Flares from a candidate Galactic magnetar suggest a missing link to dim isolated neutron stars

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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 γ-ray repeaters or anomalous X-ray pulsars. Soft γ-ray repeaters are a rare type of γ-ray transient sources that are occasionally detected as bursters in the high-energy sky2,3,4. No optical counterpart to the γ-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 γ-ray repeaters/anomalous X-ray pulsars and dim isolated neutron stars.

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Figure 1: Optical and X-ray light curves of SWIFT J195509+261406 (June–November 2007).
Figure 2: Deep, late observations of the SWIFT J195509+261406 field.
Figure 3: Log-normal distribution of flare fluxes for SWIFT J195509+261406.

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Acknowledgements

This work is based on observations carried out with the 0.3-m robotic telescope at the Spanish BOOTES-2 astronomical station of the Estación Experimental de La Mayora (CSIC), the 0.4-m WATCHER telescope operated by UCD at Boyden Observatory (South Africa), the 0.8-m IAC telescope at the Spanish Observatorio de Izaña of the Instituto de Astrofísica de Canarias (IAC), the 1.2-m Mercator telescope operated by the Flemish Community at the Spanish Observatorio del Roque de los Muchachos of the IAC, the 1.34-m telescope at the Tautenburg Observatory (Germany), the 1.5-m OSN telescope at the Spanish Observatorio de Sierra Nevada of the Instituto de Astrofísica de Andalucía (CSIC), the 6.0-m BTA telescope at the Special Astrophysical Observatory of the Russian Academy of Sciences, the 8.2-m VLT telescope of the European Southern Observatory at Paranal (Chile), the IRAM 30-m and Plateau de Bure Telescopes and the 100-m telescope of the Max-Planck-Institut für Radioastronomie at Effelsberg (Germany). IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). We thank both the SAO-RAS Director and the ESO Director's Discretionary Time Committee for accepting the observation a few days after the onset of the event. We also thank N. Schartel for allotting XMM-Newton DDT time for a late-time X-ray observation. We acknowledge the use of public data from the Swift data archives and the service provided by the γ-ray burst Coordinates Network (GRB) and BACODINE system, maintained by S. Barthelmy. A.J.C.T. acknowledges discussions with E. Alfaro, Y. Beletski, A. M. Belobodorov, W. Cui, S. Digel, R. Fernández-Muñoz, P. Gandhi, S. Gottlieb, M. Lyutikov, A. Merloni, M. A. Pérez-Torres, V. Reglero, E. Ros and G. Sala. P.F., D.A.K. and S.K. acknowledge financial support by DFG and D.P.R. from Junta de Andalucía. L.H. acknowledges support from IRCSET and SFI. R.H. acknowledges support from PECs and GACR. This work was partially supported by the Spanish Ministry of Science and Innovation.

Author Contributions A.J.C.T. wrote the paper. A.U.P., D.A.K., S. S. and A.C.W. prepared the figures and together with M. B., P. F., A. K., S. K., S. L. and D.P.R. participated in the data analysis. S. M. and K. L. helped in the modelling. The remaining coauthors provided observational data, discussed the results and commented on the manuscript.

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Correspondence to A. J. Castro-Tirado.

Supplementary information

Supplementary Information

This file contains: (1) Supplementary Optical Observations, (2) Supplementary Millimetre Observations, (3) Supplementary Centimetre Observations, (4) Supplementary X-Ray Observations, (5) The distance to Swift J195509+261406, (6) Optical and Near IR Data and (7) Supplementary References (PDF 1751 kb)

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