. Such a hard spectrum can be generated from electromagnetic cascades initiated by early primary $\sim 10$ TeV photons in the intergalactic space. An inter-galactic magnetic field strength of $B_{\rm IGMF}\sim 4 \times 10^{-17}$ G, comparable to limits from TeV blazars, can naturally account for the arrival time of the $\sim 400$ GeV photon as well as the HAWC non-detection. Such a strength will be stringently tested by the upcoming CTA.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The nature of dark matter remains obscure in spite of decades of experimental efforts. The mass of dark matter candidates can span a wide range, and its coupling with the Standard Model sector remains uncertain. All these unknowns make the etection of dark matter extremely challenging. Ultralight dark matter, with $m \sim10^{-22}$ eV, is proposed to reconcile the disagreements between observations and predictions from simulations of small-scale structures in the cold dark matter paradigm, while remaining consistent with other observations. Because of its large de Broglie wavelength and large local occupation number within galaxies, ultralight dark matter behaves like a coherently oscillating background field with an oscillating frequency dependent on its mass. If the dark matter particle is a spin-1 dark photon, such as the $U(1)_B$ or $U(1)_{B-L}$ gauge boson, it can induce an external oscillating force and lead to displacements of test masses. Such an effect would be observable in the form of periodic variations in the arrival times of radio pulses from highly stable millisecond pulsars. In this study, we search for evidence of ultralight dark photon dark matter (DPDM) using 14-year high-precision observations of 26 pulsars collected with the Parkes Pulsar Timing Array. While no statistically significant signal is found, we place constraints on coupling constants for the $U(1)_B$ and $U(1)_{B-L}$ DPDM. Compared with other experiments, the limits on the dimensionless coupling constant $\epsilon$ achieved in our study are improved by up to two orders of magnitude when the dark photon mass is smaller than $3\times10^{-22}$~eV ($10^{-22}$~eV) for the $U(1)_{B}$ ($U(1)_{B-L}$) scenario.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Recently, the evidence for gamma-ray emission has been found in the $Fermi$-LAT observation for the outer halo of Andromeda galaxy (M31). The dark matter (DM) annihilation offers a possible explanation on the gamma-ray radiation. In this work, we focus on the dark matter annihilation within minispikes around intermediate-mass black holes (IMBHs) with masses ranging from $100~\mathrm{M_\odot}$ to $10^6~\mathrm{M_\odot}$. When the thermal annihilation relic cross section $\left\langle \sigma v \right\rangle = 3 \times 10^{-26}~\mathrm {cm} ^{3}\;\mathrm {s} ^{-1}$ is adopted, we conduct an investigation on the population of IMBHs in the spherical halo area of M31. We find that there could be more than 65 IMBHs with masses of $ 100~ \mathrm{M_\odot}$ surrounded by the DM minispikes as the remnants of Population III stars in the M31 spherical halo, and it is almost impossible for the existence of minspikes around IMBHs with masses above $10^4~ \mathrm{M_\odot}$ which could be formed by the collapse of primordial cold gas, for both dark matter annihilation channels $b\bar{b}$ and $\tau^{+}\tau^{-}$. The properties of dark matter have been further explored with the simulation of these two scenarios for IMBHs formation.