HETG

Follow-on Science Instrument

Contract NAS8-01129

 

Monthly Status Report No. 010

December 2002

HETG Science Theme: X-Ray Binaries

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Prepared in accordance with DR 972MA-002; DPD #972

Prepared for

National Aeronautics and Space Administration

Marshall Space Flight Center, Alabama 35812

 

Center for Space Research; Massachusetts Institute of Technology; Cambridge, MA 02139



 


X-Ray Binaries Research Progress

 

Introduction to X-Ray Binaries

         X-ray binaries, as the name implies, consist of two objects: a compact object – a neutron star (NS) or black hole (BH) - and a companion star. Mass is transferred from the companion and falls toward the compact object often creating an accretion disk (AD) around it. X-ray heating and emission occur in the accretion disk and as mass leaves the accretion disk and lands on (NS) or falls into (BH) the compact object. One of the main classifications of the systems is based on the mass of the companion: for HMXBs the companion is a massive star (> 2 M_solar), and for LMXBs the companion has a mass less than or about 1 M_solar. The Figure at right shows a scientist-artist's image of a binary (taken from:

http://www.astro.soton.ac.uk/~rih/binsim.html .)

 

A variety of parameters determine the behavior of this generic system: primary mass and type (NS or BH); companion's mass and evolutionary status; orbital period; mass transfer rate and mechanism (Roche lobe in LMXB or stellar winds in HMXB); size and composition of the AD and any AD streams; brightness of continuum emission from the central region; the

existence of hotspots in the AD; a photo-ionized AD corona and/or atmosphere; jets from the AD/compact object; the strength and orientation of any magnetic field (NS); compact object rotation rate; the viewing angle of the system - to name a few!

 

However, in spite of this variation, there are some general categories for these objects that help to organize them besides the companion mass (LMXB or HMXB.) “Atoll sources” are thought to have a NS with a weak magnetic field; “z-sources” likely have a NS with a strong magnetic field. An x-ray pulsar has a high-rate rotating magnetic NS.

 

The Figure at left shows a close-up of an X-ray pulsar with its rotation axis offset from the magnetic field poles. Matter falling on the NS is funneled to the magnetic poles and may produce local “hot spots” there. Instabilities in the in-falling stream may be the source of quasi-periodic oscillations (QPOs; high-frequency modulations of the flux.)

 

 

 

These sources are too small to be resolved with current X-ray telescopes so they appear as point sources. Some extra information can be extracted from systems which we observe at high inclination, that is approaching edge-on. The orbital motion then causes our line of sight to vary as if we were walking around the system and seeing it from different angles. By studying the source spectra at these different orbital phases, including eclipses for some systems, we are better able to tease out and test the system geometry.

 

 

 

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The Figure above is from: http://imagine.gsfc.nasa.gov/docs/science/know_l2/binary_flash.html .

 

A useful, extensive reference is “Flourescent iron lines as a probe of astrophysical black hole systems”, by C.S. Reynolds and M.A. Nowak, to be published in “Physics Reports”, 2003 (astro-ph/0212065).

 



Summary of X-Ray Binaries GTO Observations and Activities

Ten XRBs are included in cycles 1-4 of the HETG GTO program. These XRBs cover a range of types as indicated in the Table below; all of them are in the Galaxy. (Extra-galactic XRBs are too dim for detailed study but note that the near-by LMC X-1 and LMC X-3 were observed in Cycle 1 of the ACIS GTO program.)

The accretion disk is a key component of XRB line emission and so we also have on-going efforts to model accretion disk emission in our science program as described further below. Other investigations include radiative transfer in stellar winds (HMXBs) and BH binary population synthesis from stellar evolution models.

Finally, because many XRBs are bright continuum sources they can be used to probe the absorption details of the interstellar medium (ISM) – this is an extensive topic itself and will be treated in detail in a future report.

 

Obs

cycle

Obsid(s)

Name

ISM?

Type*

Period

Comments

4, 1

3504, 104

4U 1626-67

---

LMXB: PS

42 m

Magnetic NS; ultra-compact

4

3505

Sco X-1

Yes

LMXB: NS,Z

18.9 h

Bright! Fe XXV, ADC, ISM (XAFS?)

3

3354

GX 349+2

Yes

LMXB: NS,Z

14.9 d?

ISM Fe-L and O edges

2

1016

Cyg X-2

Yes

LMXB: NS,Z

9.8 d

Bursts, super-orbital P, 0.5 Crab

2

1017

EXO 0748-676

---

LMXB: NS

3.8 h

Bursts, eclipses, dips

1, 2

1020, 1019, 106

SS 433

---

HMXB?: ?

13 d

Precessing jets, super-orbital P

1

103

GX 301-2

---

HMXB: PS

41.5 d

Eccentric orbit, wind accretion

1

102

Vela X-1

---

HMXB: PS

9 d

Winds

1

105

4U 1636-53

Yes

LMXB: NS,A

3.8 h

Bursts

1

107, 1511

Cyg X-1

Yes

HMXB: BH

5.6 d

Fe-line structure, wind

* Key: Z = Z source, A = Atoll source, PS = X-ray pulsar, NS = neutron star, BH = black hole



Cyg X-1: Black hole candidate – stellar wind absorption

From the papers: “The First High-Resolution X-ray Spectrum of Cygnus X-1: Soft X-ray Ionization and Absorption”, ApJ, 565, 1141 and “Highly Ionized Absorption in the X-ray Spectrum of Cyg X-1”, H.L. Marshall et al., astro-ph/0111464.

In Cyg X-1 we observe ionized absorption of the (suspected) black hole’s continuum X-ray by the gravitationally focused wind from the massive companion; the amount and character of the absorption depends on the orbital phase at which we observe it.

For our continuous clocking observation obsid 1511, we see ionized absorption from a large range of elements, e.g, Ne, Mg, and Fe in the figure below. This figure also shows that many of these lines at this particular phase are red shifted – appearing slightly to the right of the rest wavelengths indicated by the dashed lines. At this phase we expect the wind to be moving away from us towards the accreting black hole, hence the red-shift.

 

 

 

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4U 1626: An ultra-compact system – accretion disk emission

         From the paper: “Double-Peaked X-ray Lines from the Oxygen/Neon-Rich Accretion Disk in 4U 1626-67”, N.S. Schulz et al., ApJ, 563, 941.

The figures below show portions of the spectrum from 4U 1626 for the Neon and Oxygen Lyman alpha lines. Here they show a distinct doubling as well as a broadening indicating velocities of thousands of km/s. The lines and their velocities are consistent with their origin in the accretion disk surrounding the compact object.

In addition to these accretion disk signatures we also see evidence for enhanced neutral absorption by cold, local material. Under the assumption that this is condensed material previously expelled from the system (post-accretion) we can put constraints on the nature and size of the donor star. Based on the inferred abundance ratios we argue that the mass donor is a 0.02 M_solar C-O-Ne or O-Ne-Mg white dwarf with a chemically fractionated core which has previously crystallized.

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Vela X-1: Wind accreting source seen in eclipse

         From the paper: “The Ionized Stellar Wind in Vela X-1 During Eclipse”, N.S. Schulz et al., ApJ, 564, L21.

We observed the photoionized plasma from the wind of Vela X-1 in eclipse – blocking out the bright continuum. The spectrum below shows several features. One component comes from the photoionized plasma and we observe many lines from various ionization states (blue in the figure.) We also see in the spectrum a few radiative recombination continua which allow us to determine the temperature of the optically thin ionized plasma to be about 120,000 K. In contrast, there is evidence for a much cooler component in the wind that has high column densities and appears to be clumped. At this orbital phase we observe a variety of fluorescence lines from Si, S, Ar, Ca, and Fe (green curves) originating in the wind. In the case of Si and S we are also able to separate various low ionization stages for the first time.

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         From the paper: “The High-Resolution X-ray Spectrum of SS 433 Using the Chandra HETGS”, H.L. Marshall et al., ApJ, 564, 941.

         In this object many emission lines of highly ionized elements are detected with relativistic blue and red Doppler shifts, e.g., the Fe lines in the spectrum below. The emission is consistent with a thermal origin along a conical jet as diagramed here. The inferred velocity is 0.27c which is somewhat larger than the velocity seen in optical emission suggesting the X-rays originate closer-in to the jet source. Modeling the thermal emission gives electron densities dropping from 2x10^15 to 4x10^13 per cm^3 at distances of 2 to 20 x 10^10 cm.

         Surprisingly all of the X-ray emission can be accounted for by this jet emission and there is no indication of continuum emission from an accretion disk!

 

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Accretion disk modeling and EXO 0748

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Quality = 75, Smoothing = 0         From the paper: “The Structure and X-ray Recombination Emission of a Centrally Illuminated Accretion Disk Atmosphere and Corona”, M.A. Jimenez-Garate, ApJ, 581, 1297.

         The bright continuum radiation from the central object heats and creates an atmosphere above the denser accretion disk. We have carried out extensive modeling of this process to understand the structure of the atmosphere and the expected emission from it, e.g., the model spectrum below.

This modeling is relevant to EXO 0748, an edge-on low inclination source that shows dips, eclipses, and bursts. It has a short orbital period allowing many orbits to be observed. In order to study the photo-ionized layer on top of its accretion disk we observe the “dips”: when the bright central object is obscured by parts of the disk. XMM-Newton saw a warm absorber from O; we see warm absorption from Mg as well.

 

 

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X-Ray Binaries Plans and Further Work

 

• Compile a catalogue of sectra from bright X-ray binaries correct using our pileup model.

• Continue to investigate the precessional dependance of the lines in SS 433.

• Study the effect of resonance scattering in ionized stellar winds.

• Study models of photoionization in illuminated accretion disks.

• Study the X-ray spectra of black hole candidates with respect to their binary orbit.

• Look for absorption lines in X-ray burst spectra.