Master's Thesis

MASTER'S THESIS A Search for Pulsars in Ultracompact X-ray Binary Systems Bastian Hacker 2013 Master of Science (120 credits) Space Engineering - Space Master Luleå University of Technology Department of Computer Science, Electrical and Space Engineering A search for pulsars in ultracompact X-ray binary systems Master Thesis in astrophysics Bastian Hacker 06/06/2013 supervisor: Matteo Bachetti advisor: Natalie Webb Galaxies, High Energy Astrophysics and Cosmology (GAHEC) group, Institut de Recherche en Astrophysique et Planétologie, Luleå tekniska universitet, Université Paul Sabatier Toulouse III Abstract We present a search for periodic pulsations in the X-ray emission of seven Ultracompact X-ray Binary (UCXB) systems, in which the coherent time domain resampling method was used to recover signals distorted by the target’s orbital motion. While other UCXBs have been observed as X-ray pulsars, our targets still lack such a detection. We demonstrated the superior sensitivity of our search method over standard approaches, enabling us to find very faint signals at the cost of a large computing effort. However no pulsations were detected. Therefore we deduced stringent upper limits for the pulsed fraction in each observation, which are on the order of 0.5%. Acknowledgements This work was carried out at IRAP, the astrophysics and planetology research institute in Toulouse, France as a final thesis in both the international SpaceMaster programme and the ASEP astrophysics master course. The possibility to conduct the project in this professional and inspiring environment is kindly appreciated. I want to thank my supervisor Matteo Bachetti for his invaluable support throughout the project. It was a real pleasure to work together and learn so much about astrophysics and computing over countless productive coffee sessions. Thank you Matteo for having been an excellent supervisor! Thanks also to my advisor Natalie Webb for her professional support in guiding the project, her precious advice and resources. Her assistance helped to improve the work substantially and was very appreciated. I’d like to thank Luciano Burderi, Tiziana di Salvo and Alessandro Riggio who laid the groundwork to apply the coherent pulsar search technique together with my supervisor Matteo Bachetti. It was their ideas that created the project and their previous work and experience that I could build on. I also want to express my gratitude to all the people who organised and carried out the study course, especially Soucail Geneviève who assisted me throughout my academic period in Toulouse and gave precious guidance. I’d like to thank my research group members for the fruitful discussions and my student colleagues for their valuable advice and distraction when needed. Last but not least I thank the CINECA supercomputing centre for granting nearly two million computing hours on their Fermi supercomputer. The project would not have been possible otherwise. 2 Contents 1 Introduction 4 1.1 Pulsars..........................................4 1.2 X-ray emission from pulsars and its detection....................5 1.3 X-ray binaries and accreting millisecond pulsars...................6 1.4 The pulse signature of a binary pulsar........................8 1.5 Search techniques....................................8 2 Methods 10 2.1 The xpsrspec software................................. 11 2.2 Supercomputing.................................... 12 2.3 Time domain resampling................................ 12 2.4 Residual timing errors................................. 13 2.4.1 Eccentricity................................... 13 2.4.2 Post-Keplerian Parameters.......................... 13 2.4.3 Varying pulsation period............................ 14 2.4.4 Positioning error................................ 14 2.5 Search grid....................................... 15 2.6 Parameter constraints................................. 18 2.7 Fourier analysis..................................... 20 2.8 Follow-up epoch folding................................ 22 3 Analysis 22 3.1 Tests with a known pulsar............................... 22 3.2 Candidate binaries................................... 24 3.3 Results.......................................... 25 4 Conclusions 27 References 28 3 1 Introduction This work presents a search for periodic pulsations in the X-ray emission of Ultracompact X-ray Binary (UCXB) systems. These systems may contain a pulsar whose signal gets smeared out due to the binary orbital motion, which makes the detection difficult. We implemented a program that recovers possible pulsation signals and used it to search seven of the most promising UCXB systems. In this section I introduce all the key concepts, in section 2 I describe the methods and steps I used in the search and in section 3 I report a test of our tools on one system with known pulsations and the results of our search. 1.1 Pulsars Pulsars (pulsating stars) are neutron stars (NSs) emitting fast pulsations. They were first discovered serendipitously in 1967 through radio observations (Hewish et al., 1968). Soon after their discovery it was established that pulsars must be rapidly rotating, highly magnetised NSs (Gold, 1968; Pacini, 1968). They have theoretical masses of the order of 1.4M (solar masses) packed into a very small sphere with a radius of around 10 km (Lattimer and Prakash, 2007). Their density is higher than inside the atomic nuclei. This makes NSs some of the most extreme objects in the universe and a unique celestial laboratory for fundamental physics. Increasing numbers of pulsars (now about 2000) have been detected, most of them within our galaxy. The majority spin at periods of the order of a second while about 10%, the so called millisecond pulsars (MSPs), spin with periods down to few milliseconds (figure 1). Much can be learned from the distribution of spin periods and their derivative in time, but for this work it is sufficient to notice that MSPs are more likely found in a binary system (about two thirds of them). 10-10 10-11 10-12 10-13 10-14 10-15 10-16 dP/dt [s/s] 10-17 10-18 radio pulsars 10-19 high energy pulsation -20 10 binary pulsars -21 10 -1 1 10-3 10-2 10 100 10 P [s] Figure 1: Distribution of pulsation periods P and spin-down rate of 1902 known pulsars from the ATNF catalogue (Manchester et al., 2005). The two distinct groups of “normal” and millisecond pulsars can be identified along with a tendency of millisecond pulsars to be found in a binary system. 4 Pulsars can have extremely stable pulsation signals that are in some cases comparable to atomic clocks. This is used to study the pulsar rotation behaviour precisely, determine the pulsar position and movement very accurately or even for precision tests of general relativity (Taylor and Weisberg, 1982; Lyne et al., 2004). Therefore pulsars are precious tools in astrophysics and the search for new and different exemplars is a worthwhile endeavour. 1.2 X-ray emission from pulsars and its detection Pulsars carry very strong magnetic fields (illustrated in figure 2, right) and generally lose rotational energy by very low-frequency magnetic dipole emission into their surrounding medium which is not observable directly. The pulsed emission of radio waves is produced by charged particles that are accelerated along the magnetic field lines and therefore emit curvature and synchrotron radiation. X-ray pulsed emission on the contrary happens when matter is accreted onto the pulsar’s surface. The infalling particles lose so much gravitational energy while they are magnetically channeled onto the magnetic poles that they form a hotspot of several million degrees on the surface which emits strong thermal X-rays (White et al., 1983). A directed beam is then formed by the opaque accretion flow, which absorbs most escaping photons except the ones near the magnetic pole direction. Both mechanisms create an apparent pulsation with the frequency of the pulsar’s rotation when the magnetic poles don’t conincide with the rotational axis and the rotating light beam grazes the line of sight towards the observer. Most pulsars are seen and detected by pulsed radio emission with large-area radio telescopes. Only a fraction of these is also visible in high energy bands such as X-rays (marked in figure 1 with crosses). On the contrary X-ray pulsars in X-ray binary systems – which are being targeted in this work – have so far been invisible in the radio domain (Lorimer, 2008) with only one recently discovered exception (Papitto et al., 2013). The dense ionized clouds around these pulsars screen the radio radiation effectively while X-rays can escape. Therefore X-ray observations are necessary to detect these pulsars and learn about them during their close binary phase. This work deals purely with X-ray signals. The X-ray radiation is detected as discrete photons at individual arrival times. All the available data is collected by satellites because the earth’s atmosphere is opaque to X-ray radiation. But even above the atmosphere one requires large collecting areas and long exposures to reach reasonable signal levels. Additionally the search for high frequency pulsations requires very accurate timing. Both of these are provided by NASA’s Figure 2: Left: Artist’s illustration of a LMXB system with the primary in blue, the secondary in yellow and an accretion disc in red. Right: Illustration of an accreting pulsar. (Credit: NASA) 5 Rossi X-ray Timing Explorer (RXTE) satellite1, which has been in operation from 1996 to 2012. Its collected observation data on the HEASARC archive2

Load more