The infrared counterpart and proper motion of magnetar SGR 0501+4516

¹ European Space Agency (ESA), European Space Research and Technology Centre (ESTEC), Keplerlaan 1, 2201 AZ, Noordwijk, The Netherlands
² Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
³ Department of Physics, University of Warwick, Gibbet Hill Road, CV4 7AL Coventry, United Kingdom
⁴ European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
⁵ Astrophysics Research Cluster, School of Mathematical and Physical Sciences, University of Sheffield, Sheffield, S3 7RH United Kingdom
⁶ Instituto de Astrofísica de Canarias, E-38205 La Laguna, Tenerife, Spain
⁷ Scuola Universitaria Superiore IUSS Pavia, Palazzo del Broletto, piazza della Vittoria 15, I-27100 Pavia, Italy
⁸ School of Physics, University College Dublin, L.M.I. Main Building, Beech Hill Road, Dublin 4, D04 P7W1 Ireland
⁹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, 21218 USA
¹⁰ AIM-CEA/DRF/Irfu/Département d’Astrophysique, CNRS, Université Paris-Saclay, Université de Paris Cité, Orme des Merisiers, F-91191 Gif-sur-Yvette, France
¹¹ University of Maryland, Baltimore County, Baltimore, MD, 21250 USA
¹² NASA Goddard Space Flight Center, Greenbelt, MD, 20771 USA
¹³ INAF-Osservatorio Astronomico di Roma, via Frascati 33, I-00078 Monte Porzio Catone, Italy
¹⁴ Department of Physics, The George Washington University, Corcoran Hall, 725 21st St NW, Washington, DC, 20052 USA
¹⁵ GWU/Astronomy, Physics and Statistics Institute of Sciences (APSIS), Washington DC, USA
¹⁶ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, via E. Bassini 15, 20133 Milano, Italy
¹⁷ Janusz Gil Institute of Astronomy, University of Zielona Góra, ul Szafrana 2, 65-265 Zielona Góra, Poland
¹⁸ Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY, 11794-3800 USA
¹⁹ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, E-08193 Barcelona, Spain
²⁰ Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castelldefels (Barcelona), Spain
²¹ Isaac Newton Group of Telescopes, Apartado de Correos 321, E-38700 Santa Cruz de La Palma, Canary Islands, Spain
²² School of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH United Kingdom
²³ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield, AL10 9AB United Kingdom
²⁴ Astrophysics Research Centre, Keele University, Keele, ST5 5BG United Kingdom
²⁵ Mullard Space Science Laboratory, University, College London, Holmbury St Mary, Dorking, Surrey, RH5 6NT UK

^⋆ Corresponding author; ashley.chrimes@esa.int

Received: 17 December 2024
Accepted: 17 February 2025

Abstract

Aims. Soft gamma repeaters (SGRs) are highly magnetised neutron stars (magnetars) notable for their gamma-ray and X-ray outbursts. We used near-infrared (NIR) imaging of SGR 0501+4516 in the days, weeks, and years after its 2008 outburst to characterise the multi-wavelength emission, and to obtain a proper motion from our long temporal baseline observations.

Methods. We present short- and long-term monitoring of the IR counterpart of SGR 0501+4516 and a measurement of its proper motion. Unlike most magnetars, the source has only moderate foreground extinction with minimal crowding. Our observations began only ∼2 hours after the first activation of SGR 0501+4516 in August 2008 and continued for ∼4 years, including two epochs of Hubble Space Telescope (HST) imaging. The proper motion constraint was improved using a third HST epoch from 10 years later.

Results. The NIR and X-rays faded slowly during the first week, which was followed by a steeper power-law decay. The behaviour is satisfactorily fit by a broken power law. Three epochs of HST imaging with a 10-year baseline allowed us to determine the quiescent level and to measure a proper motion of μ = 5.4 ± 0.6 mas yr⁻¹. This corresponds to a low transverse peculiar velocity of v ≃ 51 ± 14 km s⁻¹ (at 2 kpc). The magnitude and direction of the proper motion rules out supernova remnant HB9 as the birth site. We can find no other supernova remnants or groups of massive stars within the region traversed by SGR 0501+4516 during its characteristic lifetime (∼20 kyr).

Conclusions. Our observations of SGR 0501+4516 suggest three possibilities: that some magnetars are significantly older than expected, that their progenitors produce low supernova ejecta masses, or that they can be formed through accretion-induced collapse or low-mass neutron star mergers. Although the progenitor of SGR 0501+4516 remains unclear, we propose that SGR 0501+4516 is the best Galactic candidate for a magnetar formed through a mechanism other than massive star core-collapse.