Download Broad Relativistic Iron Lines from Neutron Star LMXBs

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.