分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We describe a search for gravitational waves from compact binaries with at least one component with mass 0.2 $M_\odot$ -- $1.0 M_\odot$ and mass ratio $q \geq 0.1$ in Advanced LIGO and Advanced Virgo data collected between 1 November 2019, 15:00 UTC and 27 March 2020, 17:00 UTC. No signals were detected. The most significant candidate has a false alarm rate of 0.2 $\mathrm{yr}^{-1}$. We estimate the sensitivity of our search over the entirety of Advanced LIGO's and Advanced Virgo's third observing run, and present the most stringent limits to date on the merger rate of binary black holes with at least one subsolar-mass component. We use the upper limits to constrain two fiducial scenarios that could produce subsolar-mass black holes: primordial black holes (PBH) and a model of dissipative dark matter. The PBH model uses recent prescriptions for the merger rate of PBH binaries that include a rate suppression factor to effectively account for PBH early binary disruptions. If the PBHs are monochromatically distributed, we can exclude a dark matter fraction in PBHs $f_\mathrm{PBH} \gtrsim 0.6$ (at 90% confidence) in the probed subsolar-mass range. However, if we allow for broad PBH mass distributions we are unable to rule out $f_\mathrm{PBH} = 1$. For the dissipative model, where the dark matter has chemistry that allows a small fraction to cool and collapse into black holes, we find an upper bound $f_{\mathrm{DBH}} < 10^{-5}$ on the fraction of atomic dark matter collapsed into black holes.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We report on the near-infrared polarimetric observations of G11.11--0.12 with the 1.4 m IRSF telescope. The starlight polarization of the background stars reveals the magnetic fields in the envelope of G11.11--0.12, which are consistent in orientation with the magnetic fields obtained from submillimeter dust polarization. The magnetic fields in G11.11--0.12 are perpendicular to the filament in a large column density range, independent of the relative orientations of G11.11--0.12. The field strength on the plane of the sky in G11.11--0.12 has a typical value of $152\pm17\,\mu$G. The analyses of the magnetic fields and gas velocity dispersion indicate that the envelope of G11.11--0.12 is supersonic and sub-Alfv{\'e}nic to trans-Alfv{\'e}nic. The mass-to-flux ratio in the outer part of the envelope is $\lesssim 1$ and slightly increases to $\gtrsim 1$ closer to the filament. The weights on the relative importance of magnetic fields, turbulence and gravity indicate that gravity has been dominating the dynamical state of G11.11--0.12, with significant contribution from magnetic fields. The field strength increases slower than the gas density from the envelope to the spine of G11.11--0.12, characterized by the relation $B\propto n^{0.2}$. The observed strength and orientation of magnetic fields in G11.11--0.12 imply that supersonic gas flow is channelled by sub-Alfv{\'e}nic magnetic fields and is assembled into filaments perpendicular to the magnetic fields. The formation of low-mass stars is enhanced in the filaments with high column density, in agreement with the excess in numbers of low-mass protostars detected in one of the densest part of G11.11--0.12.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Recently, it has been proposed that the magnetic-field-induced transition (MIT) in Fe X can be used to measure coronal magnetic field strengths. Several techniques, the direct line ratio technique and the weak and strong magnetic field techniques, are developed to apply the MIT theory to spectroscopic observations taken by EUV Imaging Spectrometer (EIS) onboard Hinode. However, the suitability of coronal magnetic field measurements based on the weak and strong magnetic field techniques has not been evaluated. Besides, temperature diagnostics is also important for measuring coronal magnetic field based on the MIT theory, but how to determine the accurate formation temperature of the Fe X lines from EIS observations still needs investigation. In this study, we synthesized emissions of several spectral lines from a 3D radiation magnetohydrodynamic model of a solar active region, and then derived magnetic field strengths using different methods. We first compared the magnetic field strengths derived from the weak and strong magnetic field techniques to the values in the model. Our study suggests that both weak and strong magnetic field techniques underestimate the coronal magnetic field strength. Then we developed two methods to calculate the formation temperature of the Fe X lines. One is based on differential emission measure analyses, and the other is deriving temperature from the Fe IX and Fe XI line pairs. However, neither of the two methods can provide temperature determination for accurate coronal magnetic field measurements as those derived from the Fe X 174/175 and 184/345 {\AA} line ratios. More efforts are still needed for accurate coronal magnetic field measurements using EIS observations.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The study of the physical and chemical properties of gas infall motion in the molecular clumps helps us understand the initial stages of star formation. We used the FTS wide-sideband mode of the IRAM 30-m telescope to observe nine infall sources with significant double peaked blue line profile. The observation frequency range are 83.7 - 91.5 GHz and 99.4 - 107.2 GHz. We have obtained numbers of molecular line data. Using XCLASS, a total of 7 to 27 different molecules and isotopic transition lines have been identified in these nine sources, including carbon chain molecules such as CCH, c-C3H2 and HC3N. According to the radiation transfer model, we estimated the rotation temperatures and column densities of these sources. Chemical simulations adopting a physical model of HMSFRs are used to fit the observed molecular abundances. The comparison shows that most sources are in the early HMPO stage, with the inner temperature around several ten K.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The study of infall motion helps us to understand the initial stages of star formation. In this paper, we use the IRAM 30-m telescope to make mapping observations of 24 infall sources confirmed in previous work. The lines we use to track gas infall motions are HCO+ (1-0) and H13CO+ (1-0). All 24 sources show HCO+ emissions, while 18 sources show H13CO+ emissions. The HCO+ integrated intensity maps of 17 sources show clear clumpy structures; for the H13CO+ line, 15 sources show clumpy structures. We estimated the column density of HCO+ and H13CO+ using the RADEX radiation transfer code, and the obtained [HCO+]/[H2] and [H13CO+]/[HCO+] of these sources are about 10^-11 ~ 10^-7 and 10^-3~1, respectively. Based on the asymmetry of the line profile of the HCO+, we distinguish these sources: 19 sources show blue asymmetric profiles, and the other sources show red profiles or symmetric peak profiles. For eight sources that have double-peaked blue line profiles and signal-to-noise ratios greater than 10, the RATRAN model is used to fit their HCO^+ (1-0) lines, and to estimate their infall parameters. The mean Vin of these sources are 0.3 ~ 1.3 km/s, and the Min are about 10^-3 ~ 10^-4 Msun/yr , which are consistent with the results of intermediate or massive star formation in previous studies. The Vin estimated from the Myers model are 0.1 ~ 1.6 km/s, and the Min are within 10^-3 ~ 10^-5 Msun/yr. In addition, some identified infall sources show other star-forming activities, such as outflows and maser emissions. Especially for those sources with a double-peaked blue asymmetric profile, most of them have both infall and outflow evidence.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We report on the near-infrared polarimetric observations of RCW 120 with the 1.4 m IRSF telescope. The starlight polarization of the background stars reveals for the first time the magnetic field of RCW 120. The global magnetic field of RCW 120 is along the direction of $20^\circ$, parallel to the Galactic plane. The field strength on the plane of the sky is $100\pm26\,\mu$G. The magnetic field around the eastern shell shows evidence of compression by the HII region. The external pressure (turbulent pressure + magnetic pressure) and the gas density of the ambient cloud are minimum along the direction where RCW 120 breaks out, which explains the observed elongation of RCW 120. The dynamical age of RCW 120, depending on the magnetic field strength, is $\sim\,1.6\,\mathrm{Myr}$ for field strength of $100\,\mu$G, older than the hydrodynamic estimates. In direction perpendicular to the magnetic field, the density contrast of the western shell is greatly reduced by the strong magnetic field. The strong magnetic field in general reduces the efficiency of triggered star formation, in comparison with the hydrodynamic estimates. Triggered star formation via the "collect and collapse" mechanism could occur in the direction along the magnetic field. Core formation efficiency (CFE) is found to be higher in the southern and eastern shells of RCW 120 than in the infrared dark cloud receiving little influence from the HII region, suggesting increase in the CFE related to triggering from ionization feedback.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: An important task within the broader goal of Space Situational Awareness (SSA) is to observe changes in the orbits of satellites, where the data spans thousands of objects over long time scales (decades). The Two-Line Element (TLE) data provided by the North American Aerospace Defense Command is the most comprehensive and widely-available dataset cataloguing the orbits of satellites. This makes it a highly-attractive data source on which to perform this observation. However, when attempting to infer changes in satellite behaviour from TLE data, there are a number of potential pitfalls. These mostly relate to specific features of the TLE data which are not always clearly documented in the data sources or popular software packages for manipulating them. These quirks produce a particularly hazardous data type for researchers from adjacent disciplines (such as anomaly detection or machine learning). We highlight these features of TLE data and the resulting pitfalls in order to save future researchers from being trapped. A seperate, significant, issue is that existing contributions to manoeuvre detection from TLE data evaluate their algorithms on different satellites, making comparison between these methods difficult. Moreover, the ground-truth in these datasets is often poor quality, sometimes being based on subjective human assessment. We therefore release and describe in-depth an open, curated, benchmark dataset containing TLE data for 15 satellites alongside high-quality ground-truth manoeuvre timestamps.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: AGN bubbles could play an important role in accelerating high-energy CRs and galactic feedback. Only in nearby galaxies could we have high enough angular resolution in multi-wavelengths to study the sub-kpc environment of the AGN, where the bubbles are produced and strongly interact with the surrounding ISM. In this paper, we present the latest Chandra observations of the Virgo cluster galaxy NGC 4438, which hosts multi-scale bubbles detected in various bands. The galaxy also has low current star formation activity, so these bubbles are evidently produced by the AGN rather than a starburst. We present spatially resolved spectral analysis of the Chandra data of the $\sim3^{\prime\prime}\times5^{\prime\prime}$ ($\sim200{\rm~pc}\times350\rm~pc$) nuclear bubble of NGC 4438. The power law tail in the X-ray spectra can be most naturally explained as synchrotron emission from high-energy CR leptons. The hot gas temperature increases, while the overall contribution of the non-thermal X-ray emission decreases with the vertical distance from the galactic plane. We calculate the synchrotron cooling timescale of the CR leptons responsible for the non-thermal hard X-ray emission to be only a few tens to a few hundreds of years. The thermal pressure of the hot gas is about three times the magnetic pressure, but the current data cannot rule out the possibility that they are still in pressure balance. The spatially resolved spectroscopy presented in this paper may have important constraints on how the AGN accelerates CRs and drives outflows. We also discover a transient X-ray source only $\sim5^{\prime\prime}$ from the nucleus of NGC 4438. The source was not detected in 2002 and 2008, but became quite X-ray bright in March 2020, with an average 0.5-7 keV luminosity of $\sim10^{39}\rm~ergs~s^{-1}$.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We report the discoveries of a nuclear ring of diameter 10$\arcsec$ ($\sim$1.5 kpc) and a potential low luminosity active galactic nucleus (LLAGN) in the radio continuum emission map of the edge-on barred spiral galaxy NGC~5792. These discoveries are based on the Continuum Halos in Nearby Galaxies - an Expanded Very Large Array (VLA) Survey, as well as subsequent VLA observations of sub-arcsecond resolution. Using a mixture of H$\alpha$ and 24 $\mu$m calibration, we disentangle the thermal and non-thermal radio emission of the nuclear region, and derive a star formation rate (SFR) of $\sim 0.4~M_{\sun}$ yr$^{-1}$. We find that the nuclear ring is dominated by non-thermal synchrotron emission. The synchrotron-based SFR is about three times of the mixture-based SFR. This result indicates that the nuclear ring underwent more intense star-forming activity in the past, and now its star formation is in the low state. The sub-arcsecond VLA images resolve six individual knots on the nuclear ring. The equipartition magnetic field strength $B_{\rm eq}$ of the knots varies from 77 to 88 $\mu$G. The radio ring surrounds a point-like faint radio core of $S_{\rm 6GHz}=(16\pm4)$ $\mu$Jy with polarized lobes at the center of NGC~5792, which suggests an LLAGN with an Eddington ratio $\sim10^{-5}$. This radio nuclear ring is reminiscent of the Central Molecular Zone (CMZ) of the Galaxy. Both of them consist of a nuclear ring and LLAGN.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Measurements of the magnetic field in the stellar coronae are extremely difficult. Recently, it was proposed that the magnetic-field-induced transition (MIT) of the Fe X 257 {\AA} line can be used to measure the coronal magnetic field of the Sun. We performed forward modeling with a series of global stellar magnetohydrodynamics models to investigate the possibility of extending this method to other late-type stars. We first synthesized the emissions of several Fe X lines for each stellar model, then calculated the magnetic field strengths using the intensity ratios of Fe X 257 {\AA} to several other Fe X lines based on the MIT theory. Finally, we compared the derived field strengths with those in the models, and concluded that this method can be used to measure at least the magnetic field strengths at the coronal bases of stars with a mean surface magnetic flux density about one order of magnitude higher than that of the Sun. Our investigation suggests the need of an extreme ultraviolet spectrometer to perform routine measurements of the stellar coronal magnetic field.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: It was recently proposed that the intensity ratios of several extreme ultraviolet spectral lines from the Fe X ion can be used to measure the solar coronal magnetic field based on the magnetic-field-inducedtransition (MIT) theory. To verify the suitability of this method, we performed forward modelingwith a three-dimensional radiation magnetohydrodynamic model of a solar active region. Intensities of several spectral lines from Fe X were synthesized from the model. Based on the MIT theory, intensity ratios of the MIT line Fe X 257 A to several other Fe X lines were used to derive the magnetic field strengths, which were then compared with the field strengths in the model. We also developed a new method to simultaneously estimate the coronal density and temperature from the Fe X 174/175 and 184/345 A line ratios. Using these estimates, we demonstrated that the MIT technique can provide reasonably accurate measurements of the coronal magnetic field in both on-disk and off-limb solar observations. Our investigation suggests that a spectrometer that can simultaneously observe the Fe X 174, 175, 184, 257, and 345 A lines and allow an accurate radiometric calibration for these lines is highly desired to achieve reliable measurements of the coronal magnetic field. We have also evaluatedthe impact of the uncertainty in the Fe X 3p4 3d 4D5/2 and 4D7/2 energy difference on the magnetic field measurements.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: This paper introduces the first results of observations with the Ultra-Long-Wavelength (ULW) -- Low Frequency Interferometer and Spectrometer (LFIS) on board the selenocentric satellite Longjiang-2. We present a brief description of the satellite and focus on the LFIS payload. The in-orbit commissioning confirmed a reliable operational status of the instrumentation. We also present results of a transition observation, which offers unique measurements on several novel aspects. We estimate the RFI suppression required for such a radio astronomy instrumentation at the Moon distances from Earth to be of the order of 80 dB. We analyse a method of separating Earth- and satellite-originated radio frequency interference (RFI). It is found that the RFI level at frequencies lower than a few MHz is smaller than the receiver noise floor.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The research of infall motion is a common means to study molecular cloud dynamics and the early process of star formation. Many works had been done in-depth research on infall. We searched the literature related to infall study of molecular cloud since 1994, summarized the infall sources identified by the authors. A total of 456 infall sources are catalogued. We classify them into high-mass and low-mass sources, in which the high-mass sources are divided into three evolutionary stages: prestellar, protostellar and HII region. We divide the sources into clumps and cores according to their sizes. The H$_2$ column density values range from 1.21$\times$ 10$^{21}$ to 9.75 $\times$ 10$^{24}$ cm$^{-2}$, with a median value of 4.17$\times$ 10$^{22}$ cm$^{-2}$. The H$_2$ column densities of high-mass and low-mass sources are significantly separated. The median value of infall velocity for high-mass clumps is 1.12 km s$^{-1}$, and the infall velocities of low-mass cores are virtually all less than 0.5 km s$^{-1}$. There is no obvious difference between different stages of evolution. The mass infall rates of low-mass cores are between 10$^{-7}$ and 10$^{-4}$ M$_{\odot} \text{yr}^{-1}$, and those of high-mass clumps are between 10$^{-4}$ and 10$^{-1}$ M$_{\odot} \text{yr}^{-1}$ with only one exception. We do not find that the mass infall rates vary with evolutionary stages.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The study of the physical and chemical properties of gas infall motion in the molecular clumps helps us understand the initial stages of star formation. We used the FTS wide-sideband mode of the IRAM 30-m telescope to observe nine infall sources with significant double peaked blue line profile. The observation frequency range are 83.7 - 91.5 GHz and 99.4 - 107.2 GHz. We have obtained numbers of molecular line data. Using XCLASS, a total of 7 to 27 different molecules and isotopic transition lines have been identified in these nine sources, including carbon chain molecules such as CCH, c-C3H2 and HC3N. According to the radiation transfer model, we estimated the rotation temperatures and column densities of these sources. Chemical simulations adopting a physical model of HMSFRs are used to fit the observed molecular abundances. The comparison shows that most sources are in the early HMPO stage, with the inner temperature around several ten K.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: This paper introduces the first results of observations with the Ultra-Long-Wavelength (ULW) -- Low Frequency Interferometer and Spectrometer (LFIS) on board the selenocentric satellite Longjiang-2. We present a brief description of the satellite and focus on the LFIS payload. The in-orbit commissioning confirmed a reliable operational status of the instrumentation. We also present results of a transition observation, which offers unique measurements on several novel aspects. We estimate the RFI suppression required for such a radio astronomy instrumentation at the Moon distances from Earth to be of the order of 80 dB. We analyse a method of separating Earth- and satellite-originated radio frequency interference (RFI). It is found that the RFI level at frequencies lower than a few MHz is smaller than the receiver noise floor.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Recently, it has been proposed that the magnetic-field-induced transition (MIT) in Fe X can be used to measure coronal magnetic field strengths. Several techniques, the direct line ratio technique and the weak and strong magnetic field techniques, are developed to apply the MIT theory to spectroscopic observations taken by EUV Imaging Spectrometer (EIS) onboard Hinode. However, the suitability of coronal magnetic field measurements based on the weak and strong magnetic field techniques has not been evaluated. Besides, temperature diagnostics is also important for measuring coronal magnetic field based on the MIT theory, but how to determine the accurate formation temperature of the Fe X lines from EIS observations still needs investigation. In this study, we synthesized emissions of several spectral lines from a 3D radiation magnetohydrodynamic model of a solar active region, and then derived magnetic field strengths using different methods. We first compared the magnetic field strengths derived from the weak and strong magnetic field techniques to the values in the model. Our study suggests that both weak and strong magnetic field techniques underestimate the coronal magnetic field strength. Then we developed two methods to calculate the formation temperature of the Fe X lines. One is based on differential emission measure analyses, and the other is deriving temperature from the Fe IX and Fe XI line pairs. However, neither of the two methods can provide temperature determination for accurate coronal magnetic field measurements as those derived from the Fe X 174/175 and 184/345 {\AA} line ratios. More efforts are still needed for accurate coronal magnetic field measurements using EIS observations.
分类: 光学 >> 量子光学 提交时间: 2023-02-19
摘要: Optical lattice clocks are the prospective devices that can probe many subtle physics including temporal variation of the fine structure constant ($\alpha_e$). These studies necessitate high-precision measurements of atomic clock frequency ratios to unprecedented accuracy. In contrast to the earlier claimed highest sensitive coefficient ($K$) clock transition to $\alpha_e$ in Yb [Phys. Rev. Lett. {\bf 120}, 173001 (2018)], we found the $4f^{14}6s6p\,(^3P_2) - 4f^{13}5d6s^{2}\,(^3P_2^*)$ transition of this atom can serve as the clock transition with the largest $K$ value (about $-$25(2)). Moreover, we demonstrate a scheme to attain simultaneous magic trapping conditions for this clock transition with the other two proposed clock transitions $4f^{14}6s^2\,(^1S_0) - 4f^{14}6s6p\,(^3P_2)$ and $4f^{14}6s^2\,(^1S_0) - 4f^{13}5d6s^{2}\,(^3P_2^*)$; also exhibiting large $K$ values. This magic condition can be realized by subjecting Yb atoms to a bias magnetic field at a particular polarization angle along the quantization axis in an experimental set up. Upon realization, it will serve as the most potential optical lattice clock to probe $\alpha_e$ variation.
分类: 心理学 >> 发展心理学 提交时间: 2020-10-10
摘要: Building robust letter-to-sound correspondences is a prerequisite for reading, and such audiovisual integration becomes progressively automatic with development. However, the neural mechanisms underlying the development of audiovisual integration for reading are largely unknown. This study used functional magnetic resonance imaging (fMRI) in a lexical decision task to investigate the changes of brain functional networks that support audiovisual integration for reading between normally developing children (9-12 years old) and adults (20-28 years old). The identified networks were further examined in children with developmental dyslexia (9-12 years old). Results revealed that adults enhanced connectivity in a prefrontal-superior temporal network relative to children, reflecting the attentional modulation to the development of audiovisual integration. Moreover, this network was disrupted in dyslexics, confirming its essential role in audiovisual integration for reading. This study, for the first time, elucidates the neural basis underlying the development of audiovisual integration for reading.
分类: 心理学 >> 认知心理学 提交时间: 2020-06-30
摘要: Metacognition refers to the ability to introspect our cognitive ability, which plays an essential role in guiding and optimizing our activities. However, little is known about metacognitive capacity for highly practiced motor behaviors and its neural correlates. Using structural and functional magnetic resonance imaging (MRI), the present study examined the brain substrates underlying individual differences in self-awareness of handwriting in adults, a highly practiced visuomotor skill. Results showed that adult writers generally overestimate their handwriting skill, which is more pronounced in males relative to females. The extent of overestimation of handwriting quality was positively correlated with grey matter volume in the left fusiform gyrus, right middle frontal gyrus and right precuneus. Moreover, the activation of these regions in a handwriting task was not correlation with self-awareness of handwriting, confirming that the identified connection between brain structures and handwriting self-awareness is independent of task performances. The left fusiform gyrus and right middle frontal gyrus are thought to represent domain-specific brain mechanisms for handwriting self-awareness, while the right precuneus is likely to be a domain-general brain mechanism, suggesting that the ability of introspect practiced visuomotor skills relies on both domain-general and domain-specific brain systems. Together, this study is the first to reveal the neuroanatomical correlates of a highly practiced motor behavior, extending our understanding about the neural basis of human metacognition.
分类: 生物学 >> 生物化学 提交时间: 2020-02-12
摘要: 最近,在中国武汉爆发了肺炎病例,由一种名为2019-CoV的新型冠状病毒引起。我们之前的研究中,根据中国深圳的12例2019-nCoV感染患者的临床特征,所有病例均患有肺炎,一半病例发展为急性呼吸窘迫综合征(ARDS)。本文中,我们针对这12例患者的血浆因子表达谱进行了研究。我们测试了2019-nCoV感染患者血浆中的48个因子的表达,其中38个因子与健康个体相比显着升高;2019-nCoV感染的重症患者血浆中的高细胞因子血症水平显著低于A型流感病毒H7N9感染患者,而略高于细菌感染患者。在这38个细胞因子中,有17个与2019-CoV病毒载量相关,其中的15个(M-CSF,IL-10,IFN-α2,IL-17,IL-4,IP-10,IL-7,IL-1受体拮抗剂,G-CSF,IL-12 (p40),IFN-γ,IL-1α,IL-2,HGF和PDGF-BB)与肺损伤Murray评分高度相关,可用来预测2019-nCoV感染患者的疾病严重程度。我们的研究结果表明,这15种高细胞因子可能是衡量2019-nCoV感染患者疾病严重程度的潜在生物标志物,影响这些信号传递介质的因子可能是针对新型2019-nCoV大流行的潜在药物。