UNIVERSITÀ DEGLI STUDI DI PAVIA DOTTORATO DI RICERCA IN FISICA – XXI CICLO The unique source inside the RCW103 supernova remnant: a decade of X-ray observations Silvia Entradi Supervisors: Prof. A. De Luca Prof. K. Hurley Tesi per il conseguimento del titolo Università degli Studi di Dipartimento di Fisica Istituto Nazionale di Pavia Nucleare e Teorica Astrofisica Istituto Nazionale di Fisica University of California, Nucleare Berkeley DOTTORATO DI RICERCA IN FISICA – XXI CICLO The unique source inside the RCW103 supernova remnant: a decade of X-ray observations Dissertation submitted by Silvia Entradi to obtain the degree of DOTTORE DI RICERCA IN FISICA Supervisor: Prof. Andrea De Luca Università degli Studi di Pavia, DFNT INAF-IASF, Istituto Nazionale di Astrofisica IUSS, Istituto Universitario di Studi Superiore INFN, Istituto Nazionale di Fisica Nucleare Prof. Kevin Hurley University of California, Berkeley, SSL Referee: Dr. Roberto Mignani University College London, MSSL Cover The young SNR RCW 103 and its central source 1E as observed in August 2005 by the EPIC MOS cameras onboard XMM-Newton. Photon energy is color-coded: Red corresponds to the energy range 0.5 to 0.9 keV, green to 0.9 to 1.7 keV, and blue to 1.7 to 8 keV. North is up, east is left. The unique source inside the RCW103 supernova remnant: a decade of X- ray observations. Silvia Entradi PhD thesis – University of Pavia Printed in Pavia, Italy, November 2008 ISBN: 978-88-95767-18-5 Contents Table of contents i Thesis overview v 1 Introduction 1 1.1 Isolated Neutron Stars . 1 1.1.1 Pulsars . 2 1.1.2 Rotating Radio Transients . 4 1.1.3 Anomalous X-ray Pulsars and Soft Gamma Repeaters . 6 1.1.4 X-ray Dim Isolated Neutron Stars . 8 1.1.5 Central Compact Objects . 9 1.2 1E 161348-5055 history . 11 1.2.1 Binary system . 15 1.2.2 Magnetar . 17 1.3 Infrared observations . 21 2 Magnetars 25 2.1 X-ray luminosity . 27 2.1.1 Bursting activity . 27 2.1.2 Persistent emission . 28 2.2 Transient magnetars . 29 2.2.1 XTE J1810-197 . 32 2.2.2 CXOU J1647¡45...................... 34 2.3 Variability in SGRs . 37 2.3.1 SGR 1900+14........................ 37 2.3.2 SGR 1627¡41........................ 38 2.4 Magnetars in a binary system . 40 2.4.1 Cataclysmic Variables . 42 3 XMM data 47 3.1 Observations . 47 3.2 Background spectral analysis . 48 i CONTENTS 3.3 Source spectral time-integrated analysis . 50 3.3.1 Extrapolation to infrared . 56 3.4 Phase-resolved analysis . 57 3.4.1 2001 observation . 57 3.4.2 2005 observation . 59 3.4.3 Ratio of the spectra . 65 3.4.4 First considerations . 67 4 Chandra data 69 4.1 Observations . 69 4.2 Data processing . 70 4.3 Spectral analysis . 71 4.4 Observation 970 . 76 4.5 Observation 7619 . 81 4.6 Considerations on fluxes . 82 5 Swift data 85 5.1 Observations . 85 5.2 Data processing . 85 5.3 Spectral analysis . 87 6 Temporal analysis 95 6.1 Preliminary analysis . 95 6.2 Rayleigh test . 98 6.2.1 Swift dataset . 99 6.3 Re¯ning the period with the observations from 2005 on . 107 6.3.1 Adding XMM-Newton 2005 observation . 108 6.4 Chandra monitoring . 112 6.4.1 Adding the 2005 and 2004 observations . 112 6.4.2 Lightcurves . 116 6.5 Considerations . 120 7 Discussion and Conclusions 123 7.1 Flux evolution . 123 7.1.1 The Light Curve of 1E 161348-5055: Emitted Energy . 124 7.2 Spectral evolution . 128 7.3 Cooling . 131 7.3.1 Magnetic vs. Accretion-powered Outburst . 133 7.3.2 Where Was the Outburst Energy Stored? . 137 7.4 Timing properties . 141 7.4.1 Constant period . 142 7.4.2 Pulsed pro¯le changes . 145 7.5 Conclusions . 151 ii CONTENTS A XMM-Newton 157 A.1 X-ray telescopes . 160 A.1.1 Point Spread Function . 160 A.1.2 E®ective Area . 162 A.1.3 European Photon Imaging Camera (EPIC) . 164 A.1.4 Science modes of the EPIC cameras . 166 A.1.5 Reflection Grating Spectrometers (RGS) . 167 A.1.6 Optical Monitor . 168 B Chandra 169 B.1 Instruments . 169 B.1.1 High Resolution Camera . 170 B.1.2 Advanced CCD Imaging Spectrometer . 173 B.1.3 Comparison . 177 B.1.4 High Resolution Mirror Assembly o®set Point Spread Function . 177 C Swift 181 C.1 Instruments . 181 C.1.1 Burst Alert Telescope . 182 C.1.2 X-Ray Telescope . 183 C.1.3 Ultraviolet/Optical Telescope . 185 Acknowledgements 195 iii Thesis overview Neutron stars are a very variegate class of astrophysical sources that, since their discovery in 1968 by Bell and Hewish, have been extensively studied, and are now catalogued into several classes, that include radio quiet (Central Compact Objects, Soft Gamma Repeaters, Anomalous X-ray Pulsars and X-ray Dim Isolated Neutron Stars) and radio loud neutron stars (Pulsars, Rotating Radio Transients). Nonetheless, many questions are still unanswered, especially when dealing with the life of a neutron star and the consequent evolution of its emission. For this reason the study of young neutron stars is of fundamental importance. Young neutron stars are usually found in Supernova Remnants, and thus are generally classi¯ed as Central Compact Objects, which is the less homogeneous group. The objects that compose this class are very di®erent from each other, and may represent a sample of how a young star emits. For this reason it is extremely important to study these sources, and the one I chose for my thesis is one of the most puzzling. 1E 161348-5055 is a very peculiar point-like X-ray source, which has been extensively studied in the past years. It is located very close to the geometrical center of the young (» 2000 yr) RCW103 Supernova Remnant, 3.3 kpc far away from the Earth. The source shows a unique behavior: since its discovery in 1980, when it was ¯rst detected by Einstein as a faint, unresolved source, it was clear that the object did not have a point source radio counterpart, but its emission couldn't be ascribed to a simple thermal cooling, or to the spectrum expected in case of an accreting binary system. This radio quiet source then underwent a period of intense variability: in 1994 its flux increased by a factor 4 (one order of magnitude) from its quiescence value (» 9 £ 10¡13 erg/cm2 s), recovering the pre-brightening level in four years. A second, major episode of brightening of 1E 161348-5055 happened in 2000, when it was serendipitously detected by Chandra with a very high flux: » 9 £ 10¡11 erg/cm2 s. A follow up by means of Chandra/ACIS was then scheduled. It showed clearly that the flux was decreasing with a smooth trend, yet the origin of the outburst remained unknown, ¯rst because the type of v Thesis overview source itself was not understood, second because the peak of the emission was not detected. The Chandra monitoring was integrated with two XMM-Newton obser- vations. The ¯rst one, obtained in 2001, pointed out how the pro¯le of the lightcurve had dramatically changed as a consequence of the outburst, raising several questions on how traditional models on neutron stars could explain such variability. The second one, made in 2005, was a real breakthrough. This very long observation showed with no uncertainties that 1E 161348-5055 has a period of 24000 s, that corresponds to 6.67 hr, the longest ever detected in case the source were an isolated neutron star. The comparison between the two XMM-Newton observations, which both provided a very abundant statistic, highlighted that the spectral properties of 1E 161348-5055 were varying a lot too: the spectrum of the source was softening, and was clearly a double component model. Unfortunately, though, the lightcurve of the source in 2001 was so complicated and di®erent than the 2005 pro¯le that it was impossible to make a direct comparison between the shapes. Yet in 2005 it was still unclear which kind of object was hosted at the center of the Supernova Remnant, being at least two possibilities open: an isolated neutron star, likely embedded in a ulta-high magnetic ¯eld, where the origin of the outburst would be magnetic, or a neutron star in a binary system, and the brightenings would be driven by accretion episodes. In both cases 1E 161348-5055 was expected to be a unique source: if it were an isolated neutron star, it needs to have a giant magnetic ¯eld to power the emission and the brightenings, and to brake it down to such slow spin period. If instead it were con¯rmed to be a neutron star in a binary system, its geometrical properties have to be very ¯ne tuned. The anomalous properties of 1E 161348-5055 and the major uncertainties that still a®ect our knowledge of this source justi¯ed a global reanalysis of all the available observations. The goals of this thesis are ¯rst of all to introduce in more detail 1E 161348- 5055 and the neutron stars, in chapters 1 and 2. Next, I traced the evolution of the spectral properties of 1E 161348-5055, modeling the time integrated and phase resolved spectrum, in order to derive constraints on the origin of the source (and, as a consequence, of the outbursts), in chapters 3, 4 and 5. Then, I re¯ned the period value and phase connected the huge dataset available, in chapter 6.