分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Fast radio bursts (FRBs) are bright radio transient events with durations on the order of milliseconds. The majority of FRB sources discovered so far have a single peak, with the exception of a few showing multiple-peaked profiles, the origin of which is unknown. In this work, we show that the strong lensing effect of a point mass or a point mass $+$ external shear on a single-peak FRB can produce double peaks (i.e. lensed images). In particular, the leading peak will always be more magnified and hence brighter than the trailing peak for a point-mass lens model, while the point-mass $+$ external shear lens model can produce a less magnified leading peak. We find that, for a point-mass lens model, the combination of lens mass $M$ and redshift $z_l$ in the form of $M(1+z_l)$ can be directly computed from two observables -- the delayed time $\Delta t$ and the flux ratio of the leading peak to the trailing peak $R$. For a point-mass $+$ external shear lens model, upper and lower limits in $M(1+z_l)$ can also be obtained from $\Delta t$ and $R$ for a given external shear strength. In particular, tighter lens mass constraints can be achieved when the observed $R$ is larger. Lastly, we show the process of constraining lens mass using the observed values of $\Delta t$ and $R$ of two double-peaked FRB sources, i.e. FRB 121002 and FRB 130729, as references, although the double-peaked profiles are not necessarily caused by strong lensing.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The microlensing effect has developed into a powerful technique for a diverse range of applications including exoplanet discoveries, structure of the Milky Way, constraints on MAssive Compact Halo Objects, and measurements of the size and profile of quasar accretion discs. In this paper, we consider a special type of microlensing events where the sources are fast radio bursts with $\sim$milliseconds (ms) durations for which the relative motion between the lens and source is negligible. In this scenario, it is possible to temporally resolve the individual microimages. As a result, a method beyond the inverse ray shooting (IRS) method, which only evaluates the total magnification of all microimages, is needed. We therefore implement an algorithm for identifying individual microimages and computing their magnifications and relative time delays. We validate our algorithm by comparing to analytical predictions for a single microlens case and find excellent agreement. We show that the superposition of pulses from individual microimages produces a light curve that appears as multi-peaked FRBs. The relative time delays between pulses can reach 0.1--1 ms for stellar-mass lenses and hence can already be resolved temporally by current facilities. Although not yet discovered, microlensing of FRBs will become regular events and surpass the number of quasar microlensing events in the near future when $10^{4-5}$ FRBs are expected to be discovered on a daily basis. Our algorithm provides a way of generating the microlensing light curve that can be used for constraining stellar mass distribution in distant galaxies.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： With mock strong gravitational lensing images, we investigate the performance of broken power-law (BPL) model on the mass reconstruction of galaxy-scale lenses. An end-to-end test is carried out, including the creation of mock strong lensing images, the subtraction of lens light, and the reconstruction of lensed images. Based on these analyses, we can reliably evaluate how accurate the lens mass and source light distributions can be measured. We notice that, based on lensed images alone, only the Einstein radii ($R_{\rm E}$) or the mean convergence within them can be well determined, with negligible bias (typically $<1\%$) and controllable uncertainty. Away from the Einstein radii, the radial and mean convergence profiles can hardly be constrained unless well-designed priors are applied to the BPL model. We find that, with rigid priors, the BPL model can clearly outperform the singular power-law models by recovering the lens mass distributions with small biases out to several Einstein radii (e.g., no more than $5\%$ biases for the mean convergence profiles within $3~R_{\rm E}$). We find that the source light reconstructions are sensitive to both lens light contamination and lens mass models, where the BPL model with rigid priors still performs best when there is no lens light contamination. It is shown that, by correcting for the projection effect, the BPL model is capable of estimating the aperture and luminosity weighted line-of-sight velocity dispersions to an accuracy of $\sim6\%$. These results further highlight the great potential of the BPL model in strong lensing related studies.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： With mock strong gravitational lensing images, we investigate the performance of broken power-law (BPL) model on the mass reconstruction of galaxy-scale lenses. An end-to-end test is carried out, including the creation of mock strong lensing images, the subtraction of lens light, and the reconstruction of lensed images. Based on these analyses, we can reliably evaluate how accurate the lens mass and source light distributions can be measured. We notice that, based on lensed images alone, only the Einstein radii ($R_{\rm E}$) or the mean convergence within them can be well determined, with negligible bias (typically $<1\%$) and controllable uncertainty. Away from the Einstein radii, the radial and mean convergence profiles can hardly be constrained unless well-designed priors are applied to the BPL model. We find that, with rigid priors, the BPL model can clearly outperform the singular power-law models by recovering the lens mass distributions with small biases out to several Einstein radii (e.g., no more than $5\%$ biases for the mean convergence profiles within $3~R_{\rm E}$). We find that the source light reconstructions are sensitive to both lens light contamination and lens mass models, where the BPL model with rigid priors still performs best when there is no lens light contamination. It is shown that, by correcting for the projection effect, the BPL model is capable of estimating the aperture and luminosity weighted line-of-sight velocity dispersions to an accuracy of $\sim6\%$. These results further highlight the great potential of the BPL model in strong lensing related studies.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Yiping Shu
Raoul Cañameras
Stefan Schuldt
Sherry H. Suyu
Stefan Taubenberger
Kaiki Taro Inoue
Anton T. Jaelani
摘要： We carry out a search for strong-lens systems containing high-redshift lens galaxies with the goal of extending strong-lensing-assisted galaxy evolutionary studies to earlier cosmic time. Two strong-lens classifiers are constructed from a deep residual network and trained with datasets of different lens-redshift and brightness distributions. We classify a sample of 5,356,628 pre-selected objects from the Wide-layer fields in the second public data release of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) by applying the two classifiers to their HSC $gri$-filter cutouts. Cutting off at thresholds that correspond to a false positive rate of $10^{-3}$ on our test set, the two classifiers identify 5,468 and 6,119 strong-lens candidates. Visually inspecting the cutouts of those candidates results in 735 grade-A or B strong-lens candidates in total, of which 277 candidates are discovered for the first time. This is the single largest set of galaxy-scale strong-lens candidates discovered with HSC data to date, and nearly half of it (331/735) contains lens galaxies with photometric redshifts above 0.6. Our discoveries will serve as a valuable target list for ongoing and scheduled spectroscopic surveys such as the Dark Energy Spectroscopic Instrument, the Subaru Prime Focus Spectrograph project, and the Maunakea Spectroscopic Explorer.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Shanhao Zhu
Yiping Shu
Haibo Yuan
Jian-Ning Fu
Jian Gao
Jianghua Wu
Xiangtao He
Kai Liao
Guoliang Li
Xinzhong Er
Bin Hu
摘要： As a completely independent method, the measurement of time delay of strongly lensed quasars (TDSL) are crucial to resolve the Hubble tension. Extensive monitoring is required but so far limited to a small sample of strongly lensed quasars. Together with several partner institutes, Beijing Normal University is constructing a 1.93m reflector telescope at the Muztagh-Ata site in west China, which has the world class observing conditions. The telescope will be equipped with both a three-channel imager/photometer which covers $3500-11000$ Angstrom wavelength band, and a low-medium resolution ($\lambda/\delta\lambda=500/2000/7500$) spectrograph. In this paper, we investigate the capability of Muztagh-Ata 1.93m telescope in measuring time delays of strongly lensed quasars. We generate mock strongly lensed quasar systems and light curves with microlensing effects based on five known strongly lensed quasars, i.e., RX J1131-1231, HE 0435-1223, PG 1115+080, WFI 2033-4723 and SDSS 1206+4332. In particular, RX J1131-1231 is generated with lens modeling in this work. Due to lack of enough information, we simulate the other 4 systems with the public data without lens modeling. According to simulations, for RX J1131-like systems (wide variation in time delay between images) the TDSL measurement can be achieved with the precision about $\Delta t=0.5$ day with 4 seasons campaign length and 1 day cadence. This accuracy is comparable to the up-coming TDCOSMO project. And it would be better when the campaign length keeps longer and with high cadence. As a result, the capability of Muztagh-Ata 1.93m telescope allows it to join the network of TDSL observatories. It will enrich the database for strongly lensed quasar observations and make more precise measurements of time delays, especially considering the unique coordinate of the site.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Yiping Shu
Sergey E. Koposov
N. Wyn Evans
Vasily Belokurov
Richard G. McMahon
Matthew W. Auger
Cameron A. Lemon
摘要： We present two catalogues of active galactic nucleus (AGN) candidates selected from the latest data of two all-sky surveys -- Data Release 2 (DR2) of the \emph{Gaia} mission and the unWISE catalogue of the \emph{Wide-field Infrared Survey Explorer} (\emph{WISE}). We train a random forest classifier to predict the probability of each source in the \emph{Gaia}-unWISE joint sample being an AGN, $P_{\rm RF}$, based on \emph{Gaia} astrometric and photometric measurements and unWISE photometry. The two catalogues, which we designate C75 and R85, are constructed by applying different $P_{\rm RF}$ threshold cuts to achieve an overall completeness of 75\% ($\approx$90\% at \emph{Gaia} $G\leq20$ mag) and reliability of 85\% respectively. The C75 (R85) catalogue contains 2,734,464 (2,182,193) AGN candidates across the effective 36,000 deg$^2$ sky, of which $\approx$0.91 (0.52) million are new discoveries. Photometric redshifts of the AGN candidates are derived by a random forest regressor using \emph{Gaia} and \emph{WISE} magnitudes and colours. The estimated overall photometric redshift accuracy is 0.11. Cross-matching the AGN candidates with a sample of known bright cluster galaxies, we identify a high-probability strongly-lensed AGN candidate system, SDSS\,J1326$+$4806, with a large image separation of 21\farcs06. All the AGN candidates in our catalogues will have $\sim$5-year long light curves from \emph{Gaia} by the end of the mission, and thus will be a great resource for AGN variability studies. Our AGN catalogues will also be helpful in AGN target selections for future spectroscopic surveys, especially ones in the southern hemisphere. The C75 catalogue can be downloaded at https://zenodo.org/record/6837642#.YtEgjy8RrzI.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Xiaojing Lin
Zheng Cai
Siwei Zou
Zihao Li
Zuyi Chen
Fuyan Bian
Fengwu Sun
Yiping Shu
Yunjing Wu
Mingyu Li
Jianan Li
Xiaohui Fan
J. Xavier Prochaska
Daniel Schaerer
Stephane Charlot
Daniel Espada
Miroslava Dessauges-Zavadsky
Eiichi Egami
Daniel Stark
Kirsten K. Knudsen
摘要： Direct observations of low-mass, low-metallicity galaxies at $z\gtrsim4$ provide an indispensable opportunity for detailed inspection of the ionization radiation, gas flow, and metal enrichment in sources similar to those that reionized the Universe. Combining the James Webb Space Telescope (JWST), VLT/MUSE, and ALMA, we present detailed observations of a strongly lensed, low-mass ($\approx 10^{7.6}$ ${\rm M}_\odot$) galaxy at $z=3.98$ (also see Vanzella et al. 2022). We identify strong narrow nebular emission, including CIV $\lambda\lambda1548,1550$, HeII $\lambda1640$, OIII] $\lambda\lambda1661,1666$, [NeIII] $\lambda3868$, [OII] $\lambda3727$, and Balmer series of Hydrogen from this galaxy, indicating a metal-poor HII region ($\lesssim 0.12\ {\rm Z}_\odot$) powered by massive stars. Further, we detect a metal-enriched damped Ly$\alpha$ system (DLA) associated with the galaxy with the HI column density of $N_{\rm{HI}}\approx 10^{21.8}$ cm$^{-2}$. The metallicity of the associated DLA may reach the super solar metallicity (${\gtrsim Z}_\odot$). Moreover, thanks to JWST and gravitational lensing, we present the resolved UV slope ($\beta$) map at the spatial resolution of $\approx 100$ pc at $z=4$, with steep UV slopes reaching $\beta \approx -2.5$ around three star-forming clumps. Combining with low-redshift analogs, our observations suggest that low-mass, low-metallicity galaxies, which dominate reionization, could be surrounded by a high covering fraction of the metal-enriched, neutral-gaseous clouds. This implies that the metal enrichment of low-mass galaxies is highly efficient, and further support that in low-mass galaxies, only a small fraction of ionizing radiation can escape through the interstellar or circumgalactic channels with low column-density neutral gas.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Xiaojing Lin
Zheng Cai
Siwei Zou
Zihao Li
Zuyi Chen
Fuyan Bian
Fengwu Sun
Yiping Shu
Yunjing Wu
Mingyu Li
Jianan Li
Xiaohui Fan
J. Xavier Prochaska
Daniel Schaerer
Stephane Charlot
Daniel Espada
Miroslava Dessauges-Zavadsky
Eiichi Egami
Daniel Stark
Kirsten K. Knudsen
摘要： Direct observations of low-mass, low-metallicity galaxies at $z\gtrsim4$ provide an indispensable opportunity for detailed inspection of the ionization radiation, gas flow, and metal enrichment in sources similar to those that reionized the Universe. Combining the James Webb Space Telescope (JWST), VLT/MUSE, and ALMA, we present detailed observations of a strongly lensed, low-mass ($\approx 10^{7.6}$ ${\rm M}_\odot$) galaxy at $z=3.98$ (also see Vanzella et al. 2022). We identify strong narrow nebular emission, including CIV $\lambda\lambda1548,1550$, HeII $\lambda1640$, OIII] $\lambda\lambda1661,1666$, [NeIII] $\lambda3868$, [OII] $\lambda3727$, and Balmer series of Hydrogen from this galaxy, indicating a metal-poor HII region ($\lesssim 0.12\ {\rm Z}_\odot$) powered by massive stars. Further, we detect a metal-enriched damped Ly$\alpha$ system (DLA) associated with the galaxy with the HI column density of $N_{\rm{HI}}\approx 10^{21.8}$ cm$^{-2}$. The metallicity of the associated DLA may reach the super solar metallicity (${\gtrsim Z}_\odot$). Moreover, thanks to JWST and gravitational lensing, we present the resolved UV slope ($\beta$) map at the spatial resolution of $\approx 100$ pc at $z=4$, with steep UV slopes reaching $\beta \approx -2.5$ around three star-forming clumps. Combining with low-redshift analogs, our observations suggest that low-mass, low-metallicity galaxies, which dominate reionization, could be surrounded by a high covering fraction of the metal-enriched, neutral-gaseous clouds. This implies that the metal enrichment of low-mass galaxies is highly efficient, and further support that in low-mass galaxies, only a small fraction of ionizing radiation can escape through the interstellar or circumgalactic channels with low column-density neutral gas.
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