Telescope Array Research Articles

Complete X-ray census of M dwarfs in the solar neighborhood

Context. X-ray emission is the most sensitive diagnostic of magnetic activity in M dwarfs and, hence, of the dynamo in low-mass stars. Moreover it is crucial for quantifying the influence of the stellar irradiation on the evolution of planet atmospheres. Aims. We have embarked in a systematic study of the X-ray emission in a volume-limited sample of M dwarf stars to explore the full range of activity levels present in their coronae. We look to obtain a better understanding of the conditions in their outer atmospheres and their possible impact on the circumstellar environment. Methods. Based on a recent catalog of Gaia objects within 10 pc from the Sun, we identified all its stars with spectral types between M0 and M4 and we carried out a systematic search for X-ray measurements of this sample. To this end, we used both archival data from ROSAT, XMM-Newton, and the ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Russian Spektrum-Roentgen-Gamma mission, as well as our own dedicated XMM-Newton observations. To make inferences on the properties of the M dwarf corona, we compared the range of their observed X-ray emission levels to the flux radiated by the Sun from different types of magnetic structures: coronal holes, background corona, active regions, and cores of active regions. In this work, we focus on the properties of stars with the faintest X-ray emission. Results. At the current state of our project, with more than 90% of the 10 pc M dwarf sample observed in the X-ray, there is only one star that has had no detections: GJ 745 A. With an upper limit luminosity of log Lx (erg s−1) < 25.4 and an X-ray surface flux of log FX,SURF (erg cm−2 s−1) < 3.6, GJ 745 A defines the lower boundary of the X-ray emission level for M dwarfs. Together with its proper motion companion (GJ 745 B), it is the only star in this volume-complete sample located in the range of X-ray surface flux that corresponds to the faintest solar coronal structures, namely: coronal holes. The fact that the ultra-low X-ray emission level of GJ 745 B (log Lx (erg s−1) = 25.6 and log FX,SURF (erg cm−2 s−1) = 3.8) is entirely attributed to flaring activity indicates that while its corona is dominated by “holes,” at least one magnetically active structure is present. This structure determines the total X-ray brightness and the coronal temperature of the star.

Pulsar Discovery Prospect of FASTA

The Five-hundred-meter Aperture Spherical radio Telescope (FAST) has discovered more than 650 new pulsars, which account for 20% of our known Galactic pulsar population. In this paper, we estimate the prospect of a pulsar survey with a radio telescope array to be planned—the FAST Array (FASTA), consists of six “FAST-type” telescopes. Such a sensitive radio telescope array would be a powerful instrument in probing the pulsar population deep into our Galaxy as well as in nearby galaxies. We simulate the FASTA pulsar discovery prospects with different Galactic pulsar population models and instrumental parameter combinations. We find that FASTA could detect tens of thousands of canonical pulsars and well-over thousands of millisecond pulsars. We also estimate the potential yield if the FASTA is used to search for pulsars from the nearby spiral galaxy M31, and find that it would probably discover around a hundred new radio pulsars.

Lightning Interferometry with the Long Wavelength Array

The Long Wavelength Array is a radio telescope array located at the Sevilleta National Wildlife Refuge in La Joya, New Mexico, well suited and situated for the observation of lightning. The array consists of 256 high-sensitivity dual polarization antennas arranged in a 100 m diameter. This paper demonstrates some of the capabilities that the array brings to the study of lightning. Once 32 or more antennas are used to image lightning radio sources, virtually every integration period longer than the impulse response of the array includes at least one identifiable lightning emitter, independent of the integration period used. The use of many antennas also allows multiple simultaneous lightning radio sources to be imaged at sub-microsecond timescales; for the flash examined, 51% of the images contained more than one lightning source. Finally, by using many antennas to image lightning sources, the array is capable of locating sources fainter than the galactic background radio noise level, yielding possibly the most sensitive radio maps of lightning to date. This incredible sensitivity enables, for the first time, the emissions originating from the positive leader tips of natural in-cloud lightning to be detected and located. The tip emission is distinctly different from needle emission and is most likely due to positive breakdown.

Exploring the Impact of Galactic Interactions and Mergers on the Central Star Formation of APEX/EDGE–CALIFA Galaxies

Galactic interactions and subsequent mergers are a paramount channel for galaxy evolution. In this work, we use the data from 236 star-forming CALIFA galaxies with integrated molecular gas observations in their central region (approximately within an effective radius)—from the APEX millimeter telescope and the CARMA millimeter telescope array. This sample includes isolated (126 galaxies) and interacting galaxies in different merging stages (110 galaxies; from pairs, merging, and postmerger galaxies). We show that the impact of interactions and mergers in the center of galaxies is revealed as an increase in the fraction of molecular gas (compared to isolated galaxies). Furthermore, our results suggest that the change in star formation efficiency is the main driver for both an enhancement and/or suppression of the central star formation—except in merging galaxies where the enhanced star formation appears to be driven by an increase of molecular gas. We suggest that gravitational torques due to the interaction and subsequent merger transport cold molecular gas inwards, increasing the gas fraction without necessarily increasing star formation.

First High‐Speed Video Camera Observations of a Lightning Flash Associated With a Downward Terrestrial Gamma‐Ray Flash

AbstractIn this paper, we present the first high‐speed video observation of a cloud‐to‐ground lightning flash and its associated downward‐directed Terrestrial Gamma‐ray Flash (TGF). The optical emission of the event was observed by a high‐speed video camera running at 40,000 frames per second in conjunction with the Telescope Array Surface Detector, Lightning Mapping Array, interferometer, electric‐field fast antenna, and the National Lightning Detection Network. The cloud‐to‐ground flash associated with the observed TGF was formed by a fast downward leader followed by a very intense return stroke peak current of −154 kA. The TGF occurred while the downward leader was below cloud base, and even when it was halfway in its propagation to ground. The suite of gamma‐ray and lightning instruments, timing resolution, and source proximity offer us detailed information and therefore a unique look at the TGF phenomena.

Gamma-ray emission from spectrally resolved cosmic rays in galaxies

ABSTRACT Cosmic rays (CRs) are ubiquitous in the interstellar medium (ISM) of nearby galaxies, but many of their properties are not well-constrained. Gamma-ray observations provide a powerful tool in this respect, allowing us to constrain both the interaction of CR protons with the ISM and their transport properties. To help better understand the link between observational signatures and CR physics, we use a series of magneto-hydrodynamical arepo simulations of isolated galaxies performed using spectrally resolved CR transport in every computational cell, with subsequent gamma-ray emission calculated using the Crayon + (Cosmic RAY emissiON) code. In each of our simulated haloes, modelling the energy-dependent spatial diffusion of CRs leads to a more extended distribution of high-energy (~100 GeV) gamma rays compared to that predicted by a ‘grey’ steady-state model, which is especially visible in the corresponding emission maps and radial profiles. Despite this, the total gamma-ray spectra can often be well approximated by the steady-state model, although recovering the same spectral index typically requires a minor variation of the energy dependence of the diffusion coefficient. Our simulations reproduce the observed shapes of the gamma-ray spectra of nearby star-forming galaxies and also match recent observations of the far infrared–gamma-ray relation. We find, however, that the spectrally resolved model yields marginally smaller luminosities for lower star formation rates compared to grey simulations of CRs. Our work highlights the importance of modelling spectrally resolved CR transport for an accurate prediction of spatially resolved high-energy gamma-ray emission, as will be probed by the upcoming Cherenkov Telescope Array observatory.

Development of a SiPM Pixel prototype for the Large-Sized Telescope of the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA) will be the next generation ground-based gamma-ray observatory. CTA consists of different telescope types of which the largest ones (Large-Sized Telescopes, LSTs) cover the lower energy range, between 20GeV and 200GeV. The first LST is currently being commissioned at the Roque de los Muchachos Observatory, La Palma, Canary Islands. Its camera has 1855 photomultipliers (PMTs) with 1.5 inch cathodes. Silicon Photomultipliers (SiPMs) are increasingly becoming valid alternatives to PMTs also in gamma-ray astronomy. In the context of the LST project, there is an effort to study a novel Advanced Camera, equipped with SiPMs and a completely redesigned electronics based on a fully digital approach. To study and develop solutions on the sensors of these camera, we built a prototype camera module with a fully re-designed pre-amplifying stage and sensor bias control while re-using the digitizing and triggering stages of the existing LST camera module. We report on the design choices made to achieve the highest performance in terms of timing and charge resolution and the laboratory measurements validating those choices.

Vibration rejection of phased array telescope systems via disturbance-propagation-characteristics-based feedforward control.

Vibration rejection is one of the key techniques to stabilize the line of sight (LOS) for phased array telescope systems. Conventionally, feedback control based on image sensors is mainly used to correct the tip/tilt errors caused by disturbances and to keep the LOS stable. However, it is restricted by the sampling rate and time delay of image sensors, leading to a limited closed-loop bandwidth. Disturbances in the middle and high frequencies are hard to suppress. In this paper, disturbance-propagation-characteristics-based feedforward control is proposed to overcome these problems. A theoretical imaging model of the phased array telescope is developed to analyze the LOS disruption caused by disturbance. In addition, to improve the disturbance suppression bandwidth and correction accuracy of the system, the disturbance propagation characteristics of the phased array telescope system are analyzed. Combined with the disturbance feedforward, targeted compensation is achieved for the sub-apertures. Finally, a comparative experiment is carried out based on the self-developed Fizeau phased array telescope system to verify the superiority of the proposed method.

Phased Array Feed Experiments and Galactic H i Observations

The Phased Array Feed (PAF) is considered as one of the next generation receivers for radio telescopes, which can significantly enlarge the instantaneous Field-of-View of large aperture single dish radio telescopes and enable more flexible observing configurations. Study efforts on PAF development for radio telescopes have been made for more than two decades and have become more and more applicable. We report the development of an ambient-temperature 19 element L-band PAF system and the experimental results including its far field beam pattern and system temperature measurement, which achieve the expectations. Implementing the aperture array beam-forming method, we demonstrate a wide-field Galactic H i observations in the radio camera mode. The results indicate that this system might be applicable for strong Galactic transient detections. This system could be directly equipped to large telescopes like the Five-hundred-meter Aperture Spherical radio Telescope (FAST) and FAST array in the future.

Probing the IC/CMB interpretation for the X-ray knots of AGNs through VHE observations

ABSTRACT The detection of hard X-ray spectra (spectral index <2) from the kiloparsec-scale jet of active galactic nuclei cannot be accounted for by the synchrotron emission mechanism from the electron distribution responsible for the radio/optical emission. Alternate explanations are the inverse Compton scattering of cosmic microwave background photons (IC/CMB) or synchrotron emission from a second electron population. When the X-ray emission is interpreted as an IC/CMB process, the Compton spectrum often peaks at GeV energy and many sources are predicted to be Fermi candidate sources. The absence of significant gamma-ray flux from some of these galaxies by Fermi disfavours the IC/CMB interpretation of the high-energy emission. We extend this study to predict the very-high-energy (VHE) gamma-ray emission due to the IC/CMB model, which can be investigated by the Cherenkov Telescope Array Observatory (CTAO). The model parameters deciding the broad-band spectral energy distribution are estimated using an analytical approximation of the emissivity functions. The emission model is extrapolated to VHE and then compared with the CTAO sensitivity. For this particular study, we have selected 18 knots with harder X-ray spectra and for which the IC/CMB model for X-ray emission has been suggested.

Probing the dark dimension with Auger data

By combining swampland conjectures with observational data, it was recently pointed out that our universe could stretch off in an asymptotic region of the string landscape of vacua. Within this framework, the cosmological hierarchy problem (i.e. the smallness of the cosmological constant in Planck units: Λ∼10−122MPl4) can be naturally resolved by the addition of one mesoscopic (dark) dimension of size ∼λΛ−1/4∼1μm. The Planck scale of the higher dimensional theory, MUV∼λ−1/3Λ1/12MPl2/3∼1010GeV, is tantalizingly close to the energy above which the Telescope Array (TA) and the Pierre Auger collaborations found conclusive evidence for a sharp cutoff of the flux of ultra-high-energy cosmic rays (UHECRs). It was recently suggested that since physics becomes strongly coupled to gravity beyond MUV, universal features deep-rooted in the dark dimension could control the energy cutoff of the source spectra ∝E−γexp(−E/MUV), where E is the cosmic ray energy and γ a free parameter. Conversely, in the absence of phenomena inborn within the dark dimension, we would expect a high variance of the cosmic ray maximum energy Emax characterizing the source spectra ∝E−γexp(−E/Emax), reflecting the many different properties inherent to the most commonly assumed UHECR accelerators. The most recent analysis of Auger and TA data exposed strong evidence for a correlation between UHECRs and nearby starburst galaxies, with a global significance post-trial of 4.7σ. Since these galaxies are in our cosmic backyard, the flux attenuation factor due to cosmic ray interactions en route to Earth turns out to be negligible. This reasoning implies that for each source, the shape of the observed spectrum should roughly match the emission spectrum by the starburst, providing a unique testing ground for the dark dimension hypothesis. Using Auger data, we carry out a maximum likelihood analysis to characterize the shape of the UHECR emission from the galaxies dominating the anisotropy signal. We show that the observed spectra from these sources could be universal only if λ≲10−3.

Photon-ALP oscillations at CTA energies

Axions and axion-like-particles (ALPs) are well motivated beyond the standard model particles that can explain a variety of unsolved problems in physics, such as the strong CP problem and the nature of dark matter. These particles are characterised by their two-photon coupling, which leads to so-called photon-ALP oscillation as photons propagate through an external magnetic field. Such oscillations lead to characteristic signatures in the energy spectrum of high-energy photons from astrophysical sources, allowing us to probe the existence of ALPs and possibly dark matter. We review the signatures from ALPs in photon spectra and discuss a new method that can be used to directly search for the energy dependence of the oscillations. The focus is on photons at TeV-energies relevant for the upcoming Cherenkov Telescope Array (CTA).

Polarised radio pulsations from a new T-dwarf binary

Brown dwarfs display Jupiter-like auroral phenomena such as magnetospheric Hα emission and coherent radio emission. Coherent radio emission is a probe of magnetospheric acceleration mechanisms and it provides a direct measurement of the magnetic field strength at the emitter’s location, both of which are difficult to access by other means. Observations of the coldest brown dwarfs (spectral types T and Y) are particularly interesting as their magnetospheric phenomena may be very similar to those in gas-giant exoplanets. Here we present 144 MHz radio and infrared adaptive optics observations of the brown dwarf WISEP J101905.63+652954.2 made using the Low Frequency Array (LOFAR) and Keck telescopes, respectively. The radio data show pulsed, highly circularly polarised emission which yields a rotation rate of 0.32 ± 0.03 h−1. The infrared imaging reveals the source to be a binary with a projected separation of 423.0 ± 1.6 mas between components of spectral type T5.5 ± 0.5 and T7.0 ± 0.5. With a simple ‘toy model’, we show that the radio emission can, in principle, be powered by the interaction between the two dwarfs with a mass-loss rate of at least 25 times the Jovian value. WISEP J101905.63+652954.2 is interesting because it is the first pulsed methane dwarf detected in a low radio-frequency search. Unlike previous gigahertz-frequency searches that were only sensitive to objects with kiloGauss fields, our low-frequency search is sensitive to surface magnetic fields of ≈50 G and above which might reveal the coldest radio-loud objects down to planetary mass scales.

Nearby active galactic nuclei and starburst galaxies as sources of the measured UHECRs anisotropy signal

The Pierre Auger and the Telescope Array observatories have measured independent and statistical significant anisotropy in the arrival direction of ultra-high-energy cosmic rays (UHECR). Three hotspot regions with relative excess of events and a dipole signal have been identified in different regions of the sky and energy ranges. In this paper, we investigate the conditions under which these anisotropy signal could be generated by nearby (<23 Mpc) active galactic nuclei (AGN) and/or starburst galaxies (SBG). We studied a wide range of possibilities including injected nuclei (p, He, N, Si, and Fe), three UHECR luminosity proxies and three extragalactic magnetic field models. The results shows that both local AGN and SBG are needed to describe all the anisotropy signal. The contribution of AGN to hotspots and to the generation of the dipole is dominant in most cases. SBG is required only to explain the hotspot measured by the Telescope Array Observatory.

Modeling the propagation of very-high-energy γ-rays with the CRbeam code: Comparison with CRPropa and ELMAG codes

Context. Very-high-energy γ-rays produce electron positron pairs in interactions with low-energy photons of extragalactic background light during propagation through the intergalactic medium. The electron-positron pairs generate secondary γ-rays detectable by γ-ray telescopes. This secondary emission can be used to detect intergalactic magnetic fields (IGMF) in the voids of large-scale structure. Aims. A new γ-ray observatory, namely, Cherenkov Telescope Array (CTA), will provide an increase in sensitivity for detections of these secondary γ-ray emission and enable the measurement of its properties for sources at cosmological distances. The interpretation of the CTA data, including the detection of IGMF and study of its properties and origins, will require precision modeling of the primary and secondary γ-ray fluxes. Methods. We assess the precision of the modeling of the secondary γ-ray emission using model calculations with publicly available Monte Carlo codes CRPropa and ELMAG and compare their predictions with theoretical expectations and with model calculations of a newly developed CRbeam code. Results. We find that model predictions of different codes differ by up to 50% for low-redshift sources, with discrepancies increasing up to order-of-magnitude level with the increasing source redshifts. We identify the origin of these discrepancies and demonstrate that after eliminating the inaccuracies found, the discrepancies between the three codes are reduced to 10% when modeling nearby sources with z ~ 0.1. We argue that the new CRbeam code provides reliable predictions for the spectral, timing, and imaging properties of the secondary γ-ray signal for both nearby and distant sources with z ~ 1. Thus, it can be used to study gamma-ray sources and IGMF with a level of precision that is appropriate for the prospective CTA study of the effects of γ-ray propagation through the intergalactic medium.

The AGN fuelling/feedback cycle in nearby radio galaxies

We present Australia Compact Telescope Array (ATCA) 21 cm observations of the nearby low-excitation radio galaxy (LERG) NGC 3100. This is the brightest galaxy of a loose group and it hosts a young (∼2 Myr) radio source. The ATCA observations reveal for the first time the presence of neutral hydrogen (H I) gas in absorption in the centre of this radio galaxy, and in emission in two low-mass galaxies of the group and in a diffuse dark cloud in the proximity of NGC 3100. The sensitivity to low-column density gas (NHI ∼ 1019 cm−2) allows us to reveal asymmetries in the periphery of most of the H I-detected galaxies, suggesting that tidal interactions may be ongoing. The diffuse cloud does not show a stellar counterpart down to 27 mag arcsec−2 and could be the remnant of these interactions. The analysis of the H I absorption complex in NGC 3100 indicates that the atomic phase of the hydrogen is distributed in the same way as its molecular phase (observed at arcsecond resolution through several carbon monoxide emission lines). We suggest that the interactions occurring within the group are causing turbulent cold gas clouds in the intra-group medium to be slowly accreted towards the centre of NGC 3100. This caused the recent formation of the cold circum-nuclear disc, which is likely sustaining the young nuclear activity.

A simulation framework for telescope array and its application in distributed reinforcement learning-based scheduling of telescope arrays

Time-domain astronomy necessitates continuous observation of celestial objects across the entire sky, with specific observation depth and cadence requirements. Telescope arrays, comprised of numerous wide-field optical telescopes, have emerged as a novel observational instrument for time-domain astronomy. However, the observation capabilities of ground-based optical telescope arrays are often constrained by various dynamic factors such as clouds, satellites, and sky background. To meet the observation requirements, active scheduling of telescopes within a telescope array is essential, leveraging telemetry data from the environment. However, due to the complexity and cost of telescope arrays, it is impractical to directly design or test algorithms using a physical telescope array. In this paper, we propose a framework for simulating telescope arrays that incorporates the majority of real observation effects. Building upon this framework, we further introduce a scheduler based on a distributed reinforcement learning framework to optimize the observation strategy of telescopes within the array. The scheduler is trained and evaluated using the simulator. Results demonstrate that the distributed control framework-based scheduler enhances the observation efficiency of the telescope array.

Current status and operation of the H.E.S.S. array of imaging atmospheric Cherenkov telescopes

The High Energy Stereoscopic System (H.E.S.S.) is an array of five imaging atmospheric Cherenkov telescopes (IACTs) to study gamma-ray emission from astrophysical objects in the Southern hemisphere. It is the only hybrid array of IACTs, composed of telescopes with different collection area and footprint, individually optimised for a specific energy range. Collectively, the array is most sensitive to gamma rays in the range of 100GeV to 100TeV. The array has been in operation since 2002 and has been upgraded with new telescopes and cameras multiple times. Recent hardware upgrades and changes in the operational procedures increased the amount of observing time, which is of key importance for time-domain science. H.E.S.S. operations saw record data taking in 2020 and 2021 and we describe the current operations with specific emphasis on system performance, operational processes and workflows, quality control, and (near) real-time extraction of science results. In light of this, we will briefly discuss the early detection of gamma-ray emission from the recurrent nova RS Oph and alert distribution to the astrophysics community.

The prototype Schwarzschild–Couder Telescope: A medium-sized telescope for the Cherenkov Telescope Array

The Schwarzschild–Couder Telescope (SCT) is a dual mirror medium-sized telescope proposed for the Cherenkov Telescope Array (CTA), the next-generation very-high energy (from about 20 GeV to 300 TeV) gamma-ray observatory. The SCT design consists of a dual-mirror optics and a high resolution camera with a field of view (FoV) of 8 degrees squared, which will allow exceptional performance in terms of angular resolution and background rejection. A prototype telescope (named pSCT) has been installed at the Fred Lawrence Whipple Observatory in Arizona, USA. Its camera is partially equipped and covers a FoV of 2.7°. The pSCT has recently successfully detected the Crab Nebula with a statistical significance of 8.6 standard deviations. The upgrade of the pSCT focal plane is now ongoing, aimed to equip the full camera with upgraded sensors and electronics, enhancing the telescope field of view from the current 2.7°to the final 8°. In this presentation, an overview of the pSCT project and obtained results will be given, together with the camera upgrade status and expected performance.

The silicon photomultiplier-based camera for the Cherenkov Telescope Array small-sized telescopes

The Cherenkov Telescope Array (CTA) is the next generation ground-based gamma-ray astronomy observatory, planned to comprise two arrays of imaging air Cherenkov telescopes (IACTs) located in the northern and southern hemispheres. Three telescope sizes are required to cover the CTA gamma-ray energy range from 20 GeV to 300 TeV. An array of several tens of Small-Sized Telescopes (SSTs) at the southern site situated in the Andes at Paranal in Chile, will provide unprecedented sensitivity above 1 TeV and up to 300 TeV, and offer the highest angular resolution of any instrument at these energies. Following a down selection from three prototype telescopes, the design finally selected for SST comprises a dual mirror Schwarzschild–Couder optic with a 4.3 m diameter primary mirror and a 1.8 m secondary mirror imaged by a silicon photomultiplier (SiPM)-based camera with a ∼9°field of view (FoV). The dual mirror optics produce a smaller plate-scale aplanatic focal plane allowing a small, low-cost camera to be employed, compared to that required for the conventional single mirror Davies-Cotton IACT design. The camera comprises an array of 2048 SiPM pixels, configured as 32 sensor and electronics modules each with an 8 × 8 pixel2 tile populated with 6 × 6 mm2 SiPM pixels. Full waveform capture on every channel is provided by the TARGET ASIC which performs the dual function of event triggering and waveform digitization of the full camera at 1 GSample/s. We describe the finalized SST camera design including its optimization for the production phase of the project anticipated to begin in 2023.