probing the extreme realm of a compact objectweb.tifr.res.in/~daim/bhattacharyya_daa_2015.pdfof...

Probing the extreme realm of a compact object

Sudip Bhattacharyya, DAA, TIFR, Mumbai

ASTROSAT website

Black hole

Neutron star

ASTROSAT

Thermonuclear burst ignited on a neutron star

http://www.universetoday.com/

Courtesy: D. Page

Courtesy: A. Spitkovsky

Source: Space Time Travel, credit: Ute Kraus

20/01/2015 DAIM2015: Sudip Bhattacharyya

www.tifr.res.in/~sudip/

Our System of Interest

Radius ~ 10 - 20 km Mass ~ 1.4 - 2.0 solar mass Core density ~ 5 -10 times the nuclear density

Neutron star vs. a city (courtesy: M.C. Miller) A cartoon of a spinning black hole (http://en.wikipedia.org/wiki/Black_hole)

A “singularity” hides behind an event horizon. No hard surface. Mass ~ 10 solar mass

Accretion disc

Neutron star or black hole



Our System of Interest

Radius ~ 10 - 20 km Mass ~ 1.4 - 2.0 solar mass Core density ~ 5 -10 times the nuclear density

Neutron star vs. a city (courtesy: M.C. Miller) A cartoon of a spinning black hole (http://en.wikipedia.org/wiki/Black_hole)

A “singularity” hides behind an event horizon. No hard surface. Mass ~ 10 solar mass

Low-mass X-ray binary (LMXB)

Primarily emits X-rays. But also emits in other wavelengths. Angular size is so small that cannot be spatially resolved.

Low-mass (≤ 1 solar mass) companion star

Accretion disk

Neutron star or black hole

The cosmic X-ray observing Instruments must be sent above the atmosphere (preferably by satellites). ASTROSAT, the first Indian dedicated astronomy satellite, which will simultaneously observe from optical band to hard X-rays (100 keV), will be launched in 2015.

ASTROSAT



The Big Questions We Ask

(1) Probing strong gravity: (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.

A measure of curvature A measure of potential

Stellar mass black holes (BH) and neutron stars (NS) give rise to the strongest gravity.

Strengths of gravity

Psaltis (2008)




(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 measure parameters (mass, radius, spin) of neutron stars to address it.

Sudip Bhattacharyya (2010)

Neutron star: surface and interior Neutron star: theoretical “mass vs. radius” curves

Courtesy: D. Page


www.tifr.res.in/~sudip/ 6


(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, X-ray 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




(1) Probing strong gravity

(2) Probing dense matter

(3) Understanding accretion-ejection mechanism

None of these big questions can be answered by experiments in laboratories. Accreting NSs and BHs (LMXBs) are ideal systems to address them.



Our Tools

Broad asymmetric iron K emission lines are observed from accreting super-massive black hole (AGN) and stellar-mass black hole (black hole LMXB) systems. They originate from the inner part of the accretion disk.

Fabian et al. (2000)

Tanaka et al. (1995)

They are a nature-given tool to measure the black hole spin and to probe the strong gravity regime.

(1) Broad relativistic spectral iron emission line from inner accretion disk

Inte

nsi

ty

Photon energy

Theoretical models of broad line for two BH spin parameter (a/M Jc/GM2) values. (Miller 2007)


www.tifr.res.in/~sudip/ 20/01/2015

DAIM2015: Sudip Bhattacharyya www.tifr.res.in/~sudip/

Our Tools

Broad asymmetric iron K emission lines are observed from accreting super-massive black hole (AGN) and stellar-mass black hole (black hole LMXB) systems. They originate from the inner part of the accretion disc.





Inte

nsi

ty

Photon energy

3 = 0 implies Kerr; 3 > 0 (3 < 0) implies more prolate (oblate) than a Kerr BH.

Bambi (2012)

Our Tools

Broad asymmetric iron K emission lines are observed from accreting super-massive black hole (AGN) and stellar-mass black hole (black hole LMXB) systems. They originate from the inner part of the accretion disc.





Inte

nsi

ty

Ser X-1

Photon energy

The first relativistic iron line from a neutron star LMXB was discovered by Sudip Bhattacharyya & Strohmayer, ApJ, 664, L103 (2007). This opened up a new way to probe the strong gravity regime around neutron stars and to measure the stellar parameters.

Subsequent research discovered such lines from >10 sources, and established this new field.



Our Tools


How to constrain neutron star equation of state (EoS) models?

Rapidly spinning neutron star structures have been calculated with full GTR effects using the formalism of Cook, Shapiro & Teukolsky.

Sudip Bhattacharyya, MNRAS, 415, 3247, (2011)

Conclusion: rinc2/GM is to be measured with better than an accuracy of 0.1 to effectively constrain EoS models.



Can the proposed LOFT satellite measure rinc2/GM with an accuracy of 0.1?

Simulation: Exposure = 10 ks Used: LOFT LAD requirement files Model: wabs(bbody+diskbb+powerlaw+diskline) Parameter values: Usual; EW=124 eV, Eline=6.83 keV, rinc2/GM=7.7 Recovered value: rinc2/GM=7.66(-0.02,+0.06)

LOFT simulation (S. Bhattacharyya) presented in LOFT Meeting (Toulouse, 2012)

LOFT will be able to measure rinc2/GM with an accuracy of 0.1.



LAXPC

CZTI

SXT

GX 339-4: Exposure: 5 ks 1.17x10-8 ergs cm-2 s-1

Relativistic spectral line in only 5 ks of simulated ASTROSAT data

Our Tools

Burst intensity vs. time

Accretion on neutron star

Energy release: Gravitational ≈ 200 MeV / nucleon Nuclear ≈ 5 MeV / nucleon

Why is unstable burning needed?

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.

Accumulation of accreted matter (H, He, etc.) on the neutron star surface for hours Unstable nuclear burning for seconds Thermonuclear X-ray burst.

(2) Thermonuclear X-ray Bursts

These bursts from the neutron star surface can be very useful to probe strong gravity, to measure the neutron star parameters, and to study accretion-ejection processes.

http://www.sciencedirect.com/science?_ob=MiamiCaptionURL&_method=retrieve&_udi=B6V3S-4Y5GY2H-1&_image=fig2&_ba=2&_user=2763128&_coverDate=04/15/2010&_rdoc=1&_fmt=full&_orig=search&_cdi=5738&_issn=02731177&_pii=S0273117710000451&view=c&_acct=C000058740&_version=1&_urlVersion=0&_userid=2763128&md5=ea77d3ae493304b2257f05c25cdf4f95



Our Tools (2) Thermonuclear X-ray Bursts

What are burst oscillations?


These are variations of observed intensity during thermonuclear X-ray bursts with periods very close to the neutron star spin period. They originate from asymmetric brightness patterns on the spinning neutron star surfaces.

Spinning neutron star

Our Tools (2) Thermonuclear X-ray Bursts

What are burst oscillations?

These are variations of observed intensity during thermonuclear X-ray bursts with periods very close to the neutron star spin period.

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. Here are two examples:

NS R/M

Like

liho

od

RXTE PCA data fitting (XTE J1814-338)

1

The vertical dashed line gives the lower limit of the stellar radius-to-mass ratio with 90% confidence.

Sudip Bhattacharyya, Strohmayer, Miller, Markwardt, ApJ, 619, 483 (2005)

Simulated data fitting

A 10 square meters (e.g., LOFT) future timing X-ray instrument is considered for simulation.

Lo, Miller, Sudip Bhattacharyya, Lamb, ApJ, 776, 19 (2013)

2



Our Tools (2) Thermonuclear X-ray Bursts: Thermonuclear flame spreading:


Flame spreading on a spinning neutron star (simulation)

Thermonuclear flame spreading, which happens at early phases of bursts, is a unique field, which brings together several branches of physics, such as astrophysics, nuclear physics, magneto-fluid dynamics, gravitation, etc., and hence is very useful to probe the extreme environments of neutron stars. The study of flame spreading is also important to measure neutron star parameters.

The decreasing trend is 99.99% significant.

Chakraborty, Sudip Bhattacharyya, ApJ, 792, 4 (2014)

In its time, only ASTROSAT will be able to detect and study burst oscillations.




(3) Kilo-Hertz Quasi-Periodic Oscillations (KHz QPO)

Our Tools

High-frequency (400-1200 Hz) quasi-periodic oscillations of X-ray intensity have been observed from many neutron star LMXBs. Their high frequencies strongly suggest that they originate within a few Schwarzschild radii of the neutron star.

Frequency (Hz)

Upper kHz QPO Lower kHz QPO

Po

we

r







Our Tools



Po

we

r

According to almost all the models, the uniquely high kHz QPO frequencies are either the accretion disk frequencies, or the combinations, beating or resonances among them, or with the neutron star spin frequency.

Frequency (Hz) Accretion disk frequencies:




Therefore, kHz QPO can be a useful tool to measure neutron star parameters, to probe the strong gravity region and to test a law of gravitation. In its time, only ASTROSAT will be able to study kHz QPO, and ASTROSAT/LAXPC, with its large area in hard X-rays, and using spectro-timing methods, will advance this field significantly.

Our Tools


Frequency (Hz)


Po

we

r



Energy (keV)

4U 1728-34 data Mukherjee & Sudip Bhattacharyya (2012)

Barret (2013) 4U 1608-52

Model (TIFR)

Model (TIFR)

Our Tools (3) Kilo-Hertz Quasi-Periodic Oscillations (KHz QPO)

Gilfanov et al. (2003) 4U 1608-52 data

Energy (keV)

Energy (keV)

RM

S am

plit

ud

e (%

) Fr

acti

on

al R

MS

amp

litu

de

Energy (keV)

Energy (keV) Ti

me

lag

(s)

10



Our Tools (4) Spectral and timing state transitions

Spectral state In

ten

sity

Fender et al. (2004)

We need to study how all features and properties (in multiwavelength: radio, IR, optical, UV, X-ray) evolve along spectral state paths to probe accretion-ejection mechanism and other scientific problems.

Black hole LMXBs

Neutron star LMXBs

Spe

ctra

l sta

te

Intensity Chakraborty, Sudip Bhattacharyya, Mukherjee, MNRAS, 418, 490 (2011)

Aql X-1

EXO 1745-248

IGR J17480-2446



Why is this an exciting time for X-ray astronomy?

2001

2003

2005

2007

2009

2011

2013

2017

2019

2021

2023

2025

2027

2029

2 015

RXTE (operated for 16+ years)

Chandra (operating for 15+ years)

XMM-Newton (operating for 15+ years)

Swift

Suzaku

NuSTAR

ASTROSAT

Astro-H

NICER



Research on accreting compact objects at DAA, TIFR • Next few years : ASTROSAT (will take the central role) + other X-ray missions (Chandra, XMM, NuSTAR, Astro-H, …) + observatories of other wavelengths (GMRT, optical Telescopes, etc.)

• In 2020s : (ASTROSAT + other major international observatories): ASTROSAT : should remain operational, should take a central role in DAA X-ray astronomy research, and should remain a major DAA activity (because running a science space mission requires a huge effort (e.g., Chandra, XMM-Newton, etc.)). LOFT (if approved) : Active participation in Science Working Groups (http://www.isdc.unige.ch/loft/index.php/loft-team/science-working-groups ; see White Papers in archive) + Exploring ways for an Indian participation. SKA : DAA members are actively participating in the Indian SKA science teams. TMT : DAA/TIFR members are exploring ways to join the Indian participation in TMT.

http://www.isdc.unige.ch/loft/index.php/loft-team/science-working-groups











Research on accreting compact objects at DAA, TIFR • Next few years : ASTROSAT (will take the central role) + other X-ray missions (Chandra, XMM, NuSTAR, Astro-H, …) + observatories of other wavelengths (GMRT, optical Telescopes, etc.)

• In 2020s : (ASTROSAT + other major international observatories): ASTROSAT : should remain operational, should take a central role in DAA X-ray astronomy research, and should remain a major DAA activity (because running a science space mission requires a huge effort (e.g., Chandra, XMM-Newton, etc.)). LOFT (if approved) : Active participation in Science Working Groups (http://www.isdc.unige.ch/loft/index.php/loft-team/science-working-groups ; see White Papers in archive) + Exploring ways for an Indian participation. SKA : DAA members are actively participating in the Indian SKA science teams. TMT : DAA/TIFR members are exploring ways to join the Indian participation in TMT.









probing the extreme realm of a compact objectweb.tifr.res.in/~daim/bhattacharyya_daa_2015.pdfof...

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