69 research outputs found
Tight bounds for online TSP on the line
We consider the online traveling salesperson problem (TSP), where requests appear online over time on the real line and need to be visited by a server initially located at the origin. We distinguish between closed and open online TSP, depending on whether the server eventually needs to return to the origin or not. While online TSP on the line is a very natural online problem that was introduced more than two decades ago, no tight competitive analysis was known to date. We settle this problem by providing tight bounds on the competitive ratios for both the closed and the open variant of the problem. In particular, for closed online TSP, we provide a 1.64-competitive algorithm,thus matching a known lower bound. For open online TSP, we give a new upper bound as well as a matching lower bound that establish the remarkable competitive ratio of 2.04. Additionally, we consider the online Dial-A-Ride problem on the line, where each request needs to be transported to a specified destination. We provide an improved non-preemptive lower bound of 1.75 for this setting, as well as an improved preemptive algorithm with competitive ratio 2.41.Finally, we generalize known and give new complexity results for the underlying offline problems. In particular, we give an algorithm with running time O(n2) for closed offline TSP on the line with release dates and show that both variants of offline Dial-A-Ride on the line are NP-hard for any capacity c≥2 of the server
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
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 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 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
Undirected Graph Exploration with Θ(log log n) Pebbles
We consider the fundamental problem of exploring an undi-rected and initially unknown graph by an agent with lit-tle memory. The vertices of the graph are unlabeled, and the edges incident to a vertex have locally distinct labels. In this setting, it is known that Θ(logn) bits of memory are necessary and sufficient to explore any graph with at most n vertices. We show that this memory requirement can be decreased significantly by making a part of the mem-ory distributable in the form of pebbles. A pebble is a device that can be dropped to mark a vertex and can be collected when the agent returns to the vertex. We show that for an agent O(log logn) distinguishable pebbles and bits of mem-ory are sufficient to explore any bounded-degree graph with at most n vertices. We match this result with a lower bound exhibiting that for any agent with sub-logarithmic memory, Ω(log logn) distinguishable pebbles are necessary for explo-ration.
First follow-up of transient events with the CTA Large Size Telescope prototype
The recent detection of a very high energy (VHE) emission from Gamma-Ray Bursts (GRBs) above 100 GeV performed by the MAGIC and H.E.S.S. collaborations, has represented a significant, long-awaited result for the VHE astrophysics community. Although these results’ scientific impact has not yet been fully exploited, the possibility to detect VHE gamma-ray signals from GRBs has always been considered crucial for clarifying the poorly known physics of these objects. Furthermore, the discovery of high-energy neutrinos and gravitational waves associated with astrophysical sources have definitively opened the era of multi-messenger astrophysics, providing unique insights into the physics of extreme cosmic accelerators. In the near future, the Cherenkov Telescope Array (CTA) will play a major role in these observations. Within this framework, the Large Size Telescopes (LSTs) will be the instruments best suited to significantly impact on short time-scale transients follow-up thanks to their fast slewing and large effective area. The observations of the early emission phase of a wide range of transient events with good sensitivity below 100 GeV will allow us to open new opportunities for time-domain astrophysics in an
energy range not affected by selective absorption processes typical of other wavelengths. In this contribution, we will report about the observational program and first transients follow-up observations performed by the LST-1 telescope currently in its commissioning phase on La Palma, Canary Islands, the CTA northern hemisphere site
Development of an advanced SiPM camera for the Large Size Telescope of the Cherenkov TelescopeArray Observatory
Silicon photomultipliers (SiPMs) have become the baseline choice for cameras of the small-sized telescopes (SSTs) of the Cherenkov Telescope Array (CTA).
On the other hand, SiPMs are relatively new to the field and covering large surfaces and operating at high data rates still are challenges to outperform photomultipliers (PMTs). The higher sensitivity in the near infra-red and longer signals compared to PMTs result in higher night sky background rate for SiPMs. However, the robustness of the SiPMs represents a unique opportunity to ensure long-term operation with low maintenance and better duty cycle than PMTs. The proposed camera for large size telescopes will feature 0.05 degree pixels, low power and fast front-end electronics and a fully digital readout. In this work, we present the status of dedicated simulations and data analysis for the performance estimation. The design features and the different strategies identified, so far, to tackle the demanding requirements and the improved performance are described
Analysis of the Cherenkov Telescope Array first Large Size Telescope real data using convolutional neural networks
The Cherenkov Telescope Array (CTA) is the future ground-based gamma-ray observatory and
will be composed of two arrays of imaging atmospheric Cherenkov telescopes (IACTs) located
in the Northern and Southern hemispheres respectively. The first CTA prototype telescope built
on-site, the Large-Sized Telescope (LST-1), is under commissioning in La Palma and has already
taken data on numerous known sources. IACTs detect the faint flash of Cherenkov light indirectly
produced after a very energetic gamma-ray photon has interacted with the atmosphere and
generated an atmospheric shower. Reconstruction of the characteristics of the primary photons
is usually done using a parameterization up to the third order of the light distribution of the
images. In order to go beyond this classical method, new approaches are being developed
using state-of-the-art methods based on convolutional neural networks (CNN) to reconstruct
the properties of each event (incoming direction, energy and particle type) directly from the
telescope images. While promising, these methods are notoriously difficult to apply to real data
due to differences (such as different levels of night sky background) between Monte Carlo (MC)
data used to train the network and real data. The GammaLearn project, based on these CNN
approaches, has already shown an increase in sensitivity on MC simulations for LST-1 as well
as a lower energy threshold. This work applies the GammaLearn network to real data acquired
by LST-1 and compares the results to the classical approach that uses random forests trained
on extracted image parameters. The improvements on the background rejection, event direction,
and energy reconstruction are discussed in this contribution
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