Hochenergie Spektren akkretierender Neutronensterne

Accreting neutron stars have been enigmatic objects from the very beginning. While the very concept of an object as small as a little town (radius ~10km) having a mass of at least 1.4 solar masses is already awesome in itself, more and more mysteries were revealed with time. If such a neutron star is forming a binary system with a stellar companion, it can accrete material from its companion (see Section 2.3). Due to the depth of the gravitational well of the neutron star, the material gains tremendous speed during the accretion process. The kinetic energy is released when the material is stopped on (or close to) the surface of the neutron star in form of hard X-rays. The discovery of pulsations proved that the neutron stars were spinning with high frequencies: the fastest rotating neutron star known nowadays is PSR 1937+21 (Ashworth et al. 1983) with a spin period of 1.5ms, very close to the break-up frequency. The pulsations also showed that the emission is not originating from the whole surface of the neutron star, but from one or two hot spots. This again revealed another striking feature of neutron stars: magnetic fields with a strength B of the order of ~10^12G. Although the strength of the magnetic fields of neutron stars was estimated to be of the order of 10^12G from the very beginning, a direct observational proof was still missing. In 1976, Trümper et al. (1978) observed a cyclotron resonant scattering feature (CRSF) in the spectrum of Hercules X-1. CRSFs are due to the quantization of the kinetic energy of electrons in B-fields of the order of 10^12G. This means that the energy of the electrons can only have discrete values: multiples of the cyclotron energy, so called Landau levels. Since photons with Ecyc=h * nu are (almost) instantly absorbed by an electron, they have a very small mean free path, and cannot escape the X-ray formation region. The photons can only leave the plasma if their energy has changed to slightly higher or lower energies due to numerous scattering processes giving rise to an absorption line like feature in the spectrum at the cyclotron energy - a CRSF. CRSFs allow a direct estimate of the magnetic field strength via the 12-B-12 rule: Ecyc = 11.6 x B/10^12G kev. If a CRSF is observed in the spectrum of a neutron star, the strength of the magnetic field can be directly assessed from the energy of the CRSF (after taking gravitational redshift into account which amounts to 25%). In this thesis I used high quality data from NASA's Rossi X-ray Timing Explorer (see Chapter 4) whose spectral broad band and timing capabilities make it an ideal instrument to study accreting pulsars. I analyzed data of Vela X-1 (Chapter 5) and GX 301-2 (Chapter 6), both consisting of a neutron star and an evolved blue supergiant companion. Both chapters are based on publications in Astronomy & Astrophysics (Kreykenbohm at al. 2002a, 2004). In the case of Vela X-1, I addressed the question of the existence of a second CRSF in the spectrum. While one line at ~50kev has been observed by many instruments, a second line at ~24kev had been reported by Kretschmar et al. (1996), while other observers (Orlandini et al. 1997) could not detect this line. Using pulse phase resolved spectroscopy (in pulse phase resolved spectroscopy, separate spectra are derived and analyzed for individual sections of the pulse phase, thus deriving the evolution of the spectral parameters over the pulse), I was able to detect the line in some phase bins, while the line was weak (or insignificant) in other bins. The observed spectral parameters together with the variability of the line also explain why it is not always possible to detect the line. In the case of GX 301-2, the CRSF at ~37kev (Mihara 1995) was already well known and no secondary line could be detected. Using phase resolved spectroscopy I, however, discovered that the CRSF is strongly variable over the pulse. The variations were not random but turned out to be strongly correlated. When the CRSF is deepest, its fractional width (sigma over energy) is also largest. This is especially interesting as Coburn et al (2002) found a similar correlation when analyzing a set of phase averaged spectra from a set of accreting X-ray pulsars. The last chapter 7 is dedicated to future works: using phase resolved spectroscopy of archival RXTE data of many more sources will allow to study the previously discussed correlation in more detail. Furthermore, data of INTEGRAL with its broad band spectral coverage and unprecedented energy resolution, will be used to study the variation of CRSFs over the pulse in previously unknown detail.Akkretierende Neutronensterne waren schon seit ihrer Entdeckung rätselhafte Objekte. Obwohl schon die Tatsache an sich, daß ein Stern mit gerademal der Größe einer kleinen Stadt (Radius ~10km) und einer Masse von mindestens 1.4 Sonnenmassen in höchsten Maße erstaunlich ist, wurden im Laufe der Zeit noch weitere bemerkenswerte Tatsachen über sie bekannt. Wenn ein Neutronenstern mit einem normalen Stern ein Doppelsternsystem bildet, kann er Material von seinem Begleiter akkretieren (siehe Abschnitt 2.3). Da das Gravitationspotential des Neutronensterns sehr tief ist, wird das Material beim Fall auf den Neutronenstern stark beschleunigt. Die kinetische Energie wird beim Aufprall auf die Oberfläche des Neutronensterns in Form von Röntgenstrahlung freigesetzt. Obwohl die ungefähre Stärke des Magnetfeldes des Neutronensterns aufgrund theoretischer Überlegungen schon länger auf ungefähr 10^12G geschätzt wurde, so fehlte doch bis Mitte der 70er Jahre noch ein direkter Beweis. 1976 gelang Trümper et al. (1978) die Beobachtung einer Zyklotronresonanzlinie (engl. Cyclotron resonant scattering feature, kurz CRSF) im Spektrum von Herkules X-1. Zyklotronresonanzlinien entstehen durch die Quantisierung der Bewegungsenergie der Elektronen in starken Magnetfeldern; d.h. die Energie der Elektronen kann keine beliebigen Werte mehr annehmen, sondern nur noch vielfache der Zyklotronenergie: sie können sich nur noch auf so genannte Landau Niveaus aufhalten. Da Photonen mit Ecyc=h * nu fast instantan von einem Elektron absorbiert werden, haben sie eine extrem kleine freie Weglänge und können das Plasma im Entstehungsgebiet der Röntgenstrahlung quasi nicht verlassen. Sie können das Plasma also nur verlassen, wenn sich ihre Energie aufgrund von zahllosen Streuprozessen zu leicht höheren oder niedrigeren Werten verschiebt. Daher bildet sich im Spektrum bei der Zyklotronenergie eine Art Absorptionslinie aus. Anhand einer solchen Zyklotronresonanzlinie läßt sich mittels der 12-B-12 Regel direkt auf die Stärke des zugrunde liegenden Magnetfeldes schließen: Ecyc = 11.6 x B/10^12G kev. Wenn nun eine Zyklotronresonanzlinie im Spektrum eines Neutronensterns beobachtet wird, kann man von der Energie der Linie unter Berücksichtigung der Gravitationsrotverschiebung von ca. 25% direkt auf die Magnetfeldstärke schließen. In dieser Arbeit werden qualitativ hochwertige Daten von dem NASA Satelliten Rossi X-ray Timing Explorer (Beschreibung siehe Kapitel 4) verwendet. Dieser Satellit zeichnet sich durch seine große spektrale Bandbreite und hohe Zeitauflösung aus. Diese Eigenschaften machen ihn somit zu einem idealen Instrument für die Beobachtung von akkretierenden Röntgenpulsaren. Ich habe Daten von Vela X-1 (siehe Kapitel 5) und GX 301-2 (siehe Kapitel 6) ausgewertet; in beiden Fällen handelt es sich um Neutronensterne, die Materie von ihren entwickelten supermassiven blauen Begleitern akkretieren. Die Ergebnisse dieser Analysen wurden bzw. werden in der Zeitschrift Astronmy & Astrophysics publiziert (Kreykenbohm et al. 2002a, 2004). Im Falle von Vela X-1 widmete ich mich der Frage, ob neben der bereits bekannten Zyklotronabsorptionslinie noch eine zweite Linie existiert. Während eine Linie bei ~50kev bereits von vielen Instrumenten beobachtet worden war, so war zwar eine zweite Linie bei der Hälfte der Energie, d.h. bei ~24kev, von Kretschmar et al. (1996) und anderen beobachtet worden, von z.B. Orlandini et al. (1997) und anderen jedoch nicht. Mithilfe von Pulsphasenspektroskopie (dabei gewinnt man Spektren von einzelnen Pulsphasenabschnitten, die man separat analysiert, um so Aussagen über die Entwicklung der spektralen Parameter im Verlauf des Pulses machen zu können) gelang es mir, die Linie in einigen Phasenbreichen nachzuweisen, während sie in anderen entweder nicht vorhanden oder nicht signifikant war. Aufgrund der resultierenden Parameter und insbesondere deren Variation im Verlauf des Pulses ist verständlich, warum die Linie nicht immer beobachtet werden kann. Im Falle von GX 301-2 war nur eine Linie bei ~37kev (Mihara 1995) bekannt und ich konnte auch keine weitere Linie im Spektren entdecken. Mithilfe der Pulsphasenspektroskopie entdeckte ich jedoch, daß die bereits bekannte Linie im Verlauf des Pulses sehr stark variiert. Die Variationen der spektralen Parameter waren weiterhin nicht zufällig, sondern stark miteinander korreliert: in den Phasenbereichen, in denen die Zyklotronresonanzlinie am tiefsten ist, ist ihre Partialbreite (d.h. Breite geteilt durch Energie) ebenfalls am größten. Diese Korrelation ist deshalb besonders interessant, da Coburn et al. (2002) ebenfalls eine solche Korrelation fand, als er allerdings phasengemittelte Spektren von mehreren akkretierenden Röntgenpulsaren untersuchte

Evidence for different accretion regimes in GRO J1008-57

We present a comprehensive spectral analysis of the BeXRB GRO J1008-57 over a luminosity range of three orders of magnitude using NuSTAR, Suzaku and RXTE data. We find significant evolution of the spectral parameters with luminosity. In particular the photon index hardens with increasing luminosity at intermediate luminosities between $10^{36}$ $-$ $10^{37}$ erg s$^{-1}$. This evolution is stable and repeatedly observed over different outbursts. However, at the extreme ends of the observed luminosity range, we find that the correlation breaks down, with a significance level of at least $3.7\sigma$. We conclude that these changes indicate transitions to different accretion regimes, which are characterized by different deceleration processes, such as Coulomb or radiation breaking. We compare our observed luminosity levels of these transitions to theoretical predications and discuss the variation of those theoretical luminosity values with fundamental neutron star parameters. Finally, we present detailed spectroscopy of the unique "triple peaked" outburst in 2014/15 which does not fit in the general parameter evolution with luminosity. The pulse profile on the other hand is consistent with what is expected at this luminosity level, arguing against a change in accretion geometry. In summary, GRO J1008-57 is an ideal target to study different accretion regimes due to the well constrained evolution of its broad-band spectral continuum over several orders of magnitude in luminosity.Comment: 13 pages, 7 figures, 3 tables. Accepted for publication in A&

IGR J16318-4848: 7 Years of INTEGRAL Observations

Since the discovery of IGR 116318-4848 in 2003 January, INTEGRAL has accumulated more than 5.8 Ms in IBIS/ISGRI. We present the first extensive analysis of the archival INTEGRAL data (IBIS/ISGRI, and JEM-X when available) for this source, together with the observations carried out by XMM-Newton (twice in 2003, and twice in 2004) and Suzaku (2006). The source is very variable in the long-term, with periods of low activity, where the source is almost not detected, and flares with a luminosity approximately 10 times greater than its average value (5.4 cts/s). IGR 116318-4848 is a HMXB containing a sgB[e] star and a compact object (most probably a neutron star) deeply embedded in the stellar wind of the mass donor. The variability of the source (also in the short-term) can be ascribed to the wind of the optical star being very clumpy. We study the variation of the spectral parameters in time scales of INTEGRAL revolutions. The photoelectric absorption is, with NH around 10(exp 24)/ square cm, unusually high. During brighter phases the strong K-alpha iron line known from XMM-Newton and Suzaku observations is also detectable with the JEM-X instrument

EXO 2030+375 Restarts in Reverse

The Be X-ray binary pulsar EXO 2030+375, first detected in 1985, has shown a significant detected X-ray outburst at nearly every periastron passage of its 46-day orbit for the past ~25 years, with one low state accompanied by a torque reversal in the 1990s. In early 2015 the outbursts progressively became fainter and less regular while the monotonic spin-up flattened. At the same time a decrease in the H$\alpha$ line equivalent width was reported, indicating a change in the disk surrounding the mass donor. In order to explore the source behaviour in the poorly explored low-flux state with a possible transition to a state of centrifugal inhibition of accretion we have undertaken an observing campaign with Swift/XRT, NuSTAR and the Nordic Optical Telescope (NOT). This conference contribution reports the preliminary results obtained from our campaign.Comment: 11th INTEGRAL Conference Gamma-Ray Astrophysics in Multi-Wavelength Perspective, 10-14 October 2016, Amsterdam, The Netherlands. 7 page

Correlated radio--X-ray variability of Galactic Black Holes: A radio--X-ray flare in Cygnus X-1

We report on the first detection of a quasi-simultaneous radio-X-ray flare of Cygnus X-1. The detection was made on 2005 April 16 with pointed observations by the Rossi X-ray Timing Explorer and the Ryle telescope, during a phase where the black hole candidate was close to a transition from the its soft into its hard state. The radio flare lagged the X-rays by approximately 7 minutes, peaking at 3:20 hours barycentric time (TDB 2453476.63864). We discuss this lag in the context of models explaining such flaring events as the ejection of electron bubbles emitting synchrotron radiation.Comment: 4 pages, 4 figure

X-ray detection of a nova in the fireball phase

Novae are caused by runaway thermonuclear burning in the hydrogen-rich envelopes of accreting white dwarfs, which leads to a rapid expansion of the envelope and the ejection of most of its mass1,2. Theory has predicted the existence of a ‘fireball’ phase following directly on from the runaway fusion, which should be observable as a short, bright and soft X-ray flash before the nova becomes visible in the optical3,4,5. Here we report observations of a bright and soft X-ray flash associated with the classical Galactic nova YZ Reticuli 11¿h before its 9¿mag optical brightening. No X-ray source was detected 4¿h before and after the event, constraining the duration of the flash to shorter than 8¿h. In agreement with theoretical predictions4,6,7,8, the source’s spectral shape is consistent with a black-body of 3.27+0.11-0.33¿×¿105¿K (28.2+0.9-2.8¿eV), or a white dwarf atmosphere, radiating at the Eddington luminosity, with a photosphere that is only slightly larger than a typical white dwarf.Peer ReviewedPostprint (author's final draft

The accretion environment in Vela X-1 during a flaring period using XMM-Newton

We present analysis of 100 ks contiguous XMM-Newton data of the prototypical wind accretor Vela X-1. The observation covered eclipse egress between orbital phases 0.134 and 0.265, during which a giant flare took place, enabling us to study the spectral properties both outside and during the flare. This giant flare with a peak luminosity of $3.92^{+0.42}_{-0.09} \times 10^{37}$ erg s$^{-1}$ allows estimates of the physical parameters of the accreted structure with a mass of $\sim$ $10^{21}$ g. We have been able to model several contributions to the observed spectrum with a phenomenological model formed by three absorbed power laws plus three emission lines. After analysing the variations with orbital phase of the column density of each component, as well as those in the Fe and Ni fluorescence lines, we provide a physical interpretation for each spectral component. Meanwhile, the first two components are two aspects of the principal accretion component from the surface of the neutron star, and the third component seems to be the \textit{X-ray light echo} formed in the stellar wind of the companion.Comment: Accepted. Astronomy and Astrophysics, 201

Highly Structured Wind in Vela X-1

We present an in-depth analysis of the spectral and temporal behavior of a long almost uninterrupted INTEGRAL observation of Vela X-1 in Nov/Dec 2003. In addition to an already high activity level, Vela X-1 exhibited several very intense flares with a maximum intensity of more than 5 Crab in the 20 40 keV band. Furthermore Vela X-1 exhibited several off states where the source became undetectable with ISGRI. We interpret flares and off states as being due to the strongly structured wind of the optical companion: when Vela X-1 encounters a cavity in the wind with strongly reduced density, the flux will drop, thus potentially triggering the onset of the propeller effect which inhibits further accretion, thus giving rise to the off states. The required drop in density to trigger the propeller effect in Vela X-1 is of the same order as predicted by theoretical papers for the densities in the OB star winds. The same structured wind can give rise to the giant flares when Vela X-1 encounters a dense blob in the wind. Further temporal analysis revealed that a short lived QPO with a period of 6800 sec is present. The part of the light curve during which the QPO is present is very close to the off states and just following a high intensity state, thus showing that all these phenomena are related