Magnetars: Distances, Variability and Multi-Wavelength Observations by Martin Durant A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Graduate Department of Astronomy & Astrophysics University of Toronto Copyright c 2006 by Martin Durant ii Abstract Magnetars: Distances, Variability and Multi-Wavelength Observations Martin Durant Doctor of Philosophy Graduate Department of Astronomy & Astrophysics University of Toronto 2006 The Anomalous X-ray Pulsars (AXPs) represent our best opportunity to study in detail the physics of magnetars: neutron stars with amazingly strong magnetic fields, so strong, that they fundamentally affect their energetics, evolution and appearance. In order to con- straints the processes occurring in magnetar interiors and magnetospheres, I have conducted a phenomenological study of their behaviour. I have observed AXPs in the infrared, optical and X-ray bands, and measured both their variability and global spectral energy distributions. By measuring the interstellar extinction, these spectra do not have the large uncertainties of previous works, and by measuring the distances to the objects, the spectra can be fairly compared. During this work: infrared or optical counterparts have been found for all but one AXP; the brightest source (4U 0142+61) was found to vary rapidly and significantly in each waveband over years of monitoring, yet with no apparent correlations between spectral regions; the contradictions in the previous measures of extinction were resolved; and all six Galactic AXPs had their distances determined and were placed on a map of the Galactic Plane. From these studies, I find that, despite differences among AXPs and their variability in time, some patterns emerge. Perhaps most importantly, the X-ray luminosities of AXPs are very similar, as are their X-ray to infrared flux ratios. In this way, information is starting to mount with which to confront theoretical models, and understand the underlying conditions that give rise to the emission seen. iii Acknowledgements Thanks to my supervisor, Marten van Kerkwijk, whose ideas and methodology can be seen throughout this thesis. Thanks to others who helped me with my work over the last four years: the good people of McGill, Vik Dhillon, Bryan Gaesler and others. Specific thanks to those who helped to correct my thesis: Kevin, Hilding and Heather, and my external Deepto Chakrabarty. I have been dealt with well by telescopes, been glad for the existance of data archives, and had smooth computing throughout thanks to Hugh and Ross. My parents have ever been behind me, when I needed reassurance; Szonya has told me just how to deal with the academic world, and finally, for keeping me going day to day, I thank Sharon. iv Contents 1 Introduction 1 1.1 OpeningRemarks................................. 1 1.2 NeutronStarBasics............................... 2 1.3 HistoryofNeutronStarObservations . ...... 2 1.4 NewDiversity................................... 5 1.4.1 SoftGamma-rayRepeaters. 5 1.4.2 AnomalousX-rayPulsars . .. .. 5 1.4.3 DimIsolatedNeutronStars . 6 1.4.4 CentralCompactObjects . .. .. 6 1.4.5 High-BRadioCut-off? .......................... 7 1.5 TheMagnetarTheory .............................. 7 1.5.1 Origins................................... 8 1.5.2 Energetics................................. 9 1.5.3 EmissionMechanisms........................... 9 1.5.4 RadiativeProcesses. 11 1.6 The Anomalous X-ray Pulsars in Detail . ..... 12 1.6.1 TimingCharacteristics . 12 1.6.2 X-raySpectra............................... 14 1.6.3 OpticalandInfrared ........................... 14 1.6.4 RecentDevelopments........................... 14 1.6.5 Alternative Theoretical Descriptions . ...... 16 1.7 ThesisSummary ................................. 18 2 The Broad-Band Spectrum and Variability of 1E 1048 5937 19 − 2.1 Abstract...................................... 19 2.2 Introduction.................................... 19 2.3 ObservationsandAnalysis . ... 20 2.3.1 Astrometry ................................ 23 2.3.2 VLT/ISAAC ............................... 23 2.3.3 Magellan/PANIC ............................. 24 2.3.4 Magellan/MagIC ............................. 24 2.4 ResultsandImplications . ... 25 2.4.1 Variability................................. 25 2.4.2 DistanceandReddening . .. .. 30 2.4.3 SpectralEnergyDistribution. ... 31 2.4.4 X-raytoInfraredFluxRatio. 31 v 2.5 Discussion..................................... 33 3 Further Counterpart Searches 37 3.1 Abstract...................................... 37 3.2 Optical Detection of CXOU J010043.1 721134................. 37 − 3.2.1 Abstract.................................. 37 3.2.2 Introduction................................ 37 3.2.3 Archival Observation and Analysis . ... 38 3.2.4 Discussion and Conclusions . 42 3.3 A Deep Infrared Search for AXP 1E 1841 045................. 44 − 3.3.1 Abstract.................................. 44 3.3.2 Introduction................................ 44 3.3.3 Observationandanalysis. 44 3.3.4 Results................................... 45 3.4 The infrared counterpart to 1RXS J170849.0 400910............. 50 − 3.4.1 Abstract.................................. 50 3.4.2 Introduction................................ 50 3.4.3 ObservationandAnalysis . 51 3.4.4 Magellan.................................. 51 3.4.5 CFHT ................................... 53 3.4.6 VLT/NACO................................ 54 3.4.7 Results................................... 55 3.4.8 Variability................................. 55 3.4.9 Stellarcolors ............................... 56 3.4.10 X-raytoinfraredfluxratio. .. 56 3.4.11 Conclusions ................................ 58 3.4.12 Appendix:Photometry. 58 4 Multi-wavelength variability of the magnetar 4U 0142+61 63 4.1 Abstract...................................... 63 4.2 Introduction.................................... 63 4.3 Near-InfraredObservations. ..... 64 4.3.1 Gemini................................... 65 4.3.2 CFHT ................................... 65 4.3.3 Subaru................................... 66 4.4 OpticalObservations . .. .. .. 66 4.4.1 Keck.................................... 66 4.4.2 Gemini................................... 69 4.5 X-rayObservations ............................... 69 4.5.1 RXTE................................... 69 4.5.2 XMM-Newton............................... 70 4.6 Results....................................... 71 4.6.1 Variability Time-scales . .. 71 4.6.2 Correlations................................ 71 4.6.3 SpectralChangesandComponents . 71 4.7 Conclusions and Discussion . ... 75 vi 5 Extinction and intrinsic spectra of AXPs 79 5.1 Abstract...................................... 79 5.2 Introduction.................................... 79 5.3 DataReduction.................................. 82 5.4 Analysis ...................................... 84 5.5 FittingMethodandErrorDetermination . ...... 84 5.6 Results....................................... 86 5.7 IntrinsicSpectra ................................ 88 5.8 Opticalextinction............................... .. 93 5.9 The Spectral Energy Distribution of 4U 0142+61 . ....... 94 5.10Conclusions .................................... 94 6 Distances to Anomalous X-ray Pulsars using Red Clump Stars 97 6.1 Abstract...................................... 97 6.2 Introduction.................................... 97 6.3 Method ...................................... 99 6.4 RobustnessandUncertainties . .... 102 6.5 Results....................................... 107 6.6 ComparisonwithPreviousWork. 114 6.6.1 4U0142+61................................ 114 6.6.2 1E 1048.1 5937.............................. 114 6.6.3 1E2259+586− ............................... 115 6.7 Implications.................................... 116 6.8 Conclusions .................................... 121 7 Conclusions 123 7.1 Review....................................... 123 7.1.1 OpticalandInfraredCounterparts . 123 7.1.2 Variability................................. 124 7.1.3 IntrinsicSpectra ............................. 124 7.1.4 Luminosities................................ 124 7.2 Implications: From the Inside Out . ..... 124 7.3 Outlook ...................................... 127 Bibliography 129 vii viii Chapter 1 Introduction 1.1 Opening Remarks This thesis is a phenomenological study of the Anomalous X-ray Pulsars (AXPs), one of the groups of magnetars: neutron stars whose energetics and evolution is dominated by their immense magnetic fields. They offer perhaps the best chance to study the behaviour of matter in the most extreme of environments (for electromagnetic effects with relativistic particles, for instance). The AXPs as a group, have an interesting and rich range of observational characteris- tics. Although they (all six AXPs) share a number of parameters (spin, luminosity, X-ray spectrum), each object has certain behaviours and properties that make it unique. A central question then has to be, what causes the differences among the AXPs? Conversely, what intrinsic attribute of the AXPs sets them apart from the other types of neutron stars? In order to answer these questions, the properties and behaviours must first be thoroughly and systematically described. In this introduction, I present the background material relating to my research on the AXPs. First I present a brief overview of neutron stars in general, and the motivations for their study. Next, I give a brief history of the observations of neutron stars in the various wavebands. This brings the reader to the state