La Palma, Canary Islands, Spain – Between January and February 2020, the prototype Large-Sized Telescope (LST), the LST-1, observed the Crab Pulsar, the neutron star at the centre of the Crab Nebula. The telescope, which is being commissioned on the CTA-North site on the island of La Palma in the Canary Islands, was conducting engineering runs to verify the telescope performance and adjust operating parameters.
Pulsars are very rapidly rotating and strongly magnetized neutron stars that emit light in the form of two beams, which can be observed from Earth only when passing our line of sight.
Figure 1. Multiwavelength view of the Crab Nebula and the Crab pulsar – the bright spot at the centre of the image. Credit: NASA, ESA, G. Dubner (IAFE, CONICET-University of Buenos Aires) et al.; A. Loll et al.; T. Temim et al.; F. Seward et al.; VLA/NRAO/AUI/ NSF; Chandra/ CXC; Spitzer/JPL-Caltech; XMM-Newton/ESA; Hubble/STScI
While detecting the strong and steady emission or outbursts of gamma-ray sources with Imaging Atmospheric Cherenkov Telescopes (IACTs) has become routine, pulsars are much more challenging to detect due to their weak signals and the typical dominance of the foreground gamma-ray signal from the surrounding nebulae. Despite hundreds of observations hours by IACTs around the globe, only four pulsars emitting signals in the very high-energy gamma-ray regime have been discovered, so far. Now that the LST-1 has shown that it can detect the Crab pulsar, it joins the field of telescopes capable of detecting gamma-ray pulsars, validating the timestamping system and the low-energy performance of the telescope.
The data set collected includes 11.4 hours from eight observation nights. Figure 2 shows the resulting phasogram, plotting the gamma-ray events as a function of the pulsar rotation phase. In the phase regions marked as P1 and P2, more gamma rays are expected as the Crab pulsar emits towards the Earth. The emission detected in all phases (marked green in Figure 2) is a mixture of different background contributions, including the irreducible steady emission from the Crab Nebula. The pulsed signal detected with the LST-1 is marked red in Figure 2. The animation in Figure 3 highlights the pulse behaviour of the source during the different phases. For more details, the official CTA site for this news: http://www.cta-observatory.org/lst1-detects-vhe-emission-from-crab-pulsar
Figure 2: Phasogram of Crab Pulsar as measured by the LST-1. The pulsar is known to emit pulses of gamma rays during phases P1 and P2. The shown significance is calculated considering source emission from those phases (in red) and background events from phases (in grey). Credit: LST Collaboration
Figure 3: Animation of Crab pulsar’s emission as seen by the LST-1 along its different phases. Credit: Rubén López-Coto; Pulsar gif: Michael R. Gallis
The contribution of INFN to the construction and commissioning of the telescope has been crucial to achieve this detection. The Universities and INFN Sections of Padova, Pisa, Siena, Torino and Udine have been in front line for the LST construction. INFN and the University of Padova have participated in the design of the optical system of the telescope and built part of the azimuth movement mechanics and the carbon fiber ropes for supporting and anchoring the arch that supports the chamber. There are currently four members from INFN-Padova belonging to the LST Management: Prof. Alessandro de Angelis, member of the Steering Committee and Prof. Mosè Mariotti, LST National Responsible and Tension Ropes coordinator, Dr. Riccardo Rando, Research and Development coordinator and Dr. Rubén López-Coto, Data Analysis coordinator and responsible of the software development of the telescope. The most significant contribution of INFN-Padova to this result, has been achieved thanks to a special data analysis carried out by Dr. Rubén Lopez-Coto, researcher of the “Fellini” project of the Padova INFN Section (funded by Marie Skłodowska-Curie Grant Ggreement No. 754496).
Responsabile Nazionale LST
Figure 4: LST telescope. Credit: Akira Okumura