Quasars are powered by accretion onto supermassive black holes, but the problem of the duty cycle related to the episodic activity of the black holes remains open as one of the major questions of cosmological evolution of quasars. In this Letter, we obtain quasar duty cycles based on analyses of a large sample composed of 10,979 quasars with redshifts $z\le2.1$ from the Sloan Digital Sky Survey (SDSS) Data Release Three. We estimate masses of quasar black holes and obtain their mass function (MF) of the present sample. We then get the duty cycle $\bar{\delta}(z)=10^{-3}\sim 1$ based on the So{\l}tan's argument, implying that black holes are undergoing multiple episodic activity. We find that the duty cycle has a strong evolution. By comparison, we show that evolution of the duty cycle follows the history of cosmic star formation rate (SFR) density in the Universe, providing intriguing evidence for a natural connection between star formation and triggering of black hole activity. Feedback on star formation from black hole activity is briefly discussed.Comment: Accepted by ApJL, 4 pages and 2 Figure

The DArk Matter Particle Explorer (DAMPE) satellite has been successfully launched on the 17th December 2015. It is a powerful space detector designed for the identification of possible Dark Matter signatures thanks to its capability to detect electrons and photons with an unprecedented energy resolution in an energy range going from few GeV up to 10 TeV. Moreover, the DAMPE satellite will contribute to a better understanding of the propagation mechanisms of high energy cosmic rays measuring the nuclei flux up to 100 TeV. DAMPE is composed of four sub-detectors: a plastic strip scintillator, a silicon-tungsten tracker-converter (STK), a BGO imaging calorimeter and a neutron detector. The STK is made of twelve layers of single-sided AC-coupled silicon micro-strip detectors for a total silicon area of about 7 m 2 . To promote the conversion of incident photons into electron-positron pairs, tungsten foils are inserted into the supporting structure. In this document, a detailed description of the STK construction and its performance on orbit are reported.

The DArk Matter Particle Explorer (DAMPE) is a space-borne particle detector designed to probe electrons and gamma-rays in the few GeV to 10 TeV energy range, as well as cosmic-ray proton and nuclei components between 10 GeV and 100 TeV. The silicon-tungsten tracker-converter is a crucial component of DAMPE. It allows the direction of incoming photons converting into electron-positron pairs to be estimated, and the trajectory and charge (Z) of cosmic-ray particles to be identified. It consists of 768 silicon micro-strip sensors assembled in 6 double layers with a total active area of 6.6 m 2 . Silicon planes are interleaved with three layers of tungsten plates, resulting in about one radiation length of material in the tracker. Internal alignment parameters of the tracker have been determined on orbit, with non-showering protons and helium nuclei. We describe the alignment procedure and present the position resolution and alignment stability measurements. NUDBGO STK PSD z x y Figure 1: Schematic view of the DAMPE detector. Sensitive detectors and support structures are shown. The z-axis of the DAMPE coordinate system is oriented to the zenith, orthogonal to the STK planes and y points to the Sun.10 GeV to 100 TeV, with excellent energy resolution and direction precision [1,2]. The main objectives of DAMPE are the identification of possible indirect signatures of Dark Matter annihilation or decay, improving the understanding of the origin and propagation mechanisms of high energy cosmic rays and gamma-ray astronomy. It consists of four sub-detectors ( Figure 1) stacked together as follows, moving from top to bottom. First is a Plastic Scintillator-strip Detector (PSD), which measures the cosmic ray charge (Z) and provides the veto signal for charged particles in gamma-ray detection. It is followed by a Silicon-Tungsten tracKer-converter (STK), that is described in detail in the next section. Next, there is an imaging calorimeter made of 14 layers of Bismuth Germanium Oxide (BGO) bars in a hodoscopic arrangement with a total thickness of about 32 radiation lengths, which provides a precise energy measurement and particle identification for electron/hadron separation. The BGO is aided by the NeUtron Detector (NUD), a borondoped plastic scintillator detecting delayed neutrons coming from hadronic interactions at high energies, which serves to improve the electron/hadron separation power.The STK is a key component of DAMPE, allowing the trajectory and absolute ion charge (Z) of incoming particles to be reconstructed and measured respectively. Moreover, thanks to its high position resolution, the direction of incoming photons converting into electron-positron pairs in the STK's tungsten plates can be precisely reconstructed. In order to fully exploit the trajectory reconstruction capabilities of the STK, a precise alignment of the instrument is needed, as explained in this paper.The paper is organized as follows. In Section 2 the STK is briefly described. Section 3 provides the details of the on-orbit data and simulati...

The Hard X-ray Modulation Telescope (HXMT or also dubbed as Insight-HXMT) is China's first astronomical satellite. It was launched on 15 th June 2017 in JiuQuan, China and is currently in service smoothly. It was designed to perform pointing, scanning and gamma-ray burst (GRB) observations and, based on the Direct Demodulation Method (DDM), the image of the scanned sky region can be reconstructed. Here we introduce the mission and its progresses in aspects of payload, core sciences, ground calibration/facility, ground segment, data archive, software, in-orbit performance, calibration, background model, observations and preliminary results.