Aperture Spherical Radio Telescope Research Articles

Narrowly Banded Spectra with Peak Frequency around 1 GHz of FRB 20201124A: Implications for Energy Function and Radiation Physics

The radiation physics of fast radio bursts (FRBs) remains an open question. Current observations have discovered that narrowly banded bursts of FRB 20201124A are active in 0.4–2 GHz, and their spectral peak frequency ( νpobs ) is mostly toward ∼1 GHz. Utilizing a sample of 1268 bursts of FRB 20201124A detected with the Five-hundred-meter Aperture Spherical radio Telescope (FAST), we show that the 1σ spectral regime of 71.4% of the events (in-band bursts) is within the FAST bandpass. The intrinsic burst energies ( EBWeobs ) and spectral widths ( σsobs ) are well measured by fitting the spectral profile with a Gaussian function. The derived EBWeobs and σsobs distributions are lognormal and centered at logEBWeobs/erg=37.2(σ=0.76) and logσsobs/GHz=−1.16(σ=0.17) . Our Monte Carlo simulation analysis infers its intrinsic ν p distribution as a normal function centered at ν p,c = 1.16 GHz (σ = 0.22) and its intrinsic energy function as Φ(E)∝E−0.60e−E/Ec with E c = 9.49 × 1037 erg. We compare these results with that of typical repeating FRBs 20121102A and 20190520B, which are active over a broad frequency range at several specific frequencies, and discuss possible observational biases on the estimation of the event rate and energy function. Based on these results, we argue that FRB 20201124A likely occurs in a fine-tuned plasma for maser radiation at a narrow frequency range, while FRB 20121102A and FRB 20190520B could involve clumpy plasma conditions that make maser emission around several specific frequencies in a broad range.

Time-domain analysis of multi-waveband flares from AD leonis

Abstract Radio bursts of magnetically active stars reveal the intensity and activity of the stellar magnetic field. They may also be related to the planets around the stars. We monitored a radio-active star, AD Leonis, 3000 seconds per day for 17 days in November 2020, and 5000 seconds per day for 5 days in July 2023 with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Based on the simultaneous flux increases in Stokes I and Stokes V, one left-hand circular polarized radio burst is identified. The $\sim 50~{{\%}}$ degree of circular polarization indicates the burst being originated from non-thermal radiation related to the stellar magnetic field. Combining the newly discovered burst with previous observations of radio and X-ray bursts from AD Leonis, we did a periodicity analysis for the 49 bursts in total. No periodicity with confidence level >3σ is found, while a candidate period of 3.04 days at ≈2σ confidence level is presented and discussed. Results of recent FAST observations and the periodicity analysis suggest a more compact campaign of observation toward this source, from which a more optimistic result of period search could be achieved.

The Relativistic Spin Precession in the Compact Double Neutron Star System PSR J1946+2052

Abstract We observe systematic profile changes in the visible pulsar of the compact double neutron star system PSR J1946+2052 using observations with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The interpulse of PSR J1946+2052 changed from a single-peak to a double-peak shape from 2018–2021. We attribute this evolution as the result of the relativistic spin precession of the pulsar. With the high sensitivity of FAST, we also measure significant polarization for the first time, allowing us to model this with the precessional rotating vector model. Assuming, to the first order, a circular hollow-cone-like emission beam pattern and taking the validity of general relativity (GR), we derive the binary’s orbital inclination angle ( 63 ∘ − 3 ∘ + 5 ∘ ) and pulsar’s spin geometry. The pulsar’s spin vector and the orbital angular momentum vector are found to be only slightly misaligned ( 0. ∘ 21 − 0. ∘ 10 + 0. ∘ 28 ). The quoted uncertainties do not reflect the systematic uncertainties introduced by our model assumptions. By simulating future observations of profile and polarization evolution, we estimate that we could constrain the precession rate within a 43% uncertainty in 9 yr. Hence, we suggest that the system’s profile evolution could be combined with precise pulsar timing to test GR in the future.

Addition of the Local Volume sample of galaxies from the FAST HI survey

We report the discovery of 20 new dwarf galaxies in the Local Volume identified as optical counterparts to the Five-hundred-meter Aperture Spherical radio Telescope (FAST) All Sky HI Survey (FASHI) sources. The galaxies have a median stellar mass of $7.8 odot $ and a median HI mass of $1.0 odot $. Most of them are field galaxies, while three are probable members of the M\,101 and M\,106 groups. We also found seven FASHI radio sources to be probable dark HI clouds in nearby groups. Together with four other known HI clouds in the local groups, their mean-square radial velocity difference of 49 km $ with respect to the host galaxies yields an average total mass of ($2.7 M_ odot $ for these groups on the projected scale of 90 kpc.

Constraining axion-like particles dark matter in Coma Berenices with FAST

Axions and axion-like particles (ALPs) appear in many extensions of the Standard Model and are being investigated as promising dark matter (DM) candidates. One viable methodology for their detection involves the investigation of the line-like radio emissions from the dwarf spheroidal galaxy, potentially originating from the radiative decay of ALPs or the conversion of ALPs in the magnetic field. In this work, we constrain the properties of ALPs using the 2-hour radio observation of Coma Berenices through the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The 95% upper limits of the ALP-photon coupling are calculated for the ALP decay and conversion scenarios, respectively. Note that the sensitive ALP masses for FAST range from ∼μeV to tens of μeV, where ALP can explain the DM abundance naturally. However, our limits are weaker than those of the CAST helioscope, which can provide an independent and complementary check on the ALP non-detection for ground experiments. Furthermore, we evaluate the expected sensitivity on the ALP of FAST with its full designed bandwidth (70 MHz - 3 GHz) for 100 hours of observation time. Our results indicate that, even with the exceptional sensitivity of the FAST, it is challenging to surpass the existing experimental constraints on ALP DM using radio observation of dSphs, unless the possible enhancements of ALP signals by compact stars in dSphs are considered.

Dwarf Pulses of 10 Pulsars Detected by FAST

How pulsars radiate is a long-standing problem. Detailed polarization measurements of individual pulses shed light on currently unknown emission processes. Recently, based on supersensitive observations, dwarf pulses have been recognized as weak narrow pulses often appearing during the nulling state. In this study, we report the detection of dwarf pulses from 10 pulsars, PSRs B0525+21, B1237+25, J1538+2345, J1824−0127, J1851−0053, B1901+10, J1939+10, B1944+17, B2000+40, and J2112+4058, based on observations conducted with the Five-hundred-meter Aperture Spherical radio Telescope. Dwarf pulses of five pulsars are clearly discernible in the two-dimensional distribution of pulse intensity and pulse width. For the other five pulsars, PSRs J1538+2345, J1824−0127, J1939+10, B2000+40, and J2112+4058, only a few dwarf pulses are detected from pulse stacks. The dwarf pulses can emerge in both cone and core emission components for PSR B1237+25, and the polarization angles of these dwarf pulses are mostly in the orthogonal polarization mode of normal pulses for PSR B1944+17. In general, pulsars with detected dwarf pulses tend to be located within the “death valley” region of the distribution of pulsar periods and period derivatives.

Investigation of Profile Shifting and Subpulse Movement in PSR J0344-0901 with FAST

We report two phenomena detected in PSR J0344−0901 from two observations conducted at frequencies centered at 1.25 GHz using the Five-hundred-meter Aperture Spherical Radio Telescope. The first phenomenon manifests as the pulse emission shifting to later longitudinal phases and then gradually returning to its original location. The event lasts for about 216 pulse periods, with an average shift of about 0.°7 measured at the peak of the integrated profile. Changes in the polarization position angle (PPA) are detected around the trailing edge of the profile, together with an increase in the profile width. The second phenomenon is characterized by the apparent movement of subpulses, which results in different subpulse track patterns across the profile window. For the first time in this pulsar, we identify four emission modes, each with unique subpulse movement, and determine the pattern periods for three of them. Pulse nulling was not detected. Modeling of the changes in the PPA using the rotating vector model gives an inclination angle of 75.°12 ± 3.°80 and an impact parameter of −3.°17 ± 5.°32 for this pulsar. We speculate that the subpulse movement may be related to the shifting of the pulse emission.

Periodicity of sub-pulses in a radio pulsar

Pulsars are known to manifest complex phenomena, such as nulling, sub-pulse drifting, and periodicity. Within the purview of this investigation, we have harnessed the wavelet analysis technique to scrutinize the multifaceted periodicities and sub-pulse drifting characteristics exhibited by PSR J1926-0652, discovered by the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Our analysis draws upon the rich dataset acquired from the FAST ultra-wide-bandwidth receiver (UWB), meticulously examining pulse attributes encompassing an entire pulse. It is notable that the pulse apex recurrently manifests approximately every 17.11 times the pulsar's period P, individual sub-pulses exhibit a drifting phenomenon, with a phase decrement of approximately 1.04∘ over each P. Intriguingly, the central phase of each sub-pulse track gradually increments over temporal evolution. Furthermore, the relative offset distribution between successive sub-pulse tracks emanating from the trailing and leading components remains comparatively stable, with a central tendency of approximately ∼6.87±2.56 P. Most notably, derived from the outcomes of wavelet analysis, we ascertain a negative correlation of -0.98 between the periods of sub-pulses and their drifting rates, alongside the intrinsic period of sub-pulses identified at 28.14 seconds.

Study on tiered storage algorithm based on heat correlation of astronomical data

With the surge in astronomical data volume, modern astronomical research faces significant challenges in data storage, processing, and access. The I/O bottleneck issue in astronomical data processing is particularly prominent, limiting the efficiency of data processing. To address this issue, this paper proposes a tiered storage algorithm based on the access characteristics of astronomical data. The C4.5 decision tree algorithm is employed as the foundation to implement an astronomical data access correlation algorithm. Additionally, a data copy migration strategy is designed based on tiered storage technology to achieve efficient data access. Preprocessing tests were conducted on 418GB NSRT (Nanshan Radio Telescope) formaldehyde spectral line data, showcasing that tiered storage can potentially reduce data processing time by up to 38.15%. Similarly, utilizing 802.2 GB data from FAST (Five-hundred-meter Aperture Spherical radio Telescope) observations for pulsar search data processing tests, the tiered storage approach demonstrated a maximum reduction of 29.00% in data processing time. In concurrent testing of data processing workflows, the proposed astronomical data heat correlation algorithm in this paper achieved an average reduction of 17.78% in data processing time compared to centralized storage. Furthermore, in comparison to traditional heat algorithms, it reduced data processing time by 5.15%. The effectiveness of the proposed algorithm is positively correlated with the associativity between the algorithm and the processed data. The tiered storage algorithm based on the characteristics of astronomical data proposed in this paper is poised to provide algorithmic references for large-scale data processing in the field of astronomy in the future.

Investigation of Nulling and Subpulse-drifting Properties of PSR J1649+2533 with FAST

We report the nulling and subpulse-drifting properties of PSR J1649+2533 with observations of the Five-hundred-meter Aperture Spherical Radio Telescope (FAST). The FAST observations reveal that the nulling fraction of this pulsar is about 20.9% ± 0.8% at 1250 MHz. The statistical study of the durations of the nulling and burst states shows that the burst states can persist for extended pulse periods, exceeding 100 periods in some cases, while the shortest lasts only a few pulse periods. The null states have a much shorter duration, with a maximum duration of less than 30 pulse periods. A comparative study between the pulse profiles of the first active pulse (FAP) and the last active pulse (LAP) shows that the pulse profiles of LAPs are stronger and wider than those of FAPs. An analysis of the two-dimensional fluctuation spectrum for the single-pulse stack indicates that the pulsar exhibits clear amplitude modulation and subpulse drifting. The periods are a P 3 = 2.5 ± 0.1 spin period and P 2 = 17.°0 ± 0.°5 at 1250 MHz, respectively. In addition, the multiband investigation shows that P 2 seems to increase with the increase of the observing frequency, i.e., P 2 ∝ ν 0.8±0.1. This seems to be caused by the increasing pulse-profile width with the frequency. The high-sensitivity FAST observations will enable a detailed understanding of the emission of this pulsar and provide important clues for theoretical studies of the radiation mechanism.

Upper Limits on the Radio Pulses from Magnetars and a Central Compact Object with FAST

Magnetars and central compact objects (CCOs) are subgroups of neutron stars that show a number of properties distinguished from canonical radio pulsars. We performed radio observations of three magnetars, SGR 0418+5729, 1E 2259+586, and 4U 0142+61, and a CCO, PSR J1852+0040, with the Five-hundred-meter Aperture Spherical radio Telescope at 1.25 GHz, aiming to search for radio pulsations in their quiescent states. During two observation epochs, no radio pulses have been detected toward any target above a significance of signal-to-noise ratio = 7 from either the direct folding or blind search. We provided the most stringent upper limit of radio flux (≲2–4 μJy) for the magnetars and the CCO. For the magnetars with long periods, the real upper limits are likely 1 order of magnitude larger due to the red noise. The deep radio observations suggest that these magnetars and the CCO are indeed radio-quiet sources or unfavorably beamed.

Pulse Jitter and Single-pulse Variability in Millisecond Pulsars

Understanding the jitter noise resulting from single-pulse phase and shape variations is important for the detection of gravitational waves using pulsar timing arrays. We present measurements of the jitter noise and single-pulse variability of 12 millisecond pulsars that are part of the International Pulsar Timing Array sample using the Five-hundred-meter Aperture Spherical radio Telescope. We find that the levels of jitter noise can vary dramatically among pulsars. A moderate correlation with a correlation coefficient of 0.57 between jitter noise and pulse width is detected. To mitigate jitter noise, we perform matrix template matching using all four Stokes parameters. Our results reveal a reduction in jitter noise ranging from 6.7% to 39.6%. By performing longitude-resolved fluctuation spectrum analysis, we identify periodic intensity modulations in 10 pulsars. In PSR J0030+0451, we detect single pulses with energies more than 10 times the average pulse energy, suggesting the presence of giant pulses. We also observe a periodic mode-changing phenomenon in PSR J0030+0451. We examine the achievable timing precision by selecting a subset of pulses with a specific range of peak intensity, but no significant improvement in timing precision is achievable.

A Relativistic Double Neutron Star Binary PSR J1846-0513

We report the timing analysis of PSR J1846−0513, a pulsar discovered by the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in Commensal Radio Astronomy FAST Survey. The pulsar possesses a spin period of 23.36 ms and a spin-down rate ( Ṗ ) of 1.0106(3) × 10−18 s s−1, and it is located in an eccentric orbit (e ∼0.208) with an orbital period of 0.61 days. The characteristic age and surface magnetic field of the pulsar are found to be 366.62 Myr and 4.9178 × 109 G, respectively, indicating that it is a recycled pulsar. Using over two years of timing data, we measure the periastron advance ω̇ = 0.8956(8) deg yr−1. By assuming that this effect is purely relativistic, we have estimated the total mass M = 2.6287(35)M ⊙ and obtained an upper limit for the pulsar mass and a lower limit for the companion’s mass. Our results indicate that this is a double neutron star system.

Applying Hybrid Clustering in Pulsar Candidate Sifting with Multi-modality for FAST Survey

Pulsar search is always the basis of pulsar navigation, gravitational wave detection and other research topics. Currently, the volume of pulsar candidates collected by the Five-hundred-meter Aperture Spherical radio Telescope (FAST) shows an explosive growth rate that has brought challenges for its pulsar candidate filtering system. Particularly, the multi-view heterogeneous data and class imbalance between true pulsars and non-pulsar candidates have negative effects on traditional single-modal supervised classification methods. In this study, a multi-modal and semi-supervised learning based on a pulsar candidate sifting algorithm is presented, which adopts a hybrid ensemble clustering scheme of density-based and partition-based methods combined with a feature-level fusion strategy for input data and a data partition strategy for parallelization. Experiments on both High Time Resolution Universe Survey II (HTRU2) and actual FAST observation data demonstrate that the proposed algorithm could excellently identify pulsars: On HTRU2, the precision and recall rates of its parallel mode reach 0.981 and 0.988 respectively. On FAST data, those of its parallel mode reach 0.891 and 0.961, meanwhile, the running time also significantly decreases with the increment of parallel nodes within limits. Thus, we can conclude that our algorithm could be a feasible idea for large scale pulsar candidate sifting for FAST drift scan observation.

Radio Pulsar B0950+08: Radiation in the Magnetosphere and Sparks above the Surface

We observed the nearby radio pulsar B0950+08, which has a 100% duty cycle, using the Five-hundred-meter Aperture Spherical Radio Telescope. We obtained the polarization profile for its entire rotation, which enabled us to investigate its magnetospheric radiation geometry and the sparking pattern of the polar cap. After we excluded part of the profile in which the linear polarization factor is low (≲30%) and potentially contaminated by jumps in position angle, the rest of the swing in polarization position angle fits a classical rotating vector model (RVM) well. The best-fit RVM indicates that the inclination angle, α, and the impact angle, β, of this pulsar, are 100.°5 and −33.°2, respectively, suggesting that the radio emission comes from two poles. We find that, in such RVM geometry, either the annular vacuum gap model or the core vacuum gap model would require that the radio emissions come from a high-altitude magnetosphere with heights from ∼0.25 R LC to ∼0.56 R LC, with R LC being the light cylinder radius. Both the main and interpulses’ sparking points are located away from the magnetic pole, which could relate to the physical conditions on the pulsar surface.

Studying the nulling, subpulse drifting, and moding in PSR J1921+1948 with the FAST

ABSTRACT We reported the observations of nulling, subpulse drifting, and moding of radio radiation in pulsar (PSR) B1918+19 at 1250 MHz with the Five-hundred-meter Aperture Spherical radio Telescope. The nulling fraction is 2.6 ± 0.1 per cent and no periodicity of nulling can be seen. We confirmed the existence of three different drift modes (A, B, C) and a disordered mode (N) at 1250 MHz. Unlike the other three modes where the second component of the average pulse profile dominates, the first component of the average pulse profile in mode C is brighter than the other components, the second component shifts forward and the fourth component shifts backward. The bidrifting phenomenon is observed in PSR B1918+19, the drifting of the first and second components is positive, and the drift direction of the fourth component is negative. The drifting rate of the drift bands composed of the first and second components has an obvious evolutionary trend. Mode B has a short duration and no clear trend can be seen. But for mode A, the drift rate of each drift band starts relatively fast, then slows down to a steady state, and finally increases slowly until it enters either null or mode N. Further analysis shows that the emergence and significant variation in the drifting period of multidrifting subpulse emission modes for PSR B1918+19 may be due to the aliasing effect. The interesting subpulse emission phenomenon of PSR B1918+19 at different frequencies provides a unique opportunity to understand the switching mechanism of the multidrift mode of the pulsars.

Discovery and timing of pulsar J2016+3711 in supernova remnant CTB 87 with FAST

ABSTRACT We report on our discovery of the radio pulsar, PSR J2016+3711, in supernova remnant (SNR) CTB 87, with a ∼10.8σ significance of pulses, which confirms the compact nature of the X-ray point source in CTB 87. It is the first pulsar discovered in SNRs using Five-hundred-meter Aperture Spherical radio Telescope (FAST). Its integrated radio pulse profile can be well described by a single component, with a width at 50 per cent of the peak flux density of about 28.1○ and an effective width of about 32.2○. The mean flux density at 1.25 GHz is estimated to be about 15.5 $\mu$Jy. Combined with the non-detection of the radio pulse at lower frequencies, the radio spectral index of the pulsar is constrained to be ≲2.3. We also present the timing solution based on 28 follow-up FAST observations. Our results reveal a period of 50.81 ms, period derivative of 7.2 × 10−14 s s−1, and dispersion measure of 428 pc cm−3. The strength of the equatorial surface magnetic dipole magnetic field is inferred to be about 1.9 × 1012 G. Using the ephemeris obtained from the radio observations, we searched Fermi-LAT data for gamma-ray pulsations but detected no pulsed signal. We also searched for radio pulses with FAST towards the X-ray counterpart of the gamma-ray binary HESS J1832−093 proximate to SNR G22.7−00.2 but found no signal.

Cover Picture: Astron. Nachr. 2/2024

It has been proposed that the building blocks of pulsar matter could be ‘an’, strangeons analogy of atomic nucleons but with negative strangeness. A bare strangeon star should not have a smooth surface, but be covered with small hills (‘zits’), as indicated by red spots in this schematic illustration. These zits on the pulsar surface might be responsible for the magnetospheric activity and the mysterious coherent radio emission, as discussed in the contribution by Xu and Wang, this issue, p. e230153. Zits may also be responsible for the significantly non‐symmetrical sparking discovered in the 110‐min polarization observation of pulsar PSR B0950+08 targeted with China's Five‐hundred‐meter Aperture Spherical radio Telescope, FAST. Details can be found in the contribution by Wang et al., this issue, e240010. image

Non‐symmetrical sparking may hint “zits” on a pulsar surface

AbstractPulsar electrodynamics could be relevant to the physics of stellar surface, which remains poorly understood for more than half a centenary and is difficult to probe due to the absence of direct and clear observational evidence. Nevertheless, highly sensitive telescopes (e.g., China's Five‐hundred‐meter Aperture Spherical radio Telescope, FAST) may play an essential role to solve the problem since the predicted surface condition would have quite different characteristics in some models of pulsar structure, especially after the establishment of the standard model of particle physics. For instance, small hills (or “zits”) may exist on solid strangeon star surface with rigidity, preferential discharge, that is, gap sparking, may occur around the hills in the polar cap region. In this work, with the 110‐min polarization observation of PSR B095008 targeted by FAST, we report that the gap sparking is significantly non‐symmetrical to the meridian plane on which the rotational and magnetic axes lie. It is then speculated that this asymmetry could be the result of preferential sparking around zits which might rise randomly on pulsar surface. Some polarization features of both single pulses and the mean pulse, as well as the cross‐correlation function of different emission regions, have also been presented.

Individual subpulses of PSR B1916+14 and their polarization properties

ABSTRACT The individual subpulses of pulsars are regarded as their basic emission components, providing invaluable information for our understanding of the radio emission process in the pulsar magnetosphere. Nevertheless, subpulses are overlapped with each other along the rotation phase for most pulsars, making it difficult to study the statistical properties of subpulses. Among the pulsars observed by the Five-hundred-meter Aperture Spherical radio Telescope (FAST), PSR B1916+14 has a large number of isolated well-resolved subpulses in the high-time-resolution observations, having a typical width of 0.15 ms and a high linear polarization. We find that the number distribution of subpulses contributes dominantly to the mean profile. According to the emission geometry, these emission units come from a region roughly 155 km above the polar cap in the pulsar magnetosphere, and the length-scale of the basic emission units is approximately 120 m. The deviations of the polarization position angles for these single subpulses from the standard S-shaped curve are closely related to their fractional linear and circular polarization, and the large deviations tend to come from drifting subpulses.