1. Instituto de Astrofísica de Andalucía del Consejo Superior de Investigaciones Científicas (IAA-CSIC), PO Box 03004, E-18080 Granada, Spain
2. European Southern Observatory, Casilla 19001, Santiago 19, Chile
3. Special Astrophysical Observatory of Russian Academy of Science (SAO-RAS), Nizhnij Arkhyz, Karachai-Cherkessia, 369167 Russia
4. Thüringer Landessternwarte Tautenburg, Sternwarte 5, D-07778 Tautenburg, Germany
5. Laboratoire d'Astrophysique, École Polytechnique Fédérale de Lausanne (EPFL) Observatoire, 1290 Sauverny, Switzerland
6. Institute de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, 38406 Saint Martin d'Héres, France
7. Facultad de Ciencias Experimentales, Universidad de Jaén, Campus Las Lagunillas, E-23071 Jaén, Spain
8. Department of Physics and Astronomy, The University of Leicester, Leicester, LE1 7RH, UK
9. School of Physics, University College Dublin, Dublin 4, Ireland
10. Astronomical Institute 'Anton Pannekoek', University of Amsterdam, 1098 SJ Amsterdam, The Netherlands
11. Institute de Radioastronomie Millimétrique (IRAM), Avda. Divina Pastora 7, Núcleo Central, E-18012 Granada, Spain
12. Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
13. Cork Institute of Technology, Rossa Avenue, Bishopstown (Cork), Ireland
14. Universidad de Valencia, Edif. Institutos de Investigación (GACE-ICMOL), Campus de Paterna, E-46980 Paterna (Valencia), Spain
15. Department of Astronomy, University of Texas, Austin, Texas 78712, USA
16. Astronomical Institute of the Czech Academy of Sciences, Friova 298, 25165 Ondejov, Czech Republic
17. Instituto de Astrofísica de Canarias (IAC), Via Láctea s/n, E-38205 La Laguna (Tenerife), Spain
18. Nikolaev State University, Nikolskaya 24, 54030 Nikolaev, Ukraine
19. Aryabhatta Research Institute of Observational-Sciences (ARIES), Manora Peak, NainiTal, Uttarakhand, 263129, India
20. Crimean Astrophysical Observatory, 98409 Nauchny, Ukraine
21. University of Cukurova, Department of Physics, 01330 Adana, Turkey
22. European Space Astronomy Centre (ESAC), Avenida de los Castillos s/n, Urbanizacion Villafranca del Castillo, E-28691 Villanueva de la Cañada (Madrid), Spain
23. Instituto Nacional de Técnica Aeroespacial (INTA), E-28750 Torrejón de Ardoz (Madrid), Spain

Correspondence to: A. J. Castro-Tirado¹ Correspondence and requests for materials should be addressed to A.J.C.T. (Email: ajct@iaa.es).

Abstract

Magnetars¹ are young neutron stars with very strong magnetic fields of the order of 10¹⁴–10¹⁵ G. They are detected in our Galaxy either as soft -ray repeaters or anomalous X-ray pulsars. Soft -ray repeaters are a rare type of -ray transient sources that are occasionally detected as bursters in the high-energy sky^{2, 3, 4}. No optical counterpart to the -ray flares or the quiescent source has yet been identified. Here we report multi-wavelength observations of a puzzling source, SWIFT J195509+261406. We detected more than 40 flaring episodes in the optical band over a time span of three days, and a faint infrared flare 11 days later, after which the source returned to quiescence. Our radio observations confirm a Galactic nature and establish a lower distance limit of 3.7 kpc. We suggest that SWIFT J195509+261406 could be an isolated magnetar whose bursting activity has been detected at optical wavelengths, and for which the long-term X-ray emission is short-lived. In this case, a new manifestation of magnetar activity has been recorded and we can consider SWIFT J195509+261406 to be a link between the 'persistent' soft -ray repeaters/anomalous X-ray pulsars and dim isolated neutron stars.

1. Instituto de Astrofísica de Andalucía del Consejo Superior de Investigaciones Científicas (IAA-CSIC), PO Box 03004, E-18080 Granada, Spain
2. European Southern Observatory, Casilla 19001, Santiago 19, Chile
3. Special Astrophysical Observatory of Russian Academy of Science (SAO-RAS), Nizhnij Arkhyz, Karachai-Cherkessia, 369167 Russia
4. Thüringer Landessternwarte Tautenburg, Sternwarte 5, D-07778 Tautenburg, Germany
5. Laboratoire d'Astrophysique, École Polytechnique Fédérale de Lausanne (EPFL) Observatoire, 1290 Sauverny, Switzerland
6. Institute de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, 38406 Saint Martin d'Héres, France
7. Facultad de Ciencias Experimentales, Universidad de Jaén, Campus Las Lagunillas, E-23071 Jaén, Spain
8. Department of Physics and Astronomy, The University of Leicester, Leicester, LE1 7RH, UK
9. School of Physics, University College Dublin, Dublin 4, Ireland
10. Astronomical Institute 'Anton Pannekoek', University of Amsterdam, 1098 SJ Amsterdam, The Netherlands
11. Institute de Radioastronomie Millimétrique (IRAM), Avda. Divina Pastora 7, Núcleo Central, E-18012 Granada, Spain
12. Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
13. Cork Institute of Technology, Rossa Avenue, Bishopstown (Cork), Ireland
14. Universidad de Valencia, Edif. Institutos de Investigación (GACE-ICMOL), Campus de Paterna, E-46980 Paterna (Valencia), Spain
15. Department of Astronomy, University of Texas, Austin, Texas 78712, USA
16. Astronomical Institute of the Czech Academy of Sciences, Friova 298, 25165 Ondejov, Czech Republic
17. Instituto de Astrofísica de Canarias (IAC), Via Láctea s/n, E-38205 La Laguna (Tenerife), Spain
18. Nikolaev State University, Nikolskaya 24, 54030 Nikolaev, Ukraine
19. Aryabhatta Research Institute of Observational-Sciences (ARIES), Manora Peak, NainiTal, Uttarakhand, 263129, India
20. Crimean Astrophysical Observatory, 98409 Nauchny, Ukraine
21. University of Cukurova, Department of Physics, 01330 Adana, Turkey
22. European Space Astronomy Centre (ESAC), Avenida de los Castillos s/n, Urbanizacion Villafranca del Castillo, E-28691 Villanueva de la Cañada (Madrid), Spain
23. Instituto Nacional de Técnica Aeroespacial (INTA), E-28750 Torrejón de Ardoz (Madrid), Spain

Correspondence to: A. J. Castro-Tirado¹ Correspondence and requests for materials should be addressed to A.J.C.T. (Email: ajct@iaa.es).

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