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Nature, 455, 506, 2008 Vol 455 | 25 September 2008 | doi:10.1038/nature07328 LETTERS Flares from a candidate Galactic magnetar suggest a missing link to dim isolated neutron stars A. J. Castro-Tirado1, A. de Ugarte Postigo1,2, J. Gorosabel1, M. Jelı´nek1, T. A. Fatkhullin3, V. V. Sokolov3, P. Ferrero4, D. A. Kann4, S. Klose4, D. Sluse5, M. Bremer6, J. M. Winters6, D. Nuernberger2,D.Pe´rez-Ramı´rez7,8, M. A. Guerrero1, J. French9, G. Melady9, L. Hanlon9, B. McBreen9, K. Leventis10, S. B. Markoff10, S. Leon11, A. Kraus12, F. J. Aceituno1, R. Cunniffe1,13, P. Kuba´nek1,14,S.Vı´tek1, S. Schulze4, A. C. Wilson15, R. Hudec16, M. Durant17, J. M. Gonza´lez-Pe´rez17, T. Shahbaz17, S. Guziy18, S. B. Pandey19, L. Pavlenko20, E. Sonbas3,21, S. A. Trushkin3, N. N. Bursov3, N. A. Nizhelskij3, C. Sa´nchez-Ferna´ndez22 & L. Sabau-Graziati23 Magnetars1 are young neutron stars with very strong magnetic no further flares were seen until 22 June, when a late-time, lower- fields of the order of 1014–1015 G. They are detected in our brightness flare was detected in the near-infrared using the 8.2-m Galaxy either as soft c-ray repeaters or anomalous X-ray pulsars. Very Large Telescope. A late-time observation by the XMM- Soft c-ray repeaters are a rare type of c-ray transient sources that Newton spacecraft ,173 days after the burst failed to detect the are occasionally detected as bursters in the high-energy sky2–4.No source, imposing an upper limit (3s) to any underlying X-ray flux 214 22 21 optical counterpart to the c-ray flares or the quiescent source has of LX # 3.1 3 10 erg cm s (0.2–10 keV). yet been identified. Here we report multi-wavelength observations Our 12CO (J 5 120) spectrum towards the SWIFT J1955091261406 of a puzzling source, SWIFT J1955091261406. We detected more source reveals a molecular cloud at ,30 km s21, which contributes than 40 flaring episodes in the optical band over a time span of ,50% to the total column density N(H) derived by Swift/XRT (see three days, and a faint infrared flare 11 days later, after which the Supplementary Information section 5.1). In fact, the overall Galactic source returned to quiescence. Our radio observations confirm a column density along the line of sight towards (lII,bII) 5 (63.5u, 21.0u) 21 22 Galactic nature and establish a lower distance limit of ,3.7 kpc. is N(H) 5 N(H I) 1 2N(H2) 5 (14.1 6 2.0) 3 10 cm which should We suggest that SWIFT J1955091261406 could be an isolated be compared with the X-ray absorption column derived from the Swift/ z4| 21 {2 7 z3| 21 {2 magnetar whose bursting activity has been detected at optical XRT data: 10{3 10 cm from this work, or 7:2{2 10 cm wavelengths, and for which the long-term X-ray emission is (all quoted errors here being 3s).ThereforeweconcludethatSWIFT short-lived. In this case, a new manifestation of magnetar activity J1955091261406 is located in the Galaxy and beyond this particular has been recorded and we can consider SWIFT J1955091261406 to molecular cloud at a kinematic distance of D < 3.7 kpc from the Sun. be a link between the ‘persistent’ soft c-ray repeaters/anomalous This value is consistent with ,4 kpc derived from the ‘red clump’ X-ray pulsars and dim isolated neutron stars. method (see Supplementary Information 5.2). Hereafter, we consider Following the detection5 of GRB 070610 as a single peaked c-ray a reference distance of 5 kpc. burst (GRB) lasting about 4.6 s and its bizarre X-ray counterpart6–8 To discern the nature of the source, we explored several possibil- (dubbed SWIFT J1955091261406), we mounted a multi-wavelength ities. The first is that the source resembles the ‘bursting pulsar’ GRO observing campaign (see Supplementary Information sections 1–4 J1744–28 (refs 11,12). However, Swift/BAT has not recorded any for details). Our data were collected starting ,1 min after the burst other c-ray burst from SWIFT J1955091261406 after the initial trigger time. In the first three nights of our observations, the source one. A second possibility is based on the proposed similarity to the displayed strong optical flaring activity7–9. This, together with the black hole candidate V4641 Sgr13,14. This black hole, orbiting an location of the source in the Galactic plane, supported the view that intermediate-mass companion (a B9 subgiant), was suggested as the source is hosted by the Milky Way10, and we give strong evidence the first member of the ‘fast X-ray novae’ group15, and it has been for this here. proposed that SWIFT J1955091261406 is a member of this class7. The flares from SWIFT J1955091261406 had durations ranging However, several lines of evidence indicate otherwise. First, the lack from tens of seconds to a few minutes and flux amplitudes up to of further detections of the baseline (non-flaring) flux during the about 100 times the ‘outburst’ baseline flux (or $104 times the qui- outburst phase at c-ray (Swift/BAT), millimetre (during the out- escent state). After 13 June, the activity decayed abruptly (Fig. 1) and burst, ,0.6 mJy, 3s; Supplementary Information 2) and centimetre 1Instituto de Astrofı´sica de Andalucı´a del Consejo Superior de Investigaciones Cientı´ficas (IAA-CSIC), PO Box 03004, E-18080 Granada, Spain. 2European Southern Observatory, Casilla 19001, Santiago 19, Chile. 3Special Astrophysical Observatory of Russian Academy of Science (SAO-RAS), Nizhnij Arkhyz, Karachai-Cherkessia, 369167 Russia. 4Thu¨ringer Landessternwarte Tautenburg, Sternwarte 5, D-07778 Tautenburg, Germany. 5Laboratoire d’Astrophysique, E´cole Polytechnique Fe´de´rale de Lausanne (EPFL) Observatoire, 1290 Sauverny, Switzerland. 6Institute de Radioastronomie Millime´trique (IRAM), 300 rue de la Piscine, 38406 Saint Martin d’He´res, France. 7Facultad de Ciencias Experimentales, Universidad de Jae´n, Campus Las Lagunillas, E-23071 Jae´n, Spain. 8Department of Physics and Astronomy, The University of Leicester, Leicester, LE1 7RH, UK. 9School of Physics, University College Dublin, Dublin 4, Ireland. 10Astronomical Institute ‘Anton Pannekoek’, University of Amsterdam, 1098 SJ Amsterdam, The Netherlands. 11Institute de Radioastronomie Millime´trique (IRAM), Avda. Divina Pastora 7, Nu´cleo Central, E-18012 Granada, Spain. 12Max-Planck-Institut fu¨r Radioastronomie, Auf dem Hu¨gel 69, D-53121 Bonn, Germany. 13Cork Institute of Technology, Rossa Avenue, Bishopstown (Cork), Ireland. 14Universidad de Valencia, Edif. Institutos de Investigacio´n (GACE-ICMOL), Campus de Paterna, E-46980 Paterna (Valencia), Spain. 15Department of Astronomy, University of Texas, Austin, Texas 78712, USA. 16Astronomical Institute of the Czech Academy of Sciences, Fricˇova 298, 25165 Ondrˇejov, Czech Republic. 17Instituto de Astrofı´sica de Canarias (IAC), Via La´ctea s/n, E-38205 La Laguna (Tenerife), Spain. 18Nikolaev State University, Nikolskaya 24, 54030 Nikolaev, Ukraine. 19Aryabhatta Research Institute of Observational-Sciences (ARIES), Manora Peak, NainiTal, Uttarakhand, 263129, India. 20Crimean Astrophysical Observatory, 98409 Nauchny, Ukraine. 21University of Cukurova, Department of Physics, 01330 Adana, Turkey. 22European Space Astronomy Centre (ESAC), Avenida de los Castillos s/n, Urbanizacion Villafranca del Castillo, E-28691 Villanueva de la Can˜ada (Madrid), Spain. 23Instituto Nacional de Te´cnica Aeroespacial (INTA), E-28750 Torrejo´nde Ardoz (Madrid), Spain. 506 © 2008 Macmillan Publishers Limited. All rights reserved NATURE | Vol 455 | 25 September 2008 LETTERS a 14 a 16 18 20 -band magnitude c l 22 24 10–9 b ) –1 10–10 s 2 10–11 10–12 b –13 X-ray flux (erg cm X-ray flux (erg 10 10–14 0.01 0.10 1.00 10.00 100.00 Time since Swift/BAT detection (days) Figure 1 | Optical and X-ray light curves of SWIFT J1955091261406 (June–November 2007). a, Optical detections (Ic-band magnitudes, filled circles, with 1s error bars) are shown together with 3s upper limits (triangles). b, Swift X-ray data (0.2–10 keV, filled circles, with 1s error bars) together with the late-time 3s limit obtained with XMM-Newton (triangle). Both light curves show strong activity during the first three days, reaching N the maximum around one day after the c-ray burst and gradually decaying after the third day until the source became undetectable. The X-ray E observations made by Swift do not overlap with the times of any of the 3.5˝ optical flares that we have recorded. However, observations in both X-ray and optical agree that the strongest flaring activity is found around one day Figure 2 | Deep, late observations of the SWIFT J1955091261406 field. after the c-ray event. A short (,30-s) powerful X-ray flare, for which the flux a, Deep Ic-band image obtained with the 6.0-m Big Telescope Altazimuthal increased by a factor of Df/f < 100 on a timescale of Dt/t < 1024, was (using SCORPIO) on 12 October 2007. b, Deep H-band image obtained on followed by several optical flares of similar amplitude. The X-ray data one 30 September 2007 with the 8.2-m Very Large Telescope (using NACO) day after the giant flare event (excluding minor flaring-like activity) can be using natural guide-star adaptive optics. Both images show that the source fitted by a power-law decay F / ta with a 520.75 6 0.25, consistent with has disappeared. The location for SWIFT J1955091261406 is marked with a 22 the values seen in the decline phase of the anomalous X-ray pulsar XTE circle (error radius of 0.260). The limiting magnitudes are Ic . 23.5 and J1810-197 and the transient magnetar23 SGR 1627-41.
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