arXiv subjects

J. Tian

Publications and source records attributed to J. Tian.

At least 19 recordsLinked to original sources

arXiv2026

Discovery of 27 new Rotating Radio Transients by MeerTRAP

We present the discovery of 27 new Galactic transients made by the commensal MeerTRAP single pulse search programme at the MeerKAT telescope. We determine the location of 14 of these discoveries with arcsecond accuracy by imaging the data captured in the dedicated transient buffer. A preliminary estimate of the period was made for 8 sources using the arrival times of several pulses detected by MeerTRAP. The periods of these transients range between 0.78 s and 4 s. For the previously published MeerTRAP source MTP0040 (PSR J1357-6507), we are able to provide the position and period using new pulses detected since it was reported. We also estimate the radio fluences and discuss the burst rates derived from the commensal search detections. The accurate image-domain localizations were used to conduct follow-up observations and timing analysis. Furthermore, some follow-up observations revealed ordinary pulsar-like pulsed behavior for four of the new sources, illustrating the blurred separation between rotating radio transients and canonical pulsars. We also present coherent timing solutions for 4 sources, providing insight into their rotational properties and their place within the Galactic neutron star population.

astro-ph.HE↗

arXiv2026

A bright wideband radio burst from the isolated neutron star 2XMM J104608.7$-$594306

We present the discovery of a second coherent radio burst from the thermally emitting neutron star 2XMM J104608.7$-$594306 in our follow-up observations with the Murriyang Ultra-Wideband Low receiver. This burst shows complex morphology with multiple components and wideband emission spanning from 704 to 4032MHz. We measured a steep spectral index of $\alpha=-2.18\pm0.16$. Our polarimetric analysis demonstrates that the burst is highly polarised with a linear and circular polarisation fraction of 54% and 22%, respectively. We identified an orthogonal jump in the polarisation position angles of the burst, resembling those seen in radio pulsars. We compared this burst with the first radio burst detected from the source with MeerKAT. These two bursts detected in a total of 40 hours on source with MeerKAT and Murriyang, combined, show that 2XMM J104608.7$-$594306 can emit sporadic radio emission with luminosity jumps comparable to those seen in the bright bursts from SGR 1935+2154. This suggests that previously thought radio-quiet neutron stars such as X-ray dim isolated neutron stars and central compact objects could exhibit rare radio bursting activity.

astro-ph.HE↗

arXiv2025

Discovery of 30 Galactic radio transient pulsars with MeerTRAP

We present the discovery of 30 new Galactic sources from the MeerTRAP project, a commensal fast radio transient search programme using the MeerKAT telescope. These sources were all identified via a single pulse search. Most of them are likely to be rotating radio transients (RRATs) given their low pulse rates. Using data captured in our transient buffer we have localised nine sources in the image domain to arcsecond precision. This facilitates the timing of these sources and further follow-up with other telescopes. Using the arrival times of single pulses, we have constrained the periods of 14 sources, ranging from 121ms to 7.623s, and derived a phase-coherent timing solution for one of them. Follow-up observations of the MeerTRAP sources (including those published previously) performed with the Effelsberg telescope have detected regular but faint emission from three sources, confirming their long rotation period, including PSR J2218+2902 with a period of 17.5s, the fourth slowest in the radio pulsar population. A few of the sources exhibit interesting emission features, such as periodic microstructure in PSR J1243-0435 and possible nulling in PSR J1911-2020 and PSR J1243-0435. We find that the duty cycles of the three newly discovered pulsars are very low and follow the general trend for the duty cycle with period of known pulsars.

astro-ph.HE↗

arXiv2025

TRAPUM search for pulsars in supernova remnants and pulsar wind nebulae - II. Survey analysis and population study

We present the second and final set of TRAPUM searches for pulsars at 1284 MHz inside supernova remnants and pulsar wind nebulae with the MeerKAT telescope. No new pulsars were detected for any of the 80 targets, which include some unidentified TeV sources that could be pulsar wind nebulae. The mean upper limit on the flux density of undetected pulsars is 52 $\mu$Jy, which includes the average sensitivity loss across the coherent beam tiling pattern. This survey is the largest and most sensitive multi-target campaign of its kind. We explore the selection effects that precluded discoveries by testing the parameters of the survey iteratively against many simulated populations of young pulsars in supernova remnants. For the synthetic pulsars that were undetected, we find evidence that, after beaming effects are accounted for, about 45 per cent of pulsars are too faint, 30 per cent are too smeared by scattering, and a further 25 per cent have a modelled projected location which places them outside their supernova remnant. The simulations are repeated for the S1 subband of the MeerKAT S-band receivers, resulting in a 50-150 per cent increase in the number of discoveries compared to L-band depending on the flux density limit achieved. Therefore, higher frequency searches that can also achieve improved flux density limits are the best hope for future targeted searches. We also report updated properties for the two previous discoveries, including a polarimetry study of PSR J1831$-$0941 finding a rotation measure of 401$\pm$1 rad m$^2$.

astro-ph.HE↗

arXiv2025

Graph Learning Metallic Glass Discovery from Wikipedia

Synthesizing new materials efficiently is highly demanded in various research fields. However, this process is usually slow and expensive, especially for metallic glasses, whose formation strongly depends on the optimal combinations of multiple elements to resist crystallization. This constraint renders only several thousands of candidates explored in the vast material space since 1960. Recently, data-driven approaches armed by advanced machine learning techniques provided alternative routes for intelligent materials design. Due to data scarcity and immature material encoding, the conventional tabular data is usually mined by statistical learning algorithms, giving limited model predictability and generalizability. Here, we propose sophisticated data learning from material network representations. The node elements are encoded from the Wikipedia by a language model. Graph neural networks with versatile architectures are designed to serve as recommendation systems to explore hidden relationships among materials. By employing Wikipedia embeddings from different languages, we assess the capability of natural languages in materials design. Our study proposes a new paradigm to harvesting new amorphous materials and beyond with artificial intelligence.

cs.LG↗

arXiv2025

Constructing material network representations for intelligent amorphous alloys design

Designing high-performance amorphous alloys is demanding for various applications. But this process intensively relies on empirical laws and unlimited attempts. The high-cost and low-efficiency nature of the traditional strategies prevents effective sampling in the enormous material space. Here, we propose material networks to accelerate the discovery of binary and ternary amorphous alloys. The network topologies reveal hidden material candidates that were obscured by traditional tabular data representations. By scrutinizing the amorphous alloys synthesized in different years, we construct dynamical material networks to track the history of the alloy discovery. We find that some innovative materials designed in the past were encoded in the networks, demonstrating their predictive power in guiding new alloy design. These material networks show physical similarities with several real-world networks in our daily lives. Our findings pave a new way for intelligent materials design, especially for complex alloys.

cond-mat.mtrl-sci↗

arXiv2025

MeerKAT discovery of a hyperactive repeating fast radio burst source

We present the discovery and localisation of a repeating fast radio burst (FRB) source from the MeerTRAP project, a commensal fast radio transient search programme using the MeerKAT telescope. FRB 20240619D was first discovered on 2024 June 19 with three bursts being detected within two minutes in the MeerKAT L-band (856 - 1712MHz). We conducted follow-up observations of FRB 20240619D with MeerKAT using the Ultra-High Frequency (UHF; 544 - 1088MHz), L-band and S-band (1968 - 2843MHz) receivers one week after its discovery, and recorded a total of 249 bursts. The MeerKAT-detected bursts exhibit band-limited emission with an average fractional bandwidth of 0.31, 0.34 and 0.48 in the UHF, L-band and S-band, respectively. We find our observations are complete down to a fluence limit of ~1Jy ms, above which the cumulative burst rate follows a power law $R (>F)\propto (F/1\,\text{Jy}\,\text{ms})^\gamma$ with $\gamma=-1.6\pm0.1$ and $-1.7\pm0.1$ in the UHF and L-band, respectively. The near-simultaneous L-band, UHF and S-band observations reveal a frequency dependent burst rate with $3\times$ more bursts being detected in the L-band than in the UHF and S-band, suggesting a spectral turnover in the burst energy distribution of FRB 20240619D. Our polarimetric analysis demonstrates that most of the bursts have $\sim100\%$ linear polarisation fractions and $\sim10\%\text{--}20\%$ circular polarisation fractions. We find no optical counterpart of FRB 20240619D in the MeerLICHT optical observations simultaneous to the radio observations and set a fluence upper limit in MeerLICHT's q-band of 0.76Jy ms and an optical-to-radio fluence ratio limit of 0.034 for a 15s exposure.

astro-ph.HE↗

arXiv2025

Slow and steady: long-term evolution of the 76-second pulsar J0901$-$4046

PSR J0901$-$4046, a likely radio-loud neutron star with a period of 75.88 seconds, challenges conventional models of neutron star radio emission. Here, we showcase results from 46 hours of follow-up observations of PSR J0901$-$4046 using the MeerKAT, Murriyang, GMRT, and MWA radio telescopes. We demonstrate the intriguing stability of the source's timing solution over more than three years, leading to an RMS arrival-time uncertainty of just $\sim$10$^{-4}$ of the rotation period. Furthermore, non-detection below 500 MHz may indicate a low-frequency turnover in the source's spectrum, while no secular decline in the flux density of the source over time, as was apparent from previous observations, has been observed. Using high time-resolution MeerKAT data, we demonstrate two distinct quasi-periodic oscillation modes present in single pulses, with characteristic time scales of 73 ms and 21 ms. We also observe a statistically significant change in the relative prevalence of distinct pulse morphologies compared to previous observations, possibly indicating a shift in the magnetospheric composition over time. Finally, we show that the W$_{50}$ pulse width is nearly constant from 544-4032 MHz, consistent with zero radius-to-frequency mapping. The very short duty cycle ($\sim$1.4$^{\circ}$) is more similar to radio pulsars with periods $>$5 seconds than to radio-loud magnetars. This, along with the lack of magnetar-like outbursts or timing glitches, complicates the identification of the source with ultra-long period magnetar models.

astro-ph.HE↗

arXiv2025

A coherent radio burst from an X-ray neutron star in the Carina Nebula

The neutron star zoo comprises several sub-populations that range from energetic magnetars and thermally emitting X-ray neutron stars to radio-emitting pulsars. Despite studies over the last five decades, it has been challenging to obtain a clear physical link between the various populations of neutron stars, vital to constrain their formation and evolutionary pathways. Here we report the detection of a burst of coherent radio emission from a known radio-quiet, thermally emitting neutron star 2XMM J104608.7$-$594306in the Carina Nebula. The burst has a distinctive sharp rise followed by a decay made up of multiple components, which is unlike anything seen from other radio-emitting neutron stars. It suggests an episodic event from the neutron star surface, akin to transient radio emission seen from magnetars. The radio burst confirms that the X-ray source is a neutron star and suggests a new link between these apparently radio-quiet X-ray emitting sources and other transient or persistent radio-emitting neutron stars. It also suggests that a common physical mechanism for emission might operate over a range of magnetic field strengths and neutron star ages. We propose that 2XMM J104608.7$-$594306 straddles the boundary between young, energetic neutron stars and their evolved radio-emitting cousins and may bridge these two populations. The detection of such a radio burst also shows that other radio-quiet neutron stars may also emit such sporadic radio emission that has been missed by previous radio surveys and highlights the need for regular monitoring of this unique sub-population of neutron stars.

astro-ph.HE↗

arXiv2025

Contemporaneous optical-radio observations of a fast radio burst in a close galaxy pair

We present the MeerKAT discovery and MeerLICHT contemporaneous optical observations of the Fast Radio Burst (FRB) 20230808F, which was found to have a dispersion measure of $\mathrm{DM}=653.2\pm0.4\mathrm{\,pc\,cm^{-3}}$. FRB 20230808F has a scattering timescale $\tau_{s}=3.1\pm0.1\,\mathrm{ms}$ at $1563.6$ MHz, a rotation measure $\mathrm{RM}=169.4\pm0.2\,\mathrm{rad\,m^{-2}}$, and a radio fluence $F_{\mathrm{radio}}=1.72\pm0.01\,\mathrm{Jy\,ms}$. We find no optical counterpart in the time immediately after the FRB, nor in the three months after the FRB during which we continued to monitor the field of the FRB. We set an optical upper flux limit in MeerLICHT's $q$-band of $11.7\,\mathrm{\mu Jy}$ for a 60 s exposure which started $\sim3.4$ s after the burst, which corresponds to an optical fluence, $F_{\mathrm{opt}}$, of $0.039\,\mathrm{Jy\,ms}$ on a timescale of $\sim3.4$ s. We obtain an estimate for the $q-$band luminosity limit of $vL_{v}\sim 1.3\times10^{43}\,\mathrm{erg\,s^{-1}}$. We localise the burst to a close galaxy pair at a redshift of $z_{\mathrm{spec}}=0.3472\pm0.0002$. Our time delay of $\sim3.4$ s between the FRB arrival time and the start of our optical exposure is the shortest ever for an as yet non-repeating FRB, and hence the closest to simultaneous optical follow-up that exists for such an FRB.

astro-ph.HE↗

arXiv2024

PyMatterSim: a Python Data Analysis Library for Computer Simulations of Materials Science, Physics, Chemistry, and Beyond

Computer simulation has become one of the most important tools in scientific research in many disciplines. Benefiting from the dynamical trajectories regulated by versatile interatomic interactions, various material properties can be quantitatively characterized at the atomic scale. This greatly deepens our understanding of Nature and provides incredible insights supplementing experimental observations. Hitherto, a plethora of literature discusses the computational discoveries in studying glasses in which positional disorder is inherent in their configurations. Motivated by active research and knowledge sharing, we developed a data analysis library in Python for computational materials science research. We hope to help promote scientific progress and narrow some technical gaps for the wide communities. The toolkit mainly focuses on physical analyses of glassy properties from the open-source simulator LAMMPS. Nevertheless, the code design renders high flexibility, with functionalities extendable to other computational tools. The library provides data-driven insights for different subjects and can be incorporated into advanced machine-learning workflows. The scope of the data analysis methodologies applies not only to materials science but also to physics, chemistry, and beyond.

cond-mat.mtrl-sci↗

arXiv2024

Detection and localisation of the highly active FRB 20240114A with MeerKAT

We report observations of the highly active FRB 20240114A with MeerKAT using the Ultra-High Frequency (UHF; $544\text{--}1088$ MHz) and L-band ($856\text{--}1712$ MHz) receivers. A total of 62 bursts were detected in coherent tied-array beams using the MeerTRAP real-time transient detection pipeline. We measure a structure-optimising dispersion measure of $527.65\pm0.01\,\text{pc}\,\text{cm}^{-3}$ using the brightest burst in the sample. We find the bursts of FRB 20240114A are generally detected in part of the broad band of MeerKAT, $\sim40\%$ in the UHF and $\sim30\%$ in the L-band, indicating the band limited nature. We analyse the fluence distribution of the 44 bursts detected at UHF, constraining the fluence completeness limit to $\sim1\,$Jy ms, above which the cumulative burst rate follows a power law $R (>F)\propto (F/1\,\text{Jy}\,\text{ms})^\gamma$ with $\gamma=-1.8\pm0.2$. Using channelised telescope data captured in our transient buffer we localise FRB 20240114A in the image domain to RA = 21h27m39.86s, Dec = +04d19m45.01s with an uncertainty of 1.4 arcsec. This localisation allows us to confidently identify the host galaxy of FRB 20240114A. Also using the transient buffer data we perform a polarimetric study and demonstrate that most of the bursts have $\sim100\%$ linear polarisation fractions and up to $\sim20\%$ circular polarisation fractions. Finally, we predict the flux density of a potential persistent radio source (PRS) associated with FRB 20240114A is $\backsimeq[0.6\text{--}60]\,\mu\text{Jy}$ based on the simple relation between the luminosity of the PRS and the rotation measure arising from the FRB local environment.

astro-ph.HE↗

arXiv2023

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 $deg^2$ , 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.

astro-ph.HE↗

arXiv2023

MWA rapid follow-up of gravitational wave transients: prospects for detecting prompt radio counterparts

We present and evaluate the prospects for detecting coherent radio counterparts to gravitational wave (GW) events using Murchison Widefield Array (MWA) triggered observations. The MWA rapid-response system, combined with its buffering mode ($\sim4$ minutes negative latency), enables us to catch any radio signals produced from seconds prior to hours after a binary neutron star (BNS) merger. The large field of view of the MWA ($\sim1000\,\text{deg}^2$ at 120\,MHz) and its location under the high sensitivity sky region of the LIGO-Virgo-KAGRA (LVK) detector network, forecast a high chance of being on-target for a GW event. We consider three observing configurations for the MWA to follow up GW BNS merger events, including a single dipole per tile, the full array, and four sub-arrays. We then perform a population synthesis of BNS systems to predict the radio detectable fraction of GW events using these configurations. We find that the configuration with four sub-arrays is the best compromise between sky coverage and sensitivity as it is capable of placing meaningful constraints on the radio emission from 12.6\% of GW BNS detections. Based on the timescales of four BNS merger coherent radio emission models, we propose an observing strategy that involves triggering the buffering mode to target coherent signals emitted prior to, during or shortly following the merger, which is then followed by continued recording for up to three hours to target later time post-merger emission. We expect MWA to trigger on $\sim5\text{--}22$ BNS merger events during the LVK O4 observing run, which could potentially result in two detections of predicted coherent emission.

astro-ph.HE↗

arXiv2022

Report of the Topical Group on Higgs Physics for Snowmass 2021: The Case for Precision Higgs Physics

A future Higgs Factory will provide improved precision on measurements of Higgs couplings beyond those obtained by the LHC, and will enable a broad range of investigations across the fields of fundamental physics, including the mechanism of electroweak symmetry breaking, the origin of the masses and mixing of fundamental particles, the predominance of matter over antimatter, and the nature of dark matter. Future colliders will measure Higgs couplings to a few per cent, giving a window to beyond the Standard Model (BSM) physics in the 1-10 TeV range. In addition, they will make precise measurements of the Higgs width, and characterize the Higgs self-coupling. This report details the work of the EF01 and EF02 working groups for the Snowmass 2021 study.

hep-ph↗

arXiv2022

A targeted search for repeating fast radio bursts with the MWA

We present a targeted search for low-frequency (144--215\,MHz) FRB emission from five repeating FRBs using 23.3\,hr of archival data taken with the Murchison Widefield Array (MWA) Voltage Capture System (VCS) between 2014 September and 2020 May. This is the first time that the MWA VCS has been used to search for FRB signals from known repeaters, which enables much more sensitive FRB searches than previously performed with the standard MWA correlator mode. We performed a standard single pulse search with a temporal and spectral resolution of $400\,μ$s and 10\,kHz, respectively, over a $100\,\text{pc}\,\text{cm}^{-3}$ dispersion measure (DM) range centred at the known DM of each studied repeating FRB. No FRBs exceeding a $6σ$ threshold were detected. The fluence upper limits in the range of 32--1175\,Jy\,ms and 36--488\,Jy\,ms derived from 10 observations of FRB 20190711A and four observations of FRB 20201124A respectively, allow us to constrain the spectral indices of their bursts to $\gtrsim-1$ if these two repeaters were active during the MWA observations. If free-free absorption is responsible for our non-detection, we can constrain the size of the absorbing medium in terms of the electron temperature $T$ to $<1.00\times(T/10^4\text{K})^{-1.35}\,\text{pc}$, $<0.92\times(T/10^4\text{K})^{-1.35}\,\text{pc}$ and $<[0.22\text{--}2.50]\times(T/10^4\text{K})^{-1.35}\,\text{pc}$ for FRB 20190117A, 20190711A, and 20201124A, respectively. However, given that the activities of these repeaters are not well characterised, our non-detections could also suggest they were inactive during the MWA observations.

astro-ph.HE↗

arXiv2022

Evaluation of different recipes for chromospheric radiative losses in solar flares

Context. Radiative losses are an indispensable part in the numerical simulation of flares. Detailed calculations could be computationally expensive, especially in the chromosphere. There have been some approximate recipes for chromospheric radiative losses in flares, yet their feasibility in flare simulations needs further evaluation. Aims. We aim to evaluate the performance of different recipes for chromospheric radiative losses in flare simulations. Methods. We compare the atmospheric structure and line profiles in beam-heated flares calculated with detailed radiative losses and the approximate recipes. Results. Both GF90 and HCD22 recipes provide acceptable total radiative losses compared with detailed one, but there are discrepancies in the different atmospheric layers during the different evolutionary phases, which leads to misestimations of temperature and line intensity. The recipe of GF90 overestimates the coolings in the upper chromosphere greatly when temperature exceeds 10^5 K, which also affects the flare evolution and line asymmetries. Radiative heating in the middle chromosphere only functions in the initial stage and could be safely neglected. However, radiative heating from Lyman continuum could dominate near the transition region.

astro-ph.SR↗

arXiv2022

A novel technique for the measurement of the avalanche fluctuations of a GEM stack using a gating foil

We have developed a novel technique for the measurement of the size of avalanche fluctuations of gaseous detectors using a gating device (gating foil) prepared for the time projection chamber in the international linear collider experiment (ILD-TPC). In addition to the gating function, the gating foil is capable of controlling the average fraction of drift electrons detected after gas amplification. The signal charge width and shape (skewness) for electron-ion pairs created by a pulsed UV laser as a function of the transmission rate of the gating foil can be used to determine the relative variance of gas gain for single electrons. We present the measurement principle and the result obtained using a stack of gas electron multipliers (GEMs) operated in a gas mixture of Ar-CF$_4$(3%)-isobutane(2%) at atmospheric pressure. Also discussed is the influence of the avalanche fluctuations on the spatial resolution of the ILD-TPC.

physics.ins-det↗