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Testing general relativity with X-ray reflection spectroscopy of MCG-06-30-15

Ashutosh Tripathi

Fudan University

3rd Karl Schwarzschild Meeting, 25th July 2017

Ashutosh Tripathi Testing general relativity with X-ray reflection spectroscopy

Introduction

The theory of general relativity has been proposed by Albert Einstein in 1915 and is the standard explanation for the dynamics of spacetime. It has been tested many times in the last 60 years with experiments in Solar system and radio observations of Pulsars. Recently, there have been new methods to test this theory using electromagnetic radiation and gravitational waves. In order to test the general relativity, strong gravity region is required in which the effect can be quantified. In universe, there exist black holes which can be used to test the strong gravity.

Ashutosh Tripathi Testing general relativity with X-ray reflection spectroscopy

According to the theory, an uncharged black hole is described by the Kerr solution and only two parameters (mass M and angular momentum J) are required for complete quantification. Although within Einstein’s gravity, the Kerr metric should describe the spacetime around black hole, deviations from Kerr solutions are also possible. The electromagnetic approach analyzed the properties of electromagnetic radiation from the accretion disk close to black hole. It depends on the gas motion in strong gravity region and the photon propagation from the emission point in the disk to the faraway region where the spacetime is almost flat.

Ashutosh Tripathi Testing general relativity with X-ray reflection spectroscopy

Electromagnetic Approach to Test General Relativity

There are two leading techniques to probe the strong gravity region.

1

Continuum fitting method

2

Reflection method

Both the techniques assume Kerr black hole and can be extended to study the deviation from Kerr metric. Reflection method involves the analysis of relativistically smeared reflection spectrum of thin accretion disk. It can be easily applied to both stellar mass black hole and supermassive black hole. It is independent of the black hole mass and distance.

Ashutosh Tripathi Testing general relativity with X-ray reflection spectroscopy

Black Hole Geometry

Ashutosh Tripathi Testing general relativity with X-ray reflection spectroscopy

Theoretical Models for X-ray reflection

XILLVER provides the detailed treatment of K-shell atomic properties of ionized ions. RELCONV is the relativistic convolution code that gives the spectrum measured by distant observer given the local spectrum at any emission point in the disk assuming the Kerr metric. RELXILL is the combination of RELCONV and XILLVER. It describes the relativistic smearing of X-ray reflection spectrum. RELXILL NK is the extension of RELXILL to Johannsen metric of which Kerr metric is the special case.

Ashutosh Tripathi Testing general relativity with X-ray reflection spectroscopy

MCG-06-30-15

MCG-06-30-15 is a narrow line Seyfert 1 (NLS1) galaxy situated at a distance of 37 Mpc ( z = 0.008) There has been detection of relativistic effects in the X-ray emission line (iron Kα line) from ionized iron in the source. It is believed to be originated from the innermost region of the accretion disk. The relativistic broadening of the iron line can be a potential probe to study the deviation from Kerr metric. The data analyzed is one of the longest observation of this source (350 ks) by XMM-Newton during July-August 2001.

Ashutosh Tripathi Testing general relativity with X-ray reflection spectroscopy

Data used (ratioed against power law)

0.01 0.1 1 normalized counts s−1 keV−1 data and folded model 2 5 0.8 1 1.2 ratio Energy (keV)

Ashutosh Tripathi Testing general relativity with X-ray reflection spectroscopy

Parameters for Relxill NK

Name Description Index1 emissivity index for r < Rbr, where emissivity is r −Index1 Index2 emissivity index for r > Rbr, where emissivity is r −Index2 Rbr radius where emissivity changes from Index1 to Index2 a spin of the object Incl inclination angle measured w.r.t. normal of the disk Rin Inner radius of the disk Rout Outer radius of the disk z redshift of the source gamma (Γ) Power law Index of the primary source (E −Γ) logξ ionization parameter at the inner edge of the disk Afe Iron abundance in solar units Ecut Cut off energy of the input spectrum defpar deformation parameter α13

Ashutosh Tripathi Testing general relativity with X-ray reflection spectroscopy

Warm Absorber

Warm absorbers are the ionized absorbing gas within Active Galactic Nuclei. They contain a rich forest of lines and edges from various species of gas and dust. They incorporate several ”zones” of materials, distinct in their kinematic properties as well their column densities and ionizations, but maintained in pressure balance. XSTAR spectral synthesis package for photoionized gases is used to construct a grid of warm absorbers as a function of column density and ionization parameter. They are multiplicative models i.e., they are the absorber models that can be applied to any emission spectrum

Ashutosh Tripathi Testing general relativity with X-ray reflection spectroscopy

Model used

The model reads in xspec as follows: TBABS*WARMABS1*WARMABS2*(RELXILL NK) Relxill NK models the power law continuum and reflection component from the region near black hole and take into account the relativistic smearing of iron line. XILLVER accounts for the reflection from the disk away from the black hole where relativistic effects are not prominent. WARMABS1 and WARMABS2 are the two warm absorbers used to model the ionized gas around the source. TBABS models the galactic absorption. Emissivity index Index2 and Iron abundance Afe is frozen.

Ashutosh Tripathi Testing general relativity with X-ray reflection spectroscopy

Best fit model

0.01 0.1 1 normalized counts s−1 keV−1 data and folded model 0.9 1 1.1 ratio 2 5 −5 5 sign(data−model) × ∆ χ2 Energy (keV)

ashutoshtripathi 20−Jul−2017 11:53

Ashutosh Tripathi Testing general relativity with X-ray reflection spectroscopy

Best Fit Parameters (χ2

red = 1.04)

Components Parameters Best fit value TBABS NH (1022) 0.29 ±0.06 WARMABS1 NH (1022) 0.24 ±0.11 logξ 1.40 ±0.34 WARMABS2 NH (1022) 0.40±0.27 logξ 2.47 ±0.27 RELXILL NK Index1 2.3±0.28 Rbr 76.60 ± 121.18 a 0.92±0.12 i(deg) 3.6 ±35.8 Γ 1.98±0.03 log(ξrefl) 3.34±0.04 Refl frac 1.72±0.57 defpar

• 2.43±0.07

Ashutosh Tripathi Testing general relativity with X-ray reflection spectroscopy

Confidence Ellipses in spin-defpar plane

• 7
• 6
• 5
• 4
• 3
• 2
• 1

1 2 3 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 13

a*

Ashutosh Tripathi Testing general relativity with X-ray reflection spectroscopy

Summary and Future Work

We present constraints on deformation parameter α13 using long XMM-Newton observation of MCG-06-30-15. Other deformation parameters can also be included in the RELXILL NK and can be constrained. Other sources can be analyzed in order to have better constraints on deformation parameters. Future X-ray missions like ATHENA and LAD/eXTP can be proved useful in constraining possible deviations from the Kerr metric.

Ashutosh Tripathi Testing general relativity with X-ray reflection spectroscopy