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Abbas Askar abbas.askar@gmail University of Belgrade

X-ray Bursts: Thermonuclear Burning on Neutron Stars. Abbas Askar abbas.askar@gmail.com University of Belgrade. About Astromundus !. 2-year Erasmus Mundus Masters Course in Astronomy and Astrophysics that started in Fall 2010. 3 semesters of courses+1 semester for thesis

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Abbas Askar abbas.askar@gmail University of Belgrade

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  1. X-ray Bursts: Thermonuclear Burning on Neutron Stars AbbasAskar abbas.askar@gmail.com University of Belgrade Astronomy Workshop Presentation

  2. About Astromundus! • 2-year Erasmus Mundus Masters Course in Astronomy and Astrophysics that started in Fall 2010. • 3 semesters of courses+1 semester for thesis • Offered by a consortium of 5 Universities across 4 countries • Standard Grad School Requirements: Bachelor Degree, Application Form: Motivation Letter, 2 References , English Proficiency, CV • Many Scholarships are Available Astronomy Workshop Presentation

  3. Back to X-ray Bursts Outline • Introduction to Neutron Stars and the X-ray Burst Phenomena • Introduction to my Bachelor Research Project • Observation and Analysis • Results • Conclusions • Further Research • General Conclusions Astronomy Workshop Presentation

  4. Neutron Stars • One of the Final States of a Star • Very compact – about 10 km radius • Typical Mass > 1.4 M⊙ • Very dense – one teaspoon of neutron star material weighs as much as all the buildings in Manhattan • Spin rapidly – as fast as 600 times per second • High magnetic fields B ≈ 107 G to 1015 G • Some of the most extreme conditions of the universe exist in neutron stars. Astronomy Workshop Presentation

  5. Neutron Stars • Isolated Neutron Stars and Neutron Stars in Binary Systems • Neutron Stars have been observed in Binaries along with other stars and compact objects • LMXB and HMXB: Determined by the mass of the companion star Astronomy Workshop Presentation

  6. LMXB • Neutron Star or black hole with a secondary companion with a mass less than 1 solar mass (this could be a main sequence star, a red giant or even a white dwarf) • When the Roche Lobe of the secondary companion fills up a accretion disk forms and matter is accreted onto the compact object • These binaries are luminous in X-rays which originate from the surface of the neutron star and the inner accretion disk • Magnetic Field of Neutron stars: B ≈ 107 G to 109 G Astronomy Workshop Presentation

  7. X-ray Bursts • Matter from a Companion Star accretes onto a Neutron Star • When enough accreted material builds up (a few months)► thermonuclear reactions occur on the neutron star surface (for a few seconds) ► Results in X-ray Bursts Astronomy Workshop Presentation

  8. X-ray Bursts • Rise time ≈ 0.5 - 5 seconds • Decay time ≈ 10 - 100 seconds • Recurrence time ≈ hours to day • Energy release in 10 seconds ≈1039ergs Astronomy Workshop Presentation

  9. Introduction to my Research Project • Analysis of the onset of thermonuclear bursts • Expected to occur within a fraction of second • However, there are examples where the onset takes hundreds of seconds • RXTE • Proportional Counter Array • High Time Resolution • Standard-1 Mode with 0.125 s time resolution and no spectral resolution Astronomy Workshop Presentation

  10. Research Aims • Research Aims: • Systematically search through all RXTE-detected flashes for slow onsets • Find correlations with other parameters of the relevant neutron star Astronomy Workshop Presentation

  11. Observations and Analysis • Observational Data • Downloaded Standard-1 RXTE data for 1187 bursts cataloged by Galloway et al. 2008 from NASA archives • Observations detected from 48 sources up to 3rd June 2007 • Light Curves Astronomy Workshop Presentation

  12. Observations and Analysis 4U 1702-429 21/Feb/1999 4U 1636-536 17/Sep/2001 Astronomy Workshop Presentation

  13. Observation and Analysis • Task: • Find bursts with slow onsets from data for 1187 X-ray bursts • Devise an algorithm that can determine whether a burst observation has a slow onset Astronomy Workshop Presentation

  14. Observation and Analysis KS 1731-260 3/Oct/1998 • Fit data 150, 100 and 50 seconds before the burst to a straight line • Obtain values for gradient of the three intervals Astronomy Workshop Presentation

  15. Observation and Analysis • Chi square merit function • Filtering conditions and Light Curve Plots • Gradient of the 3 intervals should be more than 0.5 c/s-2 • Reduced chi square values for the straight line fit should be less than 2 • Gradient/Error should be greater than 3 • Gradient between the interval -50 and 0 should be greater than the gradient between -100 and 0 Astronomy Workshop Presentation

  16. Results • 28 bursts from the Galloway et al. 2008 catalogue have slow onsets • Bursts with slow onsets found in 8 out of the 48 X-ray burst sources catalogued by Galloway et al 2008. Astronomy Workshop Presentation

  17. Results Type T = transient O=burst oscillations D= dipper A=atoll source S= superburst G=global Cluster Association

  18. Results • Normalized gradients for data with slow rises 4U 1608-52 21/Nov/2001 Astronomy Workshop Presentation

  19. Results • For 4U 1636-536 • Values of normalized gradient (between -100 and 0) vary between 0.6 and 2.6

  20. Results • Comparing normalized persistent flux level • For 4U 1636-536 • Average NPF level 0.038 Astronomy Workshop Presentation

  21. Conclusions • Slow onsets found for 28 bursts from 8 sources • No more than approximately 10% of bursts have slow onsets Astronomy Workshop Presentation

  22. Further Research • Correlate data of slow onset burts with other properties of the source • Study power spectra of the slow onsets • Possible relationship between mHz Quasi-Periodic Oscillations (QPO) and slow onsets Astronomy Workshop Presentation

  23. General Conlusions • Studies of thermonuclear X-ray bursts can be very useful in order to constrain properties of neutron stars (mass, spin rate, radius etc) • Extensive observational data is available in archives that can be easily obtained and studied! Astronomy Workshop Presentation

  24. References • Review of X-ray Bursts: • Strohmayer & Bildsten, “New Views of Thermonuclear Bursts” (2003) http://arxiv.org/abs/astro-ph/0301544 • Theoretical Review: Page & Reddy, “Dense Matter in Compact Stars: Theoretical Developments and Observational Constraints” (2006) http://arxiv.org/abs/astro-ph/0608360 • Lattimer & Prakash , “Neutron Star Observations: Prognosis for Equation of State Constraints” (2006) http://arxiv.org/abs/astro-ph/0612440 • Catalogue: • Galloway D. et al, "Thermonuclear (Type I) X-Ray Bursts Observed by the Rossi X-Ray Timing Explorer." The Astrophysical Journal Supplement Series Volume 179, Issue 2(2008): 360-422 http://arxiv.org/abs/astro-ph/0608259 • mHz Oscillations: Altamirano D. et al, "Millihertz Oscillation Frequency Drift Predicts the Occurrence of Type I X-Ray Bursts." The Astrophysical Journal Volume 673, Issue 1(2008): L35-L38 Astronomy Workshop Presentation

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