Es Estimation n of the he sur urface gravitationa nal reds dshi - - PowerPoint PPT Presentation

Es Estimation n of the he sur urface gravitationa nal reds dshi hift

• f
• f a neutron
• n star with the broa
• ad spectral feature

de detected d dur during ng the he the hermonuc nuclear X-ra ray burs rst

Masachika Iwai (ISAS/JAXA)

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NSMAT 2016 21-23, Nov. Tohoku Univ.

• T. Dotani, M. Ozaki, Y. Maeda (ISAS/JAXA),
• H. Mori (NASA GSFC), and S. Saji (Nagoya Univ.)

Introduction

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Tried to detect absorption lines in the burst spectra Surface gravitational redshifts can be derived immediately.

Identification

• f the lines

Most of burst sources are rapidly spinning: ~200-600 Hz Absorption lines formed on the neutron star surface may be smeared out and may not be detected.

Corresponding to the mass-radius ratio of a neutron star

• Past studies
• Difficulties
• Breakthrough in this study

We will focus on absorption edges in the burst spectra.

Intrinsically broad!

We must carefully select a slowly spinning burst source.

Terzan 5 X-2 (11 Hz ): a transient source.

Absorption edges may be detected even from the rapidly spinning burst sources.

An X-ray burst from GRS 1747−312 detected with Suzaku

0.1 1 10 100 0×100 5×104 1×105 A Counts s-1 Seconds since 2009/09/16 07:22:51 20 40 60 80 100 50 100 150 200 Counts s-1 Seconds since 2009/09/17 01:20:07

0.5−10 keV

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• During a part of Galactic bulge mapping observations, GRS 1747−312 was

included in Suzaku field of view and a long X-ray burst was unexpectedly detected.

e-folding time: 117 +- 6 sec

• GRS 1747−312 is a transient source in the globular cluster Terzan 6

(Predehl et al. 1991; Pavlinskyet al. 1994).

A burst detected From GRS 1747−312 Light curve obtained with XIS Close-up light curve During the burst The data after 192 sec was not obtained.

• Sep. 2009

CCD camera onboard Suzaku In a low-luminosity state

Properties of the X-ray burst

0.5 1 1.5 2 50 100 150 200 χ2

ν

Seconds since 2009/09/17 01:20:07 10 20 30 R [km] 1 2 3 kTc [keV] 1 2 3 4 B Lbol [1038 erg s-1]

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ü A clear PRE (PhotosphericRadius Expansion) ü The burst spectra are well-reproduced by the blackbody model.

Burst parameters and their time-variations Large deviations A clear PRE occured In initial 80 sec L〜LEdd

Photospheric radius

ü No PRE ü Large deviations due to a broad feature at > 6 keV were observed.

9.5 kpc ,9.5 kpc

0.5 1 1.5 2 1 2 5 7 10 12 Ratio Energy [keV] 10-1 100 101 102 103 XIS0+3 XIS1 (×20) bbody model (χν

2 = 2.21)

Counts s-1 keV-1

A broad feature Average burst spectrum during time-region B

Color-temperature Bolometric luminosity Deviations from blackbody model

・ First half of the burst ・ Latter half of the burst

Photosphere returns to the neutron star surface. Poor fit!

A likely origin of the spectral feature

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• Emergence of the spectral feature was explained naturally by the change of

the ionization degrees

• Obtained parameters:

ü Continuum : ~2 keV bbodywith Nh = ~1.3E22 cm-2 ü Absorption edges : 6.1+-0.2 keV with τopt = ~1 & 7.8+-0.2 keV with τopt = ~3 ü Spin frequency : 800+-200 Hz

We tried various models and found that the double absorption edges smeared by the rapid neutron star spin is the most likely scenario.

Data/Model Energy (keV) 1 10 12 5 7

Fully ionized in the first half H-like/He-like ions in the latter half

The residual disappears!

Other scenarios cannot be rejected completely due to the lack of high-energy coverage in the latter half of the burst.

Good fit!

Combining the gravitational redshift with other parameters, we may obtain constraints on the equation of state of the neutron star matter.

0.5 1 1.5 2 2.5 8 10 12 14 16

AP4 ENG AP3 MS0 1 + z = 1 . 5 6 ± . 3 causality J1748−2446ad(716Hz) GS1 SQM1 SQM3 PAL6 FSU MS1 GM3 GR P < ∞

Mass (Mo

• )

Radius (km)

MPA1 PAL1 MS2 rotation

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・ Eedge(6.1 keV & 7.8 keV) ・ ΔEedge

Numerical results of nuclear network calculations during the burst by Weinberg et al. (2006) Adapted from Lattimer & Prakash 2007

Possible combination of ions: H-like Fe (9.28 keV) & H-like Zn (12.39 keV) Surface gravitational red shift: 1.56 +- 0.03 Ions responsible for the edges are determined by referring numerical calculations.

The observational constraints

• btained from this work

He0.1 HHe0.1 Radius (km) Mass (M◉) Mass Fraction Depth from Neutron Star surfaces (g cm-2)

Estimation of the surface gravitational redshift

Composition of burst products

Future plans & Summary

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• We need to increase number of observations of absorption

edges in the bursts.

ü We will plan to use the data of the NuSTAR satellite.

As absorption edges are not completely smeared out even for the rapidly spinning neutron star, this can be a powerful tool to measure the surface gravitational redshifts of a neutron star.

• We detected the X-ray burst from GRS 1747−312 with Suzaku

serendipitously.

• A broad spectral feature was found in the latter half of the

burst.

ü We interpret that the feature is due to double absorption edges, which are smeared by the rapid spin of the neutron star. ü If H-like Fe & Zn are responsible for the feature, the surface gravitational redshift is estimated as 1.56+-0.03.

Future plans Summary