分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We present a method by using the phase characteristics of radio observation data for pulsar search and candidate identification. The phase characteristics are relations between the pulsar signal and the phase correction in the frequency-domain, and we regard it as a new search diagnostic characteristic. Based on the phase characteristics, a search method is presented: calculating DM (dispersion measure) -- frequency data to select candidate frequencies, and then confirming of candidates by using the broadband characteristics of pulsar signals. Based on this method, we performed a search test on short observation data of M15 and M71, which were observed by Five-hundred-meter Aperture spherical radio Telescope (FAST), and some of the Galactic Plane Pulsar Snapshot survey (GPPS) data. Results show that it can get similar search results to PRESTO (PulsaR Exploration and Search TOolkit) while having a faster processing speed.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We assemble a large sample of 13,863 high-velocity stars (HiVels) with total velocity ${V}_{\rm{GSR}} \ge {\rm300} \rm{km\,s^{-1}}$, selected from the RAVE DR5, SDSS DR12, LAMOST DR8, APOGEE DR16, GALAH DR2, and $Gaia$ EDR3. In this HiVel sample, 43 sources are hypervelocity stars (HVSs) that have ${V}_{\rm{GSR}}$ exceeding their local escape velocities, $V_{\rm esc}$, 32 of which are discovered for the first time. Interestingly, all the HVSs are metal-poor and late-type, significantly different from the previous HVSs in the literature, which are largely massive, metal-rich early-type stars, identified originally by extreme radial velocity alone. This finding suggests that our newly found HVSs are ejected by different mechanisms from the previous population. To investigate their origins, for the 571 extreme HiVel stars with ${V}_{\rm{GSR}}\ge0.8V_{\rm{esc}}$ in our sample, we reconstruct their backward-integrated trajectories in the Galactic potential. According to the orbital analysis, no HVSs are found to be definitely ejected from the Galactic center (GC), while 8 late-type metal-poor HiVels are found to have a closest distance to the GC within 1 kpc. Intriguingly, 15 HiVels (including 2 HVSs) are found from their backward-integrated trajectories to have experienced a close encounter with the Sagittarius dwarf spheroidal galaxy (Sgr dSph), suggesting that they originated from this dSph. This hypothesis is supported by an analysis of the [$\alpha$/Fe]--[Fe/H] diagram. From a preliminary analysis of all the HiVels in our sample, we propose a general picture: Star ejection from Galactic subsystems such as dwarf galaxies and globular clusters, either via tidal stripping or even the Hills mechanism, can be an important channel to produce HiVels/HVSs, particularly the metal-poor late-type halo population.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We present a method by using the phase characteristics of radio observation data for pulsar search and candidate identification. The phase characteristics are relations between the pulsar signal and the phase correction in the frequency-domain, and we regard it as a new search diagnostic characteristic. Based on the phase characteristics, a search method is presented: calculating DM (dispersion measure) -- frequency data to select candidate frequencies, and then confirming of candidates by using the broadband characteristics of pulsar signals. Based on this method, we performed a search test on short observation data of M15 and M71, which were observed by Five-hundred-meter Aperture spherical radio Telescope (FAST), and some of the Galactic Plane Pulsar Snapshot survey (GPPS) data. Results show that it can get similar search results to PRESTO (PulsaR Exploration and Search TOolkit) while having a faster processing speed.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Yongda Zhu
Hai-Xia Ma
Xiao-Bo Dong
Yang Huang
Tobias Mistele
Bo Peng
Qian Long
Tianqi Wang
Liang Chang
Xi Jin
摘要： Aiming at discriminating different gravitational potential models of the Milky Way, we perform tests based on the kinematic data powered by the Gaia DR2 astrometry, over a large range of $(R,z)$ locations. Invoking the complete form of Jeans equations that admit three integrals of motion, we use the independent $R$- and $z$-directional equations as two discriminators ($T_R$ and $T_z$). We apply the formula for spatial distributions of radial and vertical velocity dispersions proposed by Binney et al., and successfully extend it to azimuthal components, $\sigma_\theta(R,z)$ and $V_\theta(R,z)$; the analytic form avoids the numerical artifacts caused by numerical differentiation in Jeans-equations calculation given the limited spatial resolutions of observations, and more importantly reduces the impact of kinematic substructures in the Galactic disk. It turns out that whereas the current kinematic data are able to reject Moffat's Modified Gravity (let alone the Newtonian baryon-only model), Milgrom's MOND is still not rejected. In fact, both the carefully calibrated fiducial model invoking a spherical dark matter (DM) halo and MOND are equally consistent with the data at almost all spatial locations (except that probably both have respective problems at low-$|z|$ locations), no matter which a tracer population or which a meaningful density profile is used. Because there is no free parameter at all in the quasi-linear MOND model we use, and the baryonic parameters are actually fine-tuned in the DM context, such an effective equivalence is surprising, and might be calling forth a transcending synthesis of the two paradigms.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Inspired by our serendipitous discovery of six AGNs with varying broad-Halpha fluxes over years out of our searching for intermediate-mass black holes (IMBHs), we conduct a systematic investigation of changing-look (CL) and large-variability AGNs. We collect all the CL AGNs at z<0.15 and the reverberation mapped AGNs with strongly variable broad Halpha, and perform careful decomposition fittings to both their images and spectra. We find two observational facts: (1) The host galaxies of local CL and large-variability AGNs, mainly being Seyferts, are in the red (gas-poor) tail of the general Seyfert galaxy population. (2) In contrast, there is a significant trend that their more luminous counterparts namely CL and extremely variable quasars (CLQs and EVQs) are different: CLQs are generally in blue galaxies; in terms of the diagram of SFR and M* local CL Seyfert galaxies are located in the green valley, whereas CLQ hosts are in the star-forming main sequence. We propose explanations for those strongly variable Seyferts and quasars, respectively, under the thought that accretion disks broadly depend on nuclear fueling modes. Local large-variability and CL Seyferts are in nuclear famine mode, where cold-gas clumps can be formed stochastically in the fueling flow, and their episodic infall produces sharp peaks in the accretion-rate curve. CLQs and EVQs are in feast fueling mode, which may account for both their preference to blue galaxies and their variability pattern (high-amplitude tail of the continuous distribution). Lastly, we propose a new thinking: to search for IMBHs by optical variability in red galaxies.
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