Searching for pulsars in the Galactic centre at 3 and 2 mm

¹ Institut de Radioastronomie Millimétrique (IRAM), Avda. Divina Pastora 7, Local 20, 18012 Granada, Spain
e-mail: torne@iram.es
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France
⁴ National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100101, PR China
⁵ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK
⁶ Institut de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, 38406 St. Martin d’Hères, France
⁷ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁸ Institut für Theoretische Physik, Max-von-Laue-Strasse 1, 60438 Frankfurt, Germany
⁹ Frankfurt Institute for Advanced Studies, Ruth-Moufang-Strasse 1, 60438 Frankfurt, Germany

Received: 10 March 2021
Accepted: 25 March 2021

Abstract

Pulsars in the Galactic centre promise to enable unparalleled tests of gravity theories and black hole physics and to serve as probes of the stellar formation history and evolution and the interstellar medium in the complex central region of the Milky Way. The community has surveyed the innermost region of the galaxy for decades without detecting a population of pulsars, which is puzzling. A strong scattering of the pulsed signals in this particular direction has been argued to be a potential reason for the non-detections. Scattering has a strong inverse dependence on observing frequency, therefore an effective way to alleviate its effect is to use higher frequencies in a survey for pulsars in the Galactic centre, in particular, close to the supermassive black hole Sagittarius A*. We present the first pulsar survey at short millimetre wavelengths, using several frequency bands between 84 and 156 GHz (λ = 3.5–1.92 mm), targeted to the Galactic centre. The observations were made with the Institut de Radioastronomie Millimétrique 30m Telescope in 28 epochs between 2016 December and 2018 May. This survey is the first that is essentially unaffected by scattering and therefore unbiased in population coverage, including fast-spinning pulsars that might be out of reach of lower-frequency Galactic centre surveys. We discovered no new pulsars and relate this result mainly to the decreased flux density of pulsars at high frequencies, combined with our current sensitivity. However, we demonstrate that surveys at these extremely high radio frequencies are capable of discovering new pulsars, analyse their sensitivity limits with respect to a simulated Galactic centre pulsar population, and discuss the main challenges and possible improvements for similar surveys in the future.