X-ray observations of the accreting Be, X-ray binary pulsar A 0535+26 in outburst [Elektronische Ressource] / vorgelegt von Isabel Caballero Doménech

Sujets

• Astronomie

Informations

• Publié par: eberhard_karls_universitat_tubingen
• Publié le: 01 janvier 2009
• Nombre de lectures: 10
• Langue: English
• Poids de l'ouvrage: 6 Mo

Extrait

X-ray observations of the
accreting Be/X-ray binary pulsar
A 0535+26 in outburst
Dissertation
zur Erlangung des Grades eines
Doktors der Naturwissenschaften
der Fakultät für Mathematik und Physik
der Eberhard Karls Universität Tübingen
vorgelegt von
ISABEL CABALLERO DOMÉNECH
aus Madrid
2009Selbstverlegt von: I. Caballero Doménech, Neue Straße 16, 72070 Tübingen
Tag der mündlichen Prüfung: 20.04.2009
Dekan: Prof. Dr. W. Knapp
1. Berichterstatter: Prof. Dr. A. Santangelo
2. Berichterstatter: Prof. Dr. K. WernerAbstract
Neutron stars are compact objects, characterized byR∼ 10−14 km radius,M ∼
15 −31.4 M mass and extremely high central densities ρ ∼ 10 g cm . If they are part⊙
of a binary system, a flow of matter can take place from the companion star onto the
neutron star. The accretion of matter onto neutron stars is one of the most powerful
sources of energy in the universe. The accretion of matter takes place under extreme
8−15physical conditions, with magnetic fields in the range B ∼ 10 G, which are
impossible to reproduce on terrestrial laboratories. Therefore, accreting neutron stars
are unique laboratories to study the matter under extreme conditions.
In this thesis, X-ray observations of the accreting Be/X-ray binary A 0535+26
during a normal (type I) outburst are presented. In this system, the neutron star
orbits around the optical companion HDE 245770 in an eccentric orbit, and some-
times presents X-ray outbursts (giant or normal) associated with the passage of the
neutron star through the periastron. After more than eleven years of quiescence,
A 0535+26 showed outbursting activity in 2005. The normal outburst analyzed in
this work took place in August/September 2005, and reached a maximum X-ray flux
ofF ∼ 400 mCrab in the 5−100 keV range. The outburst, which lasted for∼ 30X
days, was observed with the RXTE and INTEGRAL observatories.
We have measured the spectrum of the source. In particular, two absorption-like
features, interpreted as fundamental and first harmonic cyclotron resonant scattering
features, have been detected at E ∼ 46 keV and E ∼ 102 keV with INTEGRAL
and RXTE. Cyclotron lines are the only direct way to measure the magnetic field
of a neutron star. Our observations have allowed to confirm the magnetic field of
12A 0535+26 at the site of the X-ray emission to beB∼ 5×10 G.
We studied the luminosity dependence of the cyclotron line in A 0535+26, and con-
trary to other sources, we found no significant variation of the cyclotron line energy
with the luminosity. Changes of the cyclotron line energy with the X-ray luminosity
are thought to be related to a change in the height of the accretion column as the mass
accretion rate varies.
A detailed timing analysis has been performed, and we find for the first time the
onset of a spin-up, at a phase close to the periastron passage, during a normal outburst,
providing evidence for an accretion disk around the neutron star. Energy-dependent
iiiiv
pulse profiles of the source have been studied and compared to historical observations.
During the rising part of the outburst a series of flares were observed. RXTE ob-
served one of these flares, and we found during the flare the energy of the fundamental
cyclotron line shifted to a significantly higher position compared to the rest of the out-
burst. Also, the energy-dependent pulse profiles during the flare were found to vary
significantly from the rest of the outburst. These differences have been interpreted in
terms of a theoretical model, based on the presence of magnetospheric instabilities at
the onset of the accretion.
We applied a decomposition method to A 0535+26 energy-dependent pulse profiles.
Basic assumptions of the method are that the asymmetry observed in the pulse pro-
files is caused by non-antipodal magnetic poles, and that the emission regions have
axisymmetric beam patterns. Using pulse profiles obtained from RXTE observations,
the contribution of the two emission regions has been disentangled. Constraints on the
geometry of the pulsar and a possible solution of the beam pattern are given. The re-
constructed beam pattern is interpreted in terms of a geometrical model that includes
relativistic light deflection.To my fatherContents
1 Introduction 1
1.1 History of X-ray astronomy . . . . . . . . . . . . . . . . . . . . . 1
1.2 Thesis outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2 Accreting X-ray pulsars 6
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1.1 X-ray binaries . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1.2 Neutron star basic properties . . . . . . . . . . . . . . . . 7
2.1.3 Accretion . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.1.4 Mass transfer mechanisms in X-ray binaries . . . . . . . . 15
2.2 Neutron star X-ray binaries . . . . . . . . . . . . . . . . . . . . . 19
2.2.1 Accretion geometry . . . . . . . . . . . . . . . . . . . . . 19
2.2.2 Disk accretion . . . . . . . . . . . . . . . . . . . . . . . . 21
2.2.3 Torque theory . . . . . . . . . . . . . . . . . . . . . . . . 22
2.2.4 Accretion into the polar caps . . . . . . . . . . . . . . . . 23
2.2.5 Spectral formation . . . . . . . . . . . . . . . . . . . . . 24
2.2.6 Magnetic fields . . . . . . . . . . . . . . . . . . . . . . . 26
2.3 Observations of neutron star X-ray binaries . . . . . . . . . . . . 28
2.3.1 Cyclotron lines . . . . . . . . . . . . . . . . . . . . . . . 28
2.3.2 Pulse profiles . . . . . . . . . . . . . . . . . . . . . . . . 33
3 The Be/X-ray binary system A 0535+26 37
3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.1.1 History of outbursts . . . . . . . . . . . . . . . . . . . . . 38
3.1.2 The source in quiescence . . . . . . . . . . . . . . . . . . 38
3.2 Timing properties . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.2.1 Pulse period evolution . . . . . . . . . . . . . . . . . . . 40
3.2.2 QPOs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.2.3 Pulse profiles . . . . . . . . . . . . . . . . . . . . . . . . 42
3.3 Spectral properties . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.4 Optical observations . . . . . . . . . . . . . . . . . . . . . . . . 43
viiviii Contents
4 RXTE and INTEGRAL observatories 46
4.1 RXTE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
4.1.1 The Proportional Counter Array PCA . . . . . . . . . . . 46
4.1.2 The High Energy X-ray Timing Experiment HEXTE . . . 48
4.1.3 The All Sky Monitor ASM . . . . . . . . . . . . . . . . 49
4.2 INTEGRAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.2.1 Imaging with INTEGRAL . . . . . . . . . . . . . . . . . 50
4.2.2 SPI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.2.3 IBIS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.2.4 JEM-X . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.2.5 OMC . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.2.6 Dithering . . . . . . . . . . . . . . . . . . . . . . . . . . 55
5 A 0535+26 observations with RXTE and INTEGRAL 57
5.1 Overview of the August/September 2005 outburst . . . . . . . . . 57
5.1.1 Optical light curve with OMC . . . . . . . . . . . . . . . 60
5.2 RXTE and INTEGRAL data analysis . . . . . . . . . . . . . . . 61
5.2.1 X-ray spectral analysis . . . . . . . . . . . . . . . . . . . 61
5.2.2 RXTE data extraction . . . . . . . . . . . . . . . . . . . 63
5.2.3 INTEGRAL data extraction . . . . . . . . . . . . . . . . 64
6 The main outburst 66
6.1 Overview: RXTE and INTEGRAL light curves . . . . . . . . . . 66
6.2 Pulse period determination . . . . . . . . . . . . . . . . . . . . . 67
6.2.1 First estimate: epoch folding . . . . . . . . . . . . . . . . 67
6.2.2 Phase connection method . . . . . . . . . . . . . . . . . . 68
6.3 Pulse profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
6.4 Spectral analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 77
6.4.1 Cyclotron lines . . . . . . . . . . . . . . . . . . . . . . . 79
6.4.2 Cyclotron line energy evolution with X-ray luminosity . . 81
6.4.3 Phase resolved spectroscopy . . . . . . . . . . . . . . . . 83
6.5 Discussion of results . . . . . . . . . . . . . . . . . . . . . . . . 85
6.5.1 Pulse period and pulse profiles . . . . . . . . . . . . . . . 85
6.5.2 Cyclotron lines . . . . . . . . . . . . . . . . . . . . . . . 89
6.5.3 Torque theory and spin-up. . . . . . . . . . . . . . . . . . 91Contents ix
7 Pre-outburst flaring activity 93
7.1 Observations during the pre-outburst flare . . . . . . . . . . . . . 93
7.1.1 Change in the energy dependent pulse profiles . . . . . . . 93
7.1.2 Change in the cyclotron line energy . . . . . . . . . . . . 94
7.2 Evidence for magnetospheric instability . . . . . . . . . . . . . . 96
7.2.1 Summary of results . . . . . . . . . . . . . . . . . . . . . 96
7.2.2 Disk accretion and mass transport in the magnetosphere. . 99
7.2.3 Low-mode magnetospheric instability in A 0535+26 . . . 102
8 Pulse profile decomposition 108
8.1 Description of the method . . . . . . . . . . . . . . . . . . . . . 108
8.1.1 Assumptions . . . . . . . . . . . . . . . . . . . . . . . . 109
8.1.2 Steps of the method . . . . . . . . . . . . . . . . . . . . . 111
8.2 Application to A 0535+26 . . . . .