On 14 July, the performance paper of the LST-1, the prototype of the Large-Sized Telescope (LST) currently under commissioning at CTAO-North on La Palma (Spain), was accepted for publication in the Astrophysical Journal (ApJ). The study, carried out by the LST Collaboration, describes the telescope’s capabilities, including key parameters such as its sensitivity and its angular and energy resolution, and validates the simulations required for data analysis. This paper is fundamental to the upcoming science publications as it sets a performance baseline for the instrument and ensures its reliability.

“In simple words, a performance paper is a handbook for how the telescope works: it shows its capabilities and limitations,” explains Abelardo Moralejo, LST-1 Analysis Software Coordinator and author of the paper. “It allows us to evaluate potential systematic errors of the instrumentation that could affect the interpretation of data. Thus, a deep understanding of the telescope’s performance, towards which this paper is an important step, ensures that the scientific results with the LST-1 are reliable and reproducible.”

In order to evaluate the LST-1’s performance, the LST Collaboration made use of a comprehensive data set from observations of the Crab Nebula spanning from November 2020 to March 2022. The Crab Nebula is the standard candle in very high-energy astronomy, a source whose luminosity is well-known and steady at those energies. Such observations allowed the team to also verify that the simulations needed during the scientific data analysis were correct.

“Analyzing data from the Crab Nebula provides valuable insights into its emission behaviour and evolution with energy and time. By comparing the results to what we expect from the source as a standard candle, we can determine the instrument’s sensitivity and precision, and correct our simulations, if necessary,” says Rubén López-Coto, LST-1 Analysis Software Deputy Coordinator and author of the paper. “The study shows that the telescope not only performs exceptionally well overall, as expected, but it also narrows the gap with other instruments at lower energy levels, thanks to its proven low-energy threshold.”

The low-energy threshold is a fundamental parameter of the LSTs, as these telescopes are responsible to cover the sensitivity of the CTAO at the lowest energies by capturing gamma rays down to 20 GeV. The performance study is complemented by observations of the Crab pulsar, the neutron star at the centre of the Crab Nebula.

“Pulsars are very challenging sources to detect due to their weak signal,” says Masahiro Teshima, Principal Investigator of the LST Collaboration. “The LST-1 can detect the two pulses of the Crab pulsar in record time. This is not only an extraordinary result, but it also demonstrates the LST-1’s capabilities in detecting faint sources at low energies, as described in the paper.”

This is the first performance paper of a telescope prototype on a CTAO site. While the LST-1’s observing capabilities as a single telescope are already remarkable, these results will only improve once more telescopes are built and begin to operate together, thus expanding our current understanding of the gamma-ray Universe.

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The LST Collaboration Publishes the LST-1 Performance Paper - CTAO