Multiwavelength study of the galactic PeVatron candidate LHAASO J2108+5157

Context. Several new ultrahigh-energy (UHE) γ-ray sources have recently been discovered by the Large High Altitude Air Shower Observatory (LHAASO) collaboration. These represent a step forward in the search for the so-called Galactic PeVatrons, the enigmatic sources of the Galactic cosmic rays up to PeV energies. However, it has been shown that multi-TeV γ-ray emission does not necessarily prove the existence of a hadronic accelerator in the source; indeed this emission could also be explained as inverse Compton scattering from electrons in a radiation-dominated environment. A clear distinction between the two major emission mechanisms would only be made possible by taking into account multi-wavelength data and detailed morphology of the source. Aims. We aim to understand the nature of the unidentified source LHAASO J2108+5157, which is one of the few known UHE sources with no very high-energy (VHE) counterpart. Methods. We observed LHAASO J2108+5157 in the X-ray band with XMM-Newton in 2021 for a total of 3.8 hours and at TeV energies with the Large-Sized Telescope prototype (LST-1), yielding 49 hours of good-quality data. In addition, we analyzed 12 years of Fermi-LAT data, to better constrain emission of its high-energy (HE) counterpart 4FGL J2108.0+5155. We used naima and jetset software packages to examine the leptonic and hadronic scenario of the multi-wavelength emission of the source. Results. We found an excess (3.7σ) in the LST-1 data at energies E > 3 TeV. Further analysis of the whole LST-1 energy range, assuming a point-like source, resulted in a hint (2.2σ) of hard emission, which can be described with a single power law with a photon index of Σ = 1.6 ± 0.2 the range of 0.3 - 100 TeV. We did not find any significant extended emission that could be related to a supernova remnant (SNR) or pulsar wind nebula (PWN) in the XMM-Newton data, which puts strong constraints on possible synchrotron emission of relativistic electrons. We revealed a new potential hard source in Fermi-LAT data with a significance of 4σ and a photon index of Σ = 1.9 ± 0.2, which is not spatially correlated with LHAASO J2108+5157, but including it in the source model we were able to improve spectral representation of the HE counterpart 4FGL J2108.0+5155. Conclusions. The LST-1 and LHAASO observations can be explained as inverse Compton-dominated leptonic emission of relativistic electrons with a cutoff energy of 100-30+70 TeV. The low magnetic field in the source imposed by the X-ray upper limits on synchrotron emission is compatible with a hypothesis of a PWN or a TeV halo. Furthermore, the spectral properties of the HE counterpart are consistent with a Geminga-like pulsar, which would be able to power the VHE-UHE emission. Nevertheless, the lack of a pulsar in the neighborhood of the UHE source is a challenge to the PWN/TeV-halo scenario. The UHE γ rays can also be explained as π0 decay-dominated hadronic emission due to interaction of relativistic protons with one of the two known molecular clouds in the direction of the source. Indeed, the hard spectrum in the LST-1 band is compatible with protons escaping a shock around a middle-aged SNR because of their high low-energy cut-off, but the origin of the HE γ-ray emission remains an open question

Proximity-centred accessibility – A conceptual debate involving planning practitioners worldwide

In recent years, the concept of proximity has garnered increasing attention in both transportation research and practice, albeit under various terms and interpretations. Among these, the concept of the 15-minute city has catalysed attention in planning practice, with recent evolution to the x-minute city and city of proximities. In research, proximity-centred accessibility has been offered as an umbrella term to express the ability to reach activities and destinations at short distances. Regardless of the terminology used, the essence of proximity lies in the ease with which one can access desired activities and destinations within reasonable travel times, independent of speed-enhancing transport modes most notably through walking.This research investigates the nuanced meanings ascribed to proximity-centred accessibility by planning practitioners globally, spanning diverse regional and local contexts. For this, we used an online survey, disseminated among over 9000 practitioners from 22 countries across 5 continents, which generated over 1300 responses. The survey explored the preferred terms for proximity-centred accessibility and their definitions, specifically emphasizing time and distance thresholds and the identification of relevant activities. By juxtaposing our findings with an earlier survey of accessibility researchers, this study also contributes to the groundwork for a conceptual framework for proximity-centred accessibility.Our findings affirm a relatively consistent interpretation of proximity among global planning practitioners, predominantly extending up to 1600 m, in accordance with earlier results for accessibility researchers. Despite some relevant dissimilarities among practitioners from megacities compared to their smaller city counterparts, or in specific countries (most notably the Netherlands), the distance that is considered proximate is the attribute that generates the most consistent results across different contexts. Also consistent was the relevance of short distances (up to 15 min walking) for activities such as primary and pre-primary schools, playgrounds, parks, food shopping, and pharmacies, reinforcing the importance of proximity to basic and caregiving activities. No term was found to be consistently meaningful across different contexts, although terms like local and neighbourhood accessibility and walking/pedestrian, or cycling accessibility, show higher preference in the global sample

Improvement of the follow-up observations of IceCube neutrinos by CTA LST

A decade has passed since high-energy astrophysical neutrinos have been discovered by IceCube, however the corresponding sources have not been fully identified yet. The reported coincidence of the high-energy IceCube-170922A with the gamma-ray blazar TXS 0506+056 is not enough to claim that blazars are the dominant high-energy neutrino emitters in the Universe. In fact, recently IceCube announced a second correlation with NGC 1068, a nearby Seyfert galaxy, which is significantly different from a gamma-emitting blazars. The hunt for counterparts of the IceCube neutrinos using gamma-ray telescopes started in 2012. Nonetheless, these efforts will continue with the next-generation gamma-ray telescopes, such as the CTA Large Size Telescopes (LSTs) and other telescopes, by means of an improved and revised observation strategy. These new observations will allow us to detect enough sources in order to elucidate the mystery of the neutrino emitters. In this contribution, we introduces the efforts made thus far in the search for gamma-ray counterpart of high-energy IceCube events using the current generation IACTs, focusing on alerts made of multiple neutrinos events, and present an idea to improve in the observational strategies with the CTA LSTs that will become operational in the coming decade. We will discuss how to reduce the bias to gamma-ray emitters in order to search for possible neutrino counterparts

Observations of the Crab Nebula and Pulsar with the Large-Sized Telescope Prototype of the Cherenkov Telescope Array

CTA (Cherenkov Telescope Array) is the next generation ground-based observatory for gamma-ray astronomy at very-high energies. The Large-Sized Telescope prototype (\LST{}) is located at the Northern site of CTA, on the Canary Island of La Palma. LSTs are designed to provide optimal performance in the lowest part of the energy range covered by CTA, down to $\simeq 20$ GeV. \LST{} started performing astronomical observations in November 2019, during its commissioning phase, and it has been taking data since then. We present the first \LST{} observations of the Crab Nebula, the standard candle of very-high energy gamma-ray astronomy, and use them, together with simulations, to assess the basic performance parameters of the telescope. The data sample consists of around 36 hours of observations at low zenith angles collected between November 2020 and March 2022. \LST{} has reached the expected performance during its commissioning period - only a minor adjustment of the preexisting simulations was needed to match the telescope behavior. The energy threshold at trigger level is estimated to be around 20 GeV, rising to $\simeq 30$ GeV after data analysis. Performance parameters depend strongly on energy, and on the strength of the gamma-ray selection cuts in the analysis: angular resolution ranges from 0.12 to 0.40 degrees, and energy resolution from 15 to 50\%. Flux sensitivity is around 1.1\% of the Crab Nebula flux above 250 GeV for a 50-h observation (12\% for 30 minutes). The spectral energy distribution (in the 0.03 - 30 TeV range) and the light curve obtained for the Crab Nebula agree with previous measurements, considering statistical and systematic uncertainties. A clear periodic signal is also detected from the pulsar at the center of the Nebula.Comment: Submitted to Ap

Galactic Center Studies with CTA-LST-1

The Galactic Center region is known to host a wide variety of very-high-energy gamma-ray sources. In 2018 the prototype of the Large-Sized Telescope (LST-1) for CTA was inaugurated, and has been regularly observing the Galactic Center since 2021. To observe the Galactic Center in the southern sky, LST-1, located in the Northern hemisphere, requires an observation mode at a low telescope elevation. In this study, we assessed the performance of LST-1 at the large zenith angle, based both on simulations and observational data for the standard candle Crab Nebula. Analyzing LST-1 data from the Galactic Center observations, we obtained the spectral energy distributions of Sagittarius A* and G0.9+0.1, which were comparable with the results from the current imaging atmospheric Cherenkov telescopes, with a broad energy coverage owing to the large-zenith-angle observation and the low energy threshold of LST-1

First results of pulsar observations with the LST-1

After the discovery of Crab, Vela, and Geminga pulsars at Very High Energies, the search for new pulsars at tens of GeV has been gaining huge importance. However, their steep spectra along with the sensitivity of the current generation of Imaging Atmospheric Cherenkov telescopes (IACTs) are limiting the capability to detect more pulsars. The LST-1 is the first prototype of the Large-Sized Telescope of the forthcoming CTA observatory with enhanced sensitivity at tens of GeV. The LST-1 started its commissioning phase in 2018, and since then it has observed the Crab pulsar regularly. Here, we show the first results of the analysis of the Crab and other pulsars taken with the LST-1. The two characteristic emission peaks of the Crab pulsar, P1, and P2, are detected with high significance showing a clear improvement in sensitivity over the previous generation of IACTs. The spectrum is reconstructed up to 450 GeV for P1 and up to 700 GeV for P2. The low energy threshold of LST-1 also allows us to measure the spectrum of the Crab pulsar in the overlapping region with the Fermi-LAT and cross-calibrate both instruments. The results obtained with the first pulsar observations with the LST-1 confirm the excellent potential of LST telescopes to study and discover new pulsars in the near future

A detailed study of the very high-energy Crab pulsar emission with the LST-1

Context. To date, three pulsars have been firmly detected by imaging atmospheric Cherenkov telescopes (IACTs). Two of them reached the TeV energy range, challenging models of very high-energy (VHE) emission in pulsars. More precise observations are needed to better characterize pulsar emission at these energies. The LST-1 is the prototype of the large-sized telescopes, which will be part of the Cherenkov Telescope Array Observatory (CTAO). Its improved performance over previous IACTs makes it well suited for studying pulsars. Aims. In this work we study the Crab pulsar emission with the LST-1, improving upon and complementing the results from other telescopes. Crab pulsar observations can also be used to characterize the potential of the LST-1 to study other pulsars and detect new ones. Methods. We analyzed a total of ~103 hours of gamma-ray observations of the Crab pulsar conducted with the LST-1 in the period from September 2020 to January 2023. The observations were carried out at zenith angles of less than 50 degrees. To characterize the Crab pulsar emission over a broader energy range, a new analysis of the Fermi/LAT data, including ~14 years of observations, was also performed. Results. The Crab pulsar phaseogram, long-term light curve, and phase-resolved spectra are reconstructed with the LST-1 from 20 GeV to 450 GeV for the first peak and up to 700 GeV for the second peak The pulsed emission is detected with a significance level of 15.2σ. The two characteristic emission peaks of the Crab pulsar are clearly detected (> 10σ), as is the so-called bridge emission between them (5.7σ). We find that both peaks are described well by power laws, with spectral indices of ~3.44 and ~3.03, respectively. The joint analysis of Fermi/LAT and LST-1 data shows a good agreement between the two instruments in their overlapping energy range. The detailed results obtained from the first observations of the Crab pulsar with the LST-1 show the potential that CTAO will have to study this type of source

GRB 221009A: Observations with LST-1 of CTAO and Implications for Structured Jets in Long Gamma-Ray Bursts

GRB 221009A is the brightest gamma-ray burst (GRB) observed to date. Extensive observations of its afterglow emission across the electromagnetic spectrum were performed, providing the first strong evidence of a jet with a nontrivial angular structure in a long GRB. We carried out an extensive observation campaign in very-high-energy (VHE) gamma rays with the first Large-Sized Telescope of the future Cherenkov Telescope Array Observatory starting on 2022 October 10, about 1 day after the burst. A dedicated analysis of the GRB 221009A data is performed to account for the different moonlight conditions under which data were recorded. We find an excess of gamma-like events with a statistical significance of 4.1σ during the observations taken 1.33 days after the burst, followed by background-compatible results for the later days. The results are compared with various models of afterglows from structured jets that are consistent with the published multiwavelength data but entail significant quantitative and qualitative differences in the VHE emission after 1 day. We disfavor models that imply VHE flux at 1 day considerably above 10−11 erg cm−2 s−1. Our late-time VHE observations can help disentangle the degeneracy among the models and provide valuable new insight into the structure of GRB jets

LST-1 observations of an enormous flare of BL Lacertae in 2021

The first prototype of LST (LST-1) for the Cherenkov Telescope Array has been in commissioning phase since 2018 and already started scientific observations with the low energy threshold around a few tens of GeV. In 2021, LST-1 observed BL Lac following the alerts based on multi-wavelength observations and detected prominent gamma-ray flares. In addition to the daily flux variability, LST-1 also detected sub-hour-scale intra-night variability reaching 3–4 times higher than the gamma-ray flux from the Crab Nebula above 100 GeV. In this proceeding, we will report the analysis results of LST-1 observations of BL Lac in 2021, especially focusing on flux variability