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Download Broad Relativistic Iron Lines from Neutron Star LMXBs
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Relativistic spectral lines and thermonuclear X-ray bursts from neutron stars Serpens X-1 XMM Figure courtesy: Anatoly Spitkovsky SB and Strohmayer (2007) Sudip Bhattacharyya Sudip Bhattacharyya Department of Astronomy and Astrophysics Tata Institute of Fundamental Research, Mumbai, India Plan 1. Basics and motivation 2. Broad iron lines 3. Broad iron lines from neutron star LMXBs 4. Thermonuclear X-ray bursts Basics and motivation Our extreme systems of interest Neutron star vs. a city (courtesy: M.C. Miller) Radius ~ 10 - 20 km Mass ~ 1.4 - 2.0 solar mass Core density ~ 5 -10 times the nuclear density Magnetic field ~ 108-1015 G Spin frequency ~ up to several hundred Hz Strengths of gravity BH A “singularity” hides behind an event horizon. No hard surface. Mass ~ 10 solar mass Dimensionless angular momentum ~ up to 1 A cartoon of a spinning black hole (http://en.wikipedia.org/wiki/Black_hole) Psaltis (2008) A measure of curvature A measure of potential Our specific system of interest: the extreme world of a Low-mass X-ray binary (LMXB) Equipotential surfaces in a binary system Low-mass X-ray binary (LMXB) (Artist’s impression) Low-mass (≤ 1 solar mass) companion star Courtesy: Bhattacharya & van den Heuvel (1991) Chandra image of KS 1731-260 Courtesy: NASA website Accretion disk Neutron star or black hole Angular size is so small that an LMXB cannot be spatially resolved. Primarily emits X-rays. But also emits in other wavelengths. Only spectral and timing methods are available to probe LMXBs. (Exception: some jets). A reasonably clean accretion process with disk extended close to the compact object Ideal to probe strong gravity and compact object properties. The Big Questions We Ask (1) Probing strong gravity: Strengths of gravity BH (a) Is the Cosmic Censorship Conjecture (Penrose 1969) valid, or, do naked singularities exist in nature? This is a fundamental problem of physics. (b) Does GTR work in strong gravity regime? Testing its predictions, such as frame-dragging. This is a fundamental problem of physics. Stellar mass black holes (BH) and neutron stars (NS) give rise to the strongest gravity. Psaltis (2008) A measure of curvature A measure of potential The Big Questions We Ask (2) Probing dense matter: What is the nature of super-dense (5-10 times the nuclear density) degenerate matter (temperature 108 K) at the neutron star core? This is a fundamental problem of physics. Particle colliders (which probe a different regime of temperature/density) cannot answer this question. One has to constrain theoretically proposed equation of state (EoS) models of neutron star cores to address this problem. This requires measurements of neutron star parameters (mass, radius, spin frequency). Neutron star: surface and interior Neutron star: theoretical “mass vs. radius” curves SB (2010) Courtesy: D. Page The Big Questions We Ask (3) Understanding accretion-ejection mechanism: Motivation: (a) This is a critical astrophysical problem: accretion-ejection is common among various kinds of objects, such as proto-stars, Xray binaries and AGN. (b) Accretion onto black holes and neutron stars is possibly the most efficient energy source in the universe. (c) A study of accretion-ejection in X-ray binaries provides an important tool to probe the strong gravity regime. What to study? The properties, size and location of various accretion and ejection components, such as, disk, jet, wind, and their dependence on source parameters, such as accretion rate. Courtesy: heasarc.gsfc.nasa.gov The Big Questions We Ask (1) Probing very strong gravity (2) Probing super-dense cold matter (3) Understanding accretion-ejection mechanism None of these big questions can be answered by experiments in laboratories. Accreting NSs and BHs (LMXBs) provide many tools (observational aspects) to address them. Strengths of ASTROSAT The first Indian dedicated astronomy satellite 1. Catching outbursts of transients [SSM] 2. Timing, including fast timing, study with a large area instrument (unique in its time) [LAXPC] 3. Reasonably good spectral and spatial resolution in soft X-rays [SXT] 4. Simultaneous observations in 0.3-100 keV; instruments with overlapping energies can be important for cross-calibration [SXT, LAXPC, CZT] 5. Simultaneous observations in optical, near UV, far-UV, soft X-ray and hard X-ray bands; good angular resolution in UV [UVIT, SXT, LAXPC, CZT] Broad iron lines Broad relativistic spectral iron emission line from inner accretion disk MCG-6-30-15: ASCA data Intensity Broad asymmetric iron K emission lines are observed from accreting supermassive black hole (AGN) and stellarmass black hole (black hole LMXB) systems. They are believed to originate from the inner part of the accretion disk. Fabian et al., PASP, 112, 1145, 2000 Tanaka et al., Nature, 375, 659, 1995 Photon energy Origin of broad iron line Main requirements: 1. A geometrically thin, optically thick accretion disk, which is radiatively efficient down to the ISCO. 2. A hard X-ray source. A reflection spectrum off the disk, in which a Fe K line is the most prominent feature due to high abundance and fluorescent yield. Right: An example of simulated reflection spectrum: A power-law X-ray continuum with photon index 2 is reflected from a cold semi-infinite slab of gas with cosmic abundances. Reynolds and Nowak (2003) Examples of relativistic lines from stellar mass black holes GX 339-4: XMM-Newton data Infers Jc/GM2 0.9 Miller et al. (2004) GRS 1650-500: Chandra and RXTE data Miller et al. (2002) Examples of relativistic lines Fabian (2015) Time lag due to reflection Example of relativistic lines from a super-massive black hole Swift J2127.4+5654 Kara et al. (2015) Measurements of parameters Measurement of observer’s inclination angle by fitting broad relativistic spectral iron emission line Intensity Theoretical models of broad line for various inclination angles (for BH spin parameter = Jc/GM2 = 0.998) Fabian et al., PASP, 112, 1145, 2000 Reynolds and Nowak (2003) Photon energy Measurement of black hole spin parameter by fitting broad relativistic spectral iron emission line Intensity Theoretical models of broad line for two BH spin parameter (a/M Jc/GM2) values. Miller (2007) These lines are nature-given tool to measure the black hole spin and to probe the strong gravity regime. Photon energy Broad Iron Lines from neutron Star LMXBs Broad Iron Lines from neutron Star LMXBs Asai et al., ApJS, 131, 571, 2000 The relativistic nature of broad iron lines from neutron star LMXBs was not established for some time. Broad Iron Lines from neutron Star LMXBs Relativistic nature at last! Serpens X-1: XMM-Newton EPIC pn data SB and Strohmayer, ApJ, 664, L103, 2007 Broad Iron Lines from neutron Star LMXBs Relativistic nature at last! Serpens X-1: XMM-Newton EPIC pn data SB and Strohmayer, ApJ, 664, L103, 2007 Suzaku data Cackett, Miller, SB et al., ApJ, 674, 415, 2008 Broad Iron Lines from neutron Star LMXBs XMM-Newton data (red) and Suzaku data (black) Cackett, .., SB et al., ApJ, 720, 205, 2010 Since 2007, the relativistic nature of broad iron lines has been confirmed for more than ten neutron star LMXBs. Serpens X-1: XMM-Newton data Pile-up cannot make a line broad. Miller, …, SB, et al. (2010) Broad Iron Lines from neutron Star LMXBs More confirmation Serpens X-1: XMM-Newton EPIC pn data Serpens X-1: NuSTAR data Miller et al. (2013) SB and Strohmayer, ApJ, 664, L103, 2007 Broad Iron Lines from neutron Star LMXBs More confirmation Serpens X-1: XMM-Newton EPIC pn data Serpens X-1: Chandra data Chiang,…, SB, et al. (2015) SB and Strohmayer, ApJ, 664, L103, 2007 Broad Iron Lines from neutron Star LMXBs RXTE data of 4U 1608-522 Extra soft lag at energies of Fe line KHz QPO frequency Barret (2013) Broad Relativistic Line as a Tool SB, MNRAS, 415, 3247, 2011 These figures indicate that rinc2/GM is to be measured with better than an accuracy of 0.1 to effectively constrain EoS models. What can ASTROSAT do? Simulation SXT LAXPC GX 339-4: Exposure: 5 ks 1.17x10-8 ergs cm-2 s-1 CZTI What can ASTROSAT do? SXT SXT GX 339-4: Exposure: 1636-536: 5 4U ks Exposure: 12.5 ks -8 ergs -2 -2 -1 1.17x10 1.8x10-9 ergs cm cm s-1 s A tiny relativistic line sits on a huge continuum. So it is essential to model various components of the continuum accurately in order to extract information fromLAXPC the line. ASTROSAT, LAXP with its 0.3-100 keV coverage will do C such accurate modeling. CZTI CZT SB & Strohmayer, ApJ, 664, L103 (2007). I Thermonuclear X-ray bursts Thermonuclear X-ray Bursts Unstable nuclear burning of accreted matter on the neutron star surface causes type I (thermonuclear) X-ray Burst light curve bursts. Accretion on neutron star Rise time ≈ 0.5 - 5 seconds Decay time ≈ 10 - 100 seconds Recurrence time ≈ hours to day Energy release in 10 seconds ≈ 1039 ergs Sun takes more than a week to release this energy. Why is unstable burning needed? Energy release: Gravitational ≈ 200 MeV / nucleon Nuclear ≈ 5 MeV / nucleon Accumulation of accreted matter for hours Unstable nuclear burning for seconds Thermonuclear X-ray burst. Fast Timing Properties of X-ray Bursts (Burst Oscillations) Burst light curve What are burst oscillations? These are millisecond period variations of observed intensity during thermonuclear X-ray bursts. What is their origin? Asymmetric brightness pattern on the spinning neutron star surfaces. Neutron star spin frequency = Burst oscillation frequency Hot spot NASA website Spinning neutron star Burst oscillation method to measure parameters SB, Strohmayer, Miller, Markwardt, ApJ, 619, 483 (2005) XTE J1814-338 (RXTE-PCA data) Physical effects included in the model: Likelihood (1) Doppler boosts; (2) special relativistic beaming; (3) gravitational red-shift; (4) light bending; (5) frame dragging. Free parameters for a chosen EoS model: (1) NS radius-to-mass ratio: R/M; (2) -position of the centre of the hot spot: c; (3) angular radius of the spot: ; (4) observer’s inclination angle w.r.t. NS spin axis: i; (5) beaming (due to NS atmosphere) parameter: n [specific intensity as a function of angle (in emitter’s frame) from surface normal is given by I() cosn() ]; (6) background. NS R/M The vertical dashed line gives the lower limit of the stellar radius-to-mass ratio with 90% confidence. Integration over “nuisance” parameters. p posterior probability density Burst oscillation during rise: thermonuclear flame spreading Spitkovsky, Levin & Ushomirsky (2002) Courtesy: A. Spitkovsky 1. Modeling of burst oscillation light curve can be useful to measure neutron star mass and radius, which is possibly the only way to address the fundamental physics of super-dense degenerate core matter of neutron stars. 2. Thermonuclear flame spreading is an interesting science on its own; it combines various fields, such as, astrophysics, nuclear physics, fluid dynamics, gravitational physics, etc., and can be useful to constrain neutron star surface parameters, such as the turbulent viscosity for flame spreading. Burst oscillation during rise: thermonuclear flame spreading? Spitkovsky, Levin & Ushomirsky (2002) Courtesy: A. Spitkovsky The decreasing trend is 99.99% significant. Chakraborty, SB, ApJ, 792, 4 (2014) Burst oscillation What can Astrosat do? (1) At this time, only Astrosat has the capability to study burst oscillations. (2) Full LAXPC area is important to increase the signal-to-noise for both burst oscillations and X-ray spectrum. Note that RXTE-PCA operated with much lesser area during most of its lifetime. (3) Most promising sources are known from RXTE observations. (4) Astrosat will model the burst and non-burst emissions in a much larger (than RXTE) photon-energy range, which will be important to accurately model the energy-dependent burst oscillation profiles. Burst oscillation What can Astrosat do? (1) At this time, only Astrosat has the capability to study burst oscillations. (2) Full LAXPC area is important to increase the signal-to-noise for both burst oscillations and X-ray spectrum. Note that RXTE-PCA operated with much lesser area during most of its lifetime. (3) Most promising sources are known from RXTE observations. (4) Astrosat will model the burst and non-burst emissions in a much larger (than RXTE) photon-energy range, which will be important to accurately model the energy-dependent burst oscillation profiles.