Aperture Spherical Telescope Research Articles

Study of Five-Hundred-Meter Aperture Spherical Telescope Feed Cabin Time-Series Prediction Studies Based on Long Short-Term Memory-Self-Attention.

The Five-hundred-meter Aperture Spherical Telescope (FAST), as the world's most sensitive single-dish radio telescope, necessitates highly accurate positioning of its feed cabin to utilize its full observational potential. Traditional positioning methods that rely on GNSS and IMU, integrated with TS devices, but the GNSS and TS devices are vulnerable to other signal and environmental disruptions, which can significantly diminish position accuracy and even cause observation to stop. To address these challenges, this study introduces a novel time-series prediction model that integrates Long Short-Term Memory (LSTM) networks with a Self-Attention mechanism. This model can hold the precision of feed cabin positioning when the measure devices fail. Experimental results show that our LSTM-Self-Attention model achieves a Mean Absolute Error (MAE) of less than 10 mm and a Root Mean Square Error (RMSE) of approximately 12 mm, with the errors across different axes following a near-normal distribution. This performance meets the FAST measurement precision requirement of 15 mm, a standard derived from engineering practices where measurement accuracy is set at one-third of the control accuracy, which is around 48 mm (according to the accuracy form the official threshold analysis on the focus cabin of FAST). This result not only compensates for the shortcomings of traditional methods in consistently solving feed cabin positioning, but also demonstrates the model's ability to handle complex time-series data under specific conditions, such as sensor failures, thus providing a reliable tool for the stable operation of highly sensitive astronomical observations.

SETI at FAST in China

Abstract Since the commencement of the first SETI observation in 2019, China’s Search for Extraterrestrial Intelligence program has garnered momentum through domestic support and international collaborations. Several observations targeting exoplanets and nearby stars have been conducted with the Five-hundred-metre Aperture Spherical Telescope (FAST). In 2023 the introduction of the Far Neighbour Project (FNP) marked a substantial leap forward, driven by the remarkable sensitivity of the FAST telescope and some of the novel observational techniques. The FNP seeks to methodically detect technosignatures from celestial bodies, including nearby stars, exoplanetary systems, Milky Way globular clusters and more. This paper provides an overview of the progress achieved by SETI in China and offers insights into the distinct phases comprising the FNP. Additionally it underscores the significance of this project’s advancement and its potential contributions to the field.

Examining the Nature of the Starless Dark Matter Halo Candidate Cloud-9 with Very Large Array Observations

Observations with the Five-hundred-meter Aperture Spherical Telescope recently detected H i 21 cm emission near M94, revealing an intriguing object, Cloud-9, without an optical counterpart. Subsequent analysis suggests that Cloud-9 is consistent with a gas-rich (M H I ≈ 106 M ⊙), starless, dark matter (DM) halo of mass M 200 ≈ 5 × 109 M ⊙. Using the Karl G. Jansky Very Large Array in D-array configuration, we present interferometric observations of Cloud-9, revealing it as a dynamically cold (W 50 ≈ 12 km s−1), nonrotating, and spatially asymmetric system, exhibiting gas compression on one side and a tail-like structure toward the other—features likely originating from ram pressure. Our observations suggest Cloud-9 is consistent with a starless ΛCDM DM halo if the gas is largely isothermal. If interpreted as a faint dwarf, Cloud-9 is similar to Leo T, a nearby gas-rich galaxy that would fall below current optical detection limits at Cloud-9's distance (d ≈ 5 Mpc). Further observations with the Hubble Space Telescope reaching magnitudes m g ≈ 30 would help identify such a galaxy or dramatically lower the current limits on its stellar mass (M gal ≲ 105 M ⊙). Cloud-9 thus stands as the firmest starless DM halo candidate to date or the faintest galaxy known at its distance.

Polarized radio emission of RRAT J1854+0306

ABSTRACT Polarized radio emission of RRAT J1854+0306 is investigated with single pulses using Five-hundred-meter Aperture Spherical Telescope. Its emission is characterized by nulls, narrow and weak pulses, and occasional wide and intense bursts with a nulling fraction of 53.2 per cent. Its burst emission is typically of one rotation, and occasionally of two or three or even five rotations at the most, but without significant periodicity. The integrated pulse profile has an ‘S’-shaped position angle curve that is superposed with orthogonal modes, from which geometry parameters are obtained. Individual pulses exhibit diverse profile morphology with single, double, or multiple peaks. The intensity and width of these pulses are highly correlated, and bright pulses generally have wide profiles with multiple peaks. These nulling behaviours, profile morphology, and polarization demonstrate that a rotating radio transient has the same physical origins as the normal pulsars.

Scaling and universality in the temporal occurrence of repeating FRBs

ABSTRACT Fast Radio Bursts (FRBs) are energetic phenomena that have significant implications for understanding fundamental physics and the Universe. Recent observations of FRB 121102, FRB 20220912A, and FRB 20201124A by the Five-hundred-meter Aperture Spherical Telescope showed high-burst rates and distinctive energy distribution and temporal properties. In this study, we examine these observations to investigate the scale invariance of the waiting times between bursts for intervals longer than approximately 1 s. Our analysis revealed a unified scaling law for these longer intervals, which is similar to the behaviour of solar flares. This discovery inspires us to suggest a dual analogy of the FRB scenario across the entire time intervals: with earthquake dynamics at subsecond scales and with solar flare dynamics beyond the one-second threshold. This threshold potentially aligns with the dynamic time-scale of neutron star crusts, offering insight of the occurrence of FRBs into the internal processes of neutron stars.

A Bigger Cloud 9? New H i Observations of the RELHIC Candidate M94- Cloud 9

We present new H i observations of the REionization-Limited H i Cloud (RELHIC) candidate, M94-CL9, detected around M94 by Zhou et al. using the Five-hundred-meter Aperture Spherical Telescope (FAST). M94-CL9's H i properties as detected by FAST are consistent with a RELHIC as noted by Benitez-Llambay & Navarro. Our observations with the Robert C. Byrd Green Bank Telescope detect greater H i emission in M94-CL9 and result in H i properties that are larger (corrected velocity width, W 50,c,t = 35.7 ± 0.6 km s−1; and integrated flux, ∫Sdv = 0.28 ± 0.04 Jy km s−1) than those found by Zhou et al. but that match those from the FAST All-Sky H i survey. These larger properties do not preclude M94-CL9 from being a RELHIC, but the wider spectral extent and spectral asymmetry reported here may be in tension with predictions of RELHIC properties.

Discovery of Four Pulsars in a Pilot Survey at Intermediate Galactic Latitudes with FAST

We present the discovery and timing results of four pulsars discovered in a pilot survey at intermediate Galactic latitudes with the Five-hundred Aperture Spherical Telescope (FAST). Among these pulsars, two belong to the category of millisecond pulsars (MSPs) with spin periods of less than 20 ms. The other two fall under the classification of “mildly recycled” pulsars, with massive white dwarfs as companions. Remarkably, this small survey, covering an area of 4.7 deg2, led to the discovery of four recycled pulsars. Such success underscores the immense potential of future surveys at intermediate Galactic latitudes. In order to assess the potential yield of MSPs, we conducted population simulations and found that both FAST and Parkes new phased array feed surveys, focusing on intermediate Galactic latitudes, have the capacity to uncover several hundred new MSPs.

Pulsar scintillation through thick and thin: bow shocks, bubbles, and the broader interstellar medium

ABSTRACT Observations of pulsar scintillation are among the few astrophysical probes of very small-scale (≲ au) phenomena in the interstellar medium (ISM). In particular, characterization of scintillation arcs, including their curvature and intensity distributions, can be related to interstellar turbulence and potentially overpressurized plasma in local ISM inhomogeneities, such as supernova remnants, H ii regions, and bow shocks. Here we present a survey of eight pulsars conducted at the Five-hundred-metre Aperture Spherical Telescope (FAST), revealing a diverse range of scintillation arc characteristics at high sensitivity. These observations reveal more arcs than measured previously for our sample. At least nine arcs are observed toward B1929+10 at screen distances spanning $\sim 90~{{\ \rm per\ cent}}$ of the pulsar’s 361 pc path length to the observer. Four arcs are observed toward B0355+54, with one arc yielding a screen distance as close as ∼105 au (<1 pc) from either the pulsar or the observer. Several pulsars show highly truncated, low-curvature arcs that may be attributable to scattering near the pulsar. The scattering screen constraints are synthesized with continuum maps of the local ISM and other well-characterized pulsar scintillation arcs, yielding a three-dimensional view of the scattering media in context.

Is a Recently Discovered H i Cloud near M94 a Starless Dark Matter Halo?

Observations with the Five-hundred-meter Aperture Spherical Telescope have revealed the presence of a marginally resolved source of 21 cm emission from a location from the M94 galaxy, without a stellar counterpart down to the surface brightness limit of the DESI Imaging Legacy Survey (∼29.15 mag arcsec−2 in the g band). The system (hereafter Cloud-9) has round column density isocontours and a line width consistent with thermal broadening from gas at T ∼ 2 × 104 K. These properties are unlike those of previously detected dark H i clouds and similar to the expected properties of REionization-Limited-H i Clouds (RELHICs), namely, starless dark matter (DM) halos filled with gas in hydrostatic equilibrium and in thermal equilibrium with the cosmic ultraviolet background. At the distance of M94, d ∼ 4.7 Mpc, we find that Cloud-9 is consistent with being a RELHIC inhabiting a Navarro–Frenk–White (NFW) DM halo of mass M 200 ∼ 5 × 109 M ⊙ and concentration c NFW ∼ 13. Although the agreement between the model and observations is good, Cloud-9 appears to be slightly, but systematically, more extended than expected for ΛCDM RELHICs. This may imply either that Cloud-9 is much closer than implied by its recessional velocity, v CL9 ∼ 300 km s−1, or that its halo density profile is flatter than NFW, with a DM mass deficit greater than a factor of 10 at radii r ≲ 1 kpc. Further observations may aid in constraining these scenarios better and help elucidate whether Cloud-9 is the first ever observed RELHIC, a cornerstone prediction of the ΛCDM model on the smallest scales.

Deep H i Mapping of Stephan’s Quintet and Its Neighborhood

We carried out deep mapping observations of the atomic hydrogen (H i) 21 cm line emission in a field centered on the famous galaxy group Stephan's Quintet (SQ), using the Five-hundred-meter Aperture Spherical Telescope (FAST) equipped with a 19-beam receiver. The final data cube reaches an H i column density sensitivity of 5σ = 2.1 × 1017 cm−2 per 20 km s−1 channel with an angular resolution of 4.′0. The discovery of a large diffuse feature of the H i emission in the outskirts of the intragroup medium of SQ was reported in a previous paper (Xu et al.). Here we present a new study of the total H i emission of SQ and the detection of several neighboring galaxies, exploiting the high sensitivity and the large sky coverage of the FAST observations. A total H i mass of M H I = 3.48 ± 0.35 × 1010 M ☉ is found for SQ, which is significantly higher than previous measurements in the literature. This indicates that, contrary to earlier claims, SQ is not H i deficient. The excessive H i gas is mainly found in the velocity ranges of 6200–6400 km s−1 and 6800–7000 km s−1, which were undetected in previous observations that are less sensitive than ours. Our results suggest that the “missing H i” in compact groups may be hidden in the low-density diffuse neutral gas instead of in the ionized gas.

Machine learning classification of repeating FRBs from FRB 121102

ABSTRACT Fast radio bursts (FRBs) are mysterious bursts in the millisecond time-scale at radio wavelengths. Currently, there is little understanding about the classification of repeating FRBs, based on difference in physics, which is of great importance in understanding their origin. Recent works from the literature focus on using specific parameters to classify FRBs to draw inferences on the possible physical mechanisms or properties of these FRB subtypes. In this study, we use publicly available 1652 repeating FRBs from FRB 121102 detected with the Five-hundred-metre Aperture Spherical Telescope (FAST), and studied them with an unsupervised machine learning model. By fine-tuning the hyperparameters of the model, we found that there is an indication for four clusters from the bursts of FRB 121102 instead of the two clusters (‘Classical’ and ‘Atypical’) suggested in the literature. Wherein, the ‘Atypical’ cluster can be further classified into three sub-clusters with distinct characteristics. Our findings show that the clustering result we obtained is more comprehensive not only because our study produced results which are consistent with those in the literature but also because our work uses more physical parameters to create these clusters. Overall, our methods and analyses produced a more holistic approach in clustering the repeating FRBs of FRB 121102.

A new source finding and tracking analysis approach for the active reflector of five-hundred-meter aperture spherical telescope

The FAST (Five-hundred-meter Aperture Spherical Telescope) is the world’s largest single-aperture radio telescope. During operation, FAST needs to follow the observed celestial body to adjust displacements, which can be divided into two processes: the source finding and tracking. In this paper, a new numerical method based on Vector Form Intrinsic Finite Element (VFIFE) is proposed to simulate the source finding and tracking process for FAST’s active reflector. The basic principle of static equilibrium and calculation flow of the sourcing and tracking analysis is proposed, and the finite element program is compiled to analyze the displacement process of the reflector cable-net structure. Then, six typical test conditions are selected to simulate the sourcing and tracking displacement process. The obtained final form in the equilibrium state adheres to the target configuration, demonstrating the applicability of the suggested approach and the correctness of calculation findings. In addition, the change in the actuator length and the structural force distribution can be simultaneously derived. These values serve as a vital theoretical foundation for FAST’s practical operation and regulation.

FEASTS: IGM Cooling Triggered by Tidal Interactions through the Diffuse H i Phase around NGC 4631

We use the single-dish radio telescope Five-hundred meter Aperture Spherical Telescope (FAST) to map the H i in the tidally interacting NGC 4631 group with a resolution of 3.′24 (7 kpc), reaching a 5σ column density limit of 1017.9 cm−2 assuming a line width of 20 km s−1. Taking the existing interferometric H i image from the Hydrogen Accretion in LOcal GAlaxieS project of Westerbork Synthesis Radio Telescope as a reference, we are able to identify and characterize a significant excess of large-scale, low-density, and diffuse H i in the group. This diffuse H i extends more than 120 kpc across, and accounts for more than one-fourth of the total H i detected by FAST in and around the galaxy NGC 4631. In the region of the tidal tails, the diffuse H i has a typical column density above 1019.5 cm−2, and is highly turbulent with a velocity dispersion of around 50 km s−1. It increases in column density with the dense H i, and tends to be associated with the kinematically hotter part of the dense H i. Through simple modeling, we find that the majority of the diffuse H i in the tail region is likely to induce cooling out of the hot intragalactic medium (IGM) instead of evaporating or being radiatively ionized. Given these relations of gas in different phases, the diffuse H i may represent a condensing phase of the IGM. Ongoing and past active tidal interactions may have produced the wide-spreading H i distribution, and triggered the gas accretion to NGC 4631 through the phase of the diffuse H i.

HEGrid: A high efficient multi-channel radio astronomical data gridding framework in heterogeneous computing environments

The challenge to fully exploit the potential of existing and upcoming scientific instruments like large single-dish radio telescopes is to process the collected massive data effectively and efficiently. As a ”quasi 2D stencil computation” with the ”Moore neighborhood pattern”, gridding is the most computationally intensive step in data reduction pipeline for radio astronomy studies, enabling astronomers to create correct sky images for further analysis. However, the existing gridding frameworks can either only run on multi-core CPU architecture or do not support high-concurrency, multi-channel data gridding. Their performance is then limited, and there are emerging needs for innovative gridding frameworks to process data from large single-dish radio telescopes like the Five-hundred-meter Aperture Spherical Telescope (FAST). To address those challenges, we developed a High Efficient Gridding framework, HEGrid, by overcoming the above limitations. HEGrid is the first effort to solve the gridding of multi-channel data from the large single-dish radio telescope by multi-pipeline concurrency in the CPU–GPU heterogeneous environment. Specifically, we propose and construct the gridding pipeline in heterogeneous computing environments and achieve multi-pipeline concurrency for high performance multi-channel processing. Furthermore, we propose pipeline-based co-optimization to alleviate the potential negative performance impact of possible intra- and inter-pipeline low computation and I/O utilization, including component share-based redundancy elimination, thread-level data reuse and overlapping I/O and computation. Our experiments are based on both simulated datasets and actual FAST observational datasets. The results show that HEGrid outperforms other state-of-the-art gridding frameworks by up to 5.5x and has robust hardware portability, including AMD Radeon Instinct GPU and NVIDIA GPU.

Forecasting the cross-correlation of the CSST galaxy survey with the FAST H i Intensity Map

ABSTRACT The cross-correlation of optical galaxies with the neutral hydrogen (H i) radiation intensity can enhance the signal-to-noise ratio (SNR) of the H i intensity measurement. In this paper, we investigate the cross-correlation of the galaxy samples obtained by the spectroscopic survey of the China Space Station Telescope (CSST) with the H i Intensity mapping (IM) survey of the Five-hundred-meter Aperture Spherical Telescope (FAST). Using the IllusitrisTNG simulation result at redshift 0.2 ∼ 0.3, we generate mock data of the CSST survey and a FAST L-band drift scan survey. The CSST spectroscopic survey can yield a sample of galaxies with a high comoving number density of $10^{-2}\, ({\ \rm Mpc}\,h^{ -1})^{-3}$ at z ∼ 0.3. We cross-correlate the foreground-removed radio intensity with the CSST galaxies, including both the whole sample, and red and blue galaxy subsamples separately. We find that in all cases the H i and optical galaxies are well correlated. The total H i abundance can be measured with a high precision from this correlation. A relative error of $\sim 0.6{{\ \rm per\ cent}}$ for $\Omega _{\rm H\, \small {I}}$ could be achieved at z ∼ 0.3 for an overlapping survey area of $10\,000\, {\ \rm deg}^2$.

Massive Galaxy Mergers Have Distinctive Global H i Profiles

Abstract The global 21 cm H i emission-line profile of a galaxy encodes valuable information on the spatial distribution and kinematics of the neutral atomic gas. Galaxy interactions significantly influence the H i disk and imprint observable features on the integrated H i line profile. In this work, we study the neutral atomic gas properties of galaxy mergers selected from the Great Observatories All-sky LIRG Survey. The H i spectra come from new observations with the Five-hundred-meter Aperture Spherical Telescope and from a collection of archival data. We quantify the H i profile of the mergers with a newly developed method that uses the curve of growth of the line profile. Using a control sample of non-merger galaxies carefully selected to match the stellar mass of the merger sample, we show that mergers have a larger proportion of single-peaked H i profiles, as well as a greater tendency for the H i central velocity to deviate from the systemic optical velocity of the galaxy. By contrast, the H i profiles of mergers are not significantly more asymmetric than those of non-mergers.

Comparison of the Parkes and FAST FRB DM distribution

ABSTRACT We model the fast radio burst (FRB) dispersion measure (DM) distribution for the Five-hundred-meter Aperture Spherical Telescope (FAST) and compare this with the four FRBs published in the literature to date. We compare the DM distribution of Parkes and FAST, taking advantage of the similarity between their multibeam receivers. Notwithstanding the limited sample size, we observe a paucity of events at low DM for all evolutionary models considered, resulting in a sharp rise in the observed cumulative distribution function in the region of 1000 pc cm−3 ≲ DM ≲ 2000 pc cm−3. These traits could be due to statistical fluctuations (0.12 ≤ p ≤ 0.22), a complicated energy distribution or break in an energy distribution power law, spatial clustering, observational bias, or outliers in the sample (e.g. an excessive DMHost as recently found for FRB 20190520B). The energy distribution in this regime is unlikely to be adequately constrained until further events are detected. Modelling suggests that FAST may be well placed to discriminate between redshift evolutionary models and to probe the helium ionization signal of the intergalactic medium.

Baseline correction for FAST radio recombination lines: A modified penalised least squares smoothing technique

Abstract A pilot project has been proceeded to map $1\, \textrm{deg}^2$ on the Galactic plane for radio recombination lines (RRLs) using the Five-hundred-metre Aperture Spherical Telescope (FAST). The motivation is to verify the techniques and reliabilities for a large-scale Galactic plane RRL survey with FAST aiming to investigate the ionised environment in the Galaxy. The data shows that the bandpass of the FAST 19 beam L-band is severely affected by radio frequency interferences and standing wave ripples, which can hardly be corrected by traditional low order polynomials. In this paper, we investigate a series of penalised least square (PLS) based baseline correction methods for radio astronomical spectra that usually contain weak signals with high level of noise. Three promising penalised least squares based methods, AsLS, arPLS, and asPLS are evaluated. Adopting their advantages, a modified method named rrlPLS is developed to optimise the baseline fitting to our RRL spectra. To check their effectiveness, the four methods are tested by simulations and further verified using observed data sets. It turns out that the rrlPLS method, with optimised parameter $\lambda=2\times10^8$ , reveals the most sensitive and reliable emission features in the RRL map. By injecting artificial line profiles into the real data cube, a further evaluation of profile distortion is conducted for rrlPLS. Comparing to simulated signals, the processed lines with low signal-to-noise ratio are less affected, of which the uncertainties are mainly caused by the rms noise. The rrlPLS method will be applied for baseline correction in future data processing pipeline of FAST RRL survey. Configured with proper parameters, the rrlPLS technique verified in this work may also be used for other spectroscopy projects.

The FAST Ultra-Deep Survey (FUDS): Observational strategy, calibration and data reduction

Abstract The FAST Ultra-Deep Survey (FUDS) is a blind survey that aims for the direct detection of H i in galaxies at redshifts $z<0.42$ . The survey uses the multibeam receiver on the Five-hundred-metre Aperture Spherical Telescope (FAST) to map six regions, each of size $0.72\ \textrm{deg}^2$ at high sensitivity ( ${\sim}50\,\mu \textrm{Jy}$ ) and high-frequency resolution (23 kHz). The survey will enable studies of the evolution of galaxies and their H i content with an eventual sample size of ${\sim}1\,000$ . We present the science goals, observing strategy, the effects of radio frequency interference at the FAST site, our mitigation strategies and the methods for calibration, data reduction and imaging as applied to initial data. The observations and reductions for the first field, FUDS0, are completed, with around 128 H i galaxies detected in a preliminary analysis. Example spectra are given in this paper, including a comparison with data from the overlapping GAL2577 field of Arecibo Ultra-Deep Survey.

Principles of Gravitational-Wave Detection with Pulsar Timing Arrays

Pulsar timing uses the highly stable pulsar spin period to investigate many astrophysical topics. In particular, pulsar timing arrays make use of a set of extremely well-timed pulsars and their time correlations as a challenging detector of gravitational waves. It turns out that pulsar timing arrays are particularly sensitive to ultra-low-frequency gravitational waves, which makes them complementary to other gravitational-wave detectors. Here, we summarize the basics, focusing especially on supermassive black-hole binaries and cosmic strings, which have the potential to form a stochastic gravitational-wave background in the pulsar timing array detection band, and the scientific goals on this challenging topic. We also briefly outline the recent interesting results of the main pulsar timing array collaborations, which have found strong evidence of a common-spectrum process compatible with a stochastic gravitational-wave background and mention some new perspectives that are particularly interesting in view of the forthcoming radio observatories such as the Five hundred-meter Aperture Spherical Telescope, the MeerKAT telescope, and the Square Kilometer Array.