(Nature, 405, pp ) on behalf of a large Collaboration

Transcription

1 Rapid flaring in a new Galactic source: a missing link between magnetars and dim isolated neutron stars? (Nature, 405, pp ) 509) Alberto J. CASTRO-TIRADO (IAA-CSIC Granada) on behalf of a large Collaboration VILLAFRANCA DEL CASTILLO, 30 OCT 2008

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3 Collaboration A.J. Castro-Tirado (1), A. de Ugarte Postigo (2,1), J. Gorosabel (1), M. Jelínek (1), T. A. Fatkhullin (3), V. V. Sokolov (3), P. Ferrero (4), D. A. Kann (4), S. Klose (4), D. Sluse (5), M. Bremer (6), J. M. Winters (6), D. Nuernberger (2), D. Pérez-Ramírez (7), M. A. Guerrero (1), J. French (8), G. Melady (8), L. Hanlon (8), B. McBreen (8), K. Leventis (9), S. Markoff (9), S. Leon (6), A. Kraus (10), F. J. Aceituno (1), R. Cunniffe (1), P. Kubánek (1) S. Vítek (1), S. Schulze (4), A. C. Wilson (11), R. Hudec (12), V. Simon (12), J. M. González-Pérez (13), T. Shahbaz (13), S. Guziy (14), S. B. Pandey (15), L. Pavlenko (16), E. Sonbas (17), S. Trushkin (3), N. Bursov (3), C. Sánchez.-Fernández (18) & L. Sabau-Graziati (19) (1) IAA-CSIC, Camino Bajo de Huétor, 50, E Granada, Spain (2) ESO, Alonso de Córdova 3107, Vitacura, Santiago, Chile (3) SAO-RAS, Nizhnij Arkhyz, Karachai-Cirkassian Rep., Russia (4) Thüringer Landessternwarte Tautenburg, Sternwarte 5, D Tautenburg, Germany (5) Laboratoire d'astrophysique, EPFL, Observatoire, 1290 Sauverny, Switzerland (6) IRAM, 300 rue de la Piscine, Saint Martin d'héres, France (7) Facultad de Ciencias Experimentales, Universidad de Jaén, Campus Las Lagunillas, E Jaén, Spain (8) School of Physics, University College, Dublin, Ireland (9) Astronomical Institute Anton Pannekoek, Univ. of Amsterdam, The Netherlands (10) MPI fur Radioastronomie, Bonn, Germany (11) Department of Astronomy, University of Texas, Austin, TX 78712, USA (12) Astronomical Institute of the Czech Academy of Sciences, Ondrejov (13) IAC, Vía Láctea s/n, La Laguna, Tenerife, Spain (14) Nikolaev State University, Nikolskaya 24, Nikolaev, Ukraine (15) ARIES, NainiTal, India (16) Crimean Astrophysical Observatory, Nauchnyv, Ukraine (17) Univ. of Cukurova, Dep. of Physics, Adana, Turkey (18) ESAC Villafranca del Castillo (Madrid), Spain (19) INTA, Ctra. de Ajalvir km. 4, Torrejón de Ardoz (Madrid), Spain

4 Outline Introduction (neutron stars, pulsars, magnetars, ) GRB : a very unusual X-ray/opt X ray/opt counterpart SWIFT J : Extragalactic or Galactic? Distance scale Its nature: a likely magnetar (but a UCXRB cannot be completely excluded) Implications: a missing link in the NS family?

5 I. Introduction Neutron stars, pulsars, magnetars,

6 Neutron star flavours Accretion-powered (mainly X-ray X binaries) Rotation-powered (ordinary pulsars) Ultra-strong magnetic field-powered (magnetars( magnetars) Rotation and magnetic field powered (one case!)

7 Ultra-strong B-powered: B Magnetars First detection on 5 Mar 1979 by Mazets et al. (1979): a tremendous, short burst of soft gamma- rays arising from the SNR N49 in LMC. Dubbed SGR ( Soft Gamma-ray Repeater ). The localization superimposed to the SNR N49 (Cline et al. 1979) made straightforward its relationship to a NS Another SGRs discovered in the next decades (half a dozen so far) An X-ray X burst arising from SGR was detected in 1993, confirming the NS origin (Murakami et al. 1993) Detection of pulsations in SGR (Kouveliotou et al. 1998) proposed a rotating NS with B~5 B x G The magnetar model (Duncan & Thompson 1992) Anomalous X-ray X pulsars join magnetars in 2000 s. A magnetic twist gives rise to X-ray emissions (from R. Duncan)

8 II. The γ-ray burst A very unusual X-ray X and optical counterpart (SWIFT J )

9 Swift detection of a GRB Detected on 10 June :52:26 UT by Swift/BAT as a normal burst, indistinguishable from cosmic GRBs (Pagani et al. GCN 6489) at galactic ccordinates (l l, b) = (63.5, -1.0) T 90 ~5s (longer and harder than soft gamma-ray repeater bursts) Photon index 1.76 ± 0.25 Fluence (2.4 ± 0.4) 10-7 erg/cm 2 (Tueller et al. GCN 6491) Swift/XRT detected an X-ray counterpart 3100s later (Pagani( et al. GCN 6490) with a column density slightly lower than the Galactic. GRB

10 The optical counterpart Stefanescu et al. (GCN 6492) reported the detection of a variable optical counterpart. de Ugarte Postigo et al. (GCN 6501) confirmed the detection with observations from the 1.5m OSN. D.A. Kann et al. (GCN 6505) suggested a Galactic origin, based on unusual flaring activity and location near the galactic plane: l=63.3º b=-1º Dubbed SWIFT J

11 Our dataset Between 1 minute and 1 year after the burst: Radio observations from RATAN-600 and 100m Effelsberg Millimetre observations from Plateau de Bure & IRAM NIR imaging from 8.2m VLT (including AO) Optical data from 1.5-m m OSN, 0.3-m m BOOTES-2, 0.4-m m WATCHER, 1.34-m m TLS, 6.0-m m BTA and 8.2- m VLT X-ray data from Swift/XRT & XMM-Newton

12 Optical and X-ray X light curves We recorded ~ 40 flare episodes Up to I~14.8, timescales of ~20sec - 7 min Amplitudes up to 7 mag Early light curve / early flares (I) 1

13 Optical light curve (I) Early light curve / early flares (II) An example of a dense series of VRI observations (duration ~75 min) (June 10, 2007). J1955 was near the detection limit, but peaks of several flares are resolved. Amplitude of the brightest R band flare is > 1 mag(r). Baseline brightness is below the detection limit. Dense series of VRI observations (duration ~65 min) (June 11, 2007). J1955 was clearly detected. The I and R data are not quite simultaneous. Significant fluctuations of the brightness are apparent (~1 mag, timescale ~15 min). Baseline brightness is welldefined both in R and I.

14 Optical light curve (II) Early light curve / early flares (III) R I Statistical distribution of R and I magnitudes in the June 11 series. The most abundant faintest magnitude and negative skewness imply that the observed brightness fluctuations have the form of peaks (spikes) from a well-defined baseline brightness. The mutual similarity of the statistical distributions of R and I magnitudes in this night series enables to determine the mean color index R-I from the mean mag in each filter. Comparison of the color index R-I from the June 11 series with the color of OAs of GRBs from Simon et al. (2004). Colors of the ensemble are de-reddened for the Galactic extinction and are in the observer frame. The color of J1955 was dereddened by A V = 4 mag, near the lower limit A V =3.6 mag of the suggested range. Further increase of A V leads to the decrease of the color index (arrow). (R-I) 0 of J1955 is thus similar to that of the ensemble of OAs and is consistent with the synchrotron emission (e.g. Sari et al. 1998). Thus thermal emission is ruled out.

15 Optical light curve (III) 2 3

16 Optical light curve (IV) Late flaring After the third day activity decreases A faint source was still detectable Last flare (nir( nir) ) is detected 11 days after

17 Optical light curve (V) Light curve- 3 (quiet level) 3 3

18 Optical light curve (VI) Baseline flux evolution The emission between flares slowly decreased until it disappeared. No detectable quiescent counterpart once the flaring emission ceased Limits: H, I > 23 (VLT, SAO, this work) R > 26.5 (Keck, Kasliwal et al. paper, ApJ 2008) I > 24.5 (Keck, Kasliwal et al. paper, ApJ 2008) June September October

19 Optical spectroscopy and Murphy s s law Attempted at the 6.0m BTA the 11th and the 12th (bad weather; green) Finally achieved on the 13th when the source was already I~23 (arrow( arrow) 6x600s BTA spectra showed not even a detection of continuum

20 III. Distance scale The H column in the line of sight vs the absorption column from the X-ray X spectrum The red giants clump method

21 Distance scale (I) G. Kanbach & A. Stefanescu

22 Distance scale (II) X-ray (Swift/XRT data): PL N H = ( ) x 1021 cm -2 A V = mag following E(B-V) = N H /5.9x10 21 cm -2 H I observations (21 cm line) (Effelsberg), 3 x 3 raster p. N H I =(8.1±1.2) 1.2) x cm -2

23 Distance scale (III) X-ray N H =( )) x A V = mag H I observations N H I = (8.1±1.2) 1.2) x cm cm -2 H2 observations (PdB) CO(J=1-0) N H2 =(3.0±0.4) 0.4) x cm -2 Cloud at a kinematic distance of 3.7 kpc producing 50% of the N H d > 3.7 kpc (tangent point) N H = N H I + 2 N H2 = (14.1±2.0) x cm -2 GALACTIC origin!

24 Red clump method Distance scale (III) (López-Corredoira et al. 2005, Durant and van Kerwijk 2006) A V = mag D~ ~ 4 kpc

25 IV. The SWIFT J nature A magnetar (very likely) OR an ultracompact low-mass X-ray X binary?

26 What is the nature of the source? (I) Observational Facts Gamma-ray burst-like event triggering the activity X-ray, optical and nir (but no radio) flaring events X-ray and optical baseline emission detected as well between the flares but no at radio or soft (15-40 kev) ) gamma-rays. Dramatic optical flaring activity for ~2 2 days No quiescent counterpart once the activity ceased (only deep limits in radio, NIR & X-rays) X Galactic origin

27 What is the nature of the source? (II) Cosmological GRB? Light curve behaviour and location rule it out. Microquasar/Fast X-ray X nova? (Uemura et al. 1999, Kasliwal et al. 2008) Unlike V4641 Sgr,, where strong gamma- ray and radio emission was detected (reaching around 1 Crab in gamma-rays and few mjy in radio, compared to < 0.01 Crab and < 0.1 mjy respectively for the SWIFT source). Bursting pulsar? GRO J , where further gamma-ray activity was observed, is also quite different. No pulsations measured in the SWIFT source for the time being.

28 What is the nature of the source? (III) An ultra-compact low-mass X-ray binary? Flares could be produced by blobs of homogeneous synchrotron-emitting plasma of size of 10 7 cm and magnetic field of strength 10 5 G. The blobs can be found in a magnetized corona (Merloni( et al. 2000) or a wind (rather( than in the outer regions of a collimated jet). This model cannot be ruled out

29 What is the nature of the source? A soft gamma-ray repeater? (IV) SGRs are magnetars (young NS with B = G) with a prolific bursting rate when they are switched-on. Flares would be due to coherent plasma bunches (Thompson & Beloborodov 2005),, leading easily to the observed optical luminosities of : (D/5 kpc) 2 erg/s

30 What is the nature of the source? (V) SGR hypothesis supported by: 1. the lognormal distribution burst fluxes. SWIFT source (Castro-Tirado et al. 2008) SGR (Woods et al. 1999)

31 What is the nature of the source? (VI) 2. similarities with other transient magnetars: -harder, intermediate-duration duration bursts with comparable energy releases to GRB also observed in magnetars SGR (Woods et al. 1999) -similar X-ray X decaying light curves SGR (Kouveliotou et al. 2003)

32 What is the nature of the source? (VII) (Stefanescu et al. 2008) 3. A possible periodicity in the X-ray X data: -similar period as seen in other magnetars (few s)

33 V. Implications A missing link in the NS family?

34 Implications A new magnifestation of magnetar activity, becoming one of the few hundred Galactic ones becoming active in few x 10 4 yr. The quiescent X-ray X luminosity : Lx < 9 x (D/5 kpc) 2 erg/s is intermediate between transient magnetars (including SGRs/AXPs): Lx = (2-4) x erg/s and dim isolated neutron stars (DINs( DINs): Lx = (2-20) 20) x erg/s The missing link between magnetars and DINs?

35 Conclusions

36 Conclusions We observed a normal GRB and found a peculiar galactic source (b( = -1.0) with intense flaring activity. Optical flares of over 4 magnitudes and durations of ~1 min (not observed in classical SGRs,, not only a high Galactic extinction problem). An ultracompact LMXB? The fifth SGR in the Galaxy? First optical couterpart detected for such object. The missing link between magnetars and DINs?