Thermonuclear Xray bursts as science driver

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Thermonuclear X-ray bursts asscience driver
Jean in t Zand
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X-ray bursts science driver requirements,
briefly
One of 12 Scientific Questions in ESAs Cosmic
Vision

• X-ray bursts are most luminous phenomenon nearest
  to the densest matter of the visible universe
• X-ray bursts are most luminous phenomenon from
  the strongest gravitation fields in the visible
  universe

• X-ray bursts are transient and the most
  interesting ones are rather unpredictable ? need
  monitor quick slewing

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Determine nature of NS matter

• By constraining M/R relation
• Through measurement of M and R
• M determined accurately in 20 cases (pulsars in
  binaries)
• Radius is difficult to measure accurately, let
  alone for those systems which have accurate mass
  measurements
• From thermal radiation Fsigma (R/d)2 Teff4
• Through measurement of M/R, M/R2
• By gravitationally redshifted lines from NS
  surface
• By pressure-broadened lines (Stark effect)
• By rotationally broadened lines
• By measuring NS cooling curves (after SN, SXT or
  XRB)
• Amount of cooling set by amount of neutrino
  emission which is dependent on nature of matter

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Slide from Deepto Chakrabarty
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Slide from Deepto Chakrabarty
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What are X-ray bursts?
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X-ray bursts thermonuclear flashes on neutron
stars
(Courtesy Andrew Cumming)

• Most luminous emission from surface of low-B NS,
  albeit short

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Fuel accumulation and ignition

• Local accretion rate in low-B NSs 10 to 105 gr
  s-1 cm-2
• After hours to days, accumulate columns of
  y106-8 gr cm-2
• Pressure (yg) builds up to ignition condition
  for nuclear fusion processes like CNO cycle,
  triple-alpha, rp-capture etc
• CNO often stable, triple-alpha not ?
  thermonuclear shell flash ? X-ray burst

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X-ray burst movie of abundance evolution(Schatz
2003)
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Nuclear burning stability versus accretion rate
Heger et al. 2006
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X-ray burst phenomenology
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Burst profiles
2S 0918-549/WFC
GX 31/WFC
4U 1812-12 / HETE-WXM
GS 1826-24/WFC
Den Hartog et al. 2003 Nakagawa et al. 2004 in
t Zand et al. 2005
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Superburst profiles
Strohmayer Bildsten 2006
in t Zand, Cornelisse Cumming 2004
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lt- intermediate bursts
lt- superbursts (carbon)
He bursts -gt
mixed H/He bursts -gt
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Spectra with RXTE

• Black body to a high degree of accuracy
• kT 0.5-2.5 keV
• Fe-K line and edge at 6 and 9 keV

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Photospheric radius expansion
RXTE work by Galloway et al. (submitted)
Ginga result by Van Paradijs et al. 1990
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82 Galactic X-ray bursters (as of September 2006)
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In t Zand et al. 2004 Galloway et al. 2005
Vanderspek 2005 Brandt et al. 2006
http//www.sron.nl/jeanz/bursterlist.html
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All 10 superbursters known sofar (Sep 06)
Purple uncertain detection Probably we have
detected already the majority of superbursters
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Map of X-ray bursters
Version 2001
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One classification of X-ray bursts that is useful
now
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Motivation EXO 0748-676
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Where it all seems to come together EXO 0748-676
Villarreal Strohmayer 2005
Cottam, Paerels Mendez 2002
Wolff et al. 2005
Oezel 2006
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Slide from Mariano Mendez
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Pursuit of confirmation of EXO results

• 2003 XMM-Newton obs. of EXO 0748-676 (570 ksec
  76 bursts no full confirmation)
• 2003 Chandra HETG obs. of EXO 0748-676 (300 ksec
  no confirmation)
• 2003 XMM-Newton obs. of GS 1826-24 (110 ksec no
  confirmation)
• 2006 Chandra obs. of GS 1826-24 (240 ksec
  unsuccesful)
• 2006 Chandra obs. of GX 354-0 (240 ksec
  performed, no results yet)
• XMM-Newton TOOs of superbursts(2 x 25 ksec
  waiting for triggers from INTEGRAL and Swift)
• Chandra TOO of one of brightest bursters 4U
  1812-12 (90 ksec waiting for a trigger)

• Results on non-PRE burst negative, pursuit is now
  for
• features in PRE bursts and superbursts

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Weinberg, Bildsten Schatz (2006) dredging up
the ashes through convection in PRE bursts ?
touches on ISM pollution
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Observatory requirements
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General requirements for X-ray burst studies

• Need triggers ? need monitoring device
• monitor sensitivity should be able to detect 2
  keV black body spectrum with bolometric flux of
  10-8 erg s-1 cm-2 within 1 s, e.g. 100 cm2 at 40
  x 40 sq deg
• Have X-ray bursters in field of view, for
  instance through considerable exposure on
  Galactic center
• Fast slew to target ? between 1 s and 1 hour. If
  slew is slower than 1 min, one looses out on gt99
  of all bursts. With 10 s one will detect at least
  10 of all fluence ? carry out Galactic center
  campaign to get sufficient triggers and
  decrease slew time
• NFI sensitivity. Best instrument sofar
  XMM-Newton 100 cm2 for RGS 1500 cm2 for EPIC
  PNMOS. Say this is threshold for EXO result on
  28 bursts, one would need 500 cm2 _at_ 1 keV for
  science in 1 burst.

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Specific requirements

• Monitor bandpass down to at least 5 keV
• Monitor spectral resolution 20 FWHM
• Monitor area few hundred cm2
• Monitor field of view 60 degrees FWZR
• Monitor angular resolution few arcminutes
• Slew speed few degrees per second, in
  combination with dedicated observation program on
  Gal. Center
• NFI bandpass 0.3-10 keV
• NFI spectral resolution 3 eV _at_ 1 keV
• NFI area as large as possible but at least..
• NFI maximum countrate 10,000 (pile up?)
• NFI field of view none
• NFI angular resolution none
• NFI time resolution ?

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Finally points of concern..

• A confirmation of the EXO result would make for a
  much stronger science case. So far, not so. Final
  results probably within a year
• All other bursters have fast rotation frequencies
  (1 at 95 Hz 16 at gt270 Hz) line broadening
  diminishes detection possibilities considerably

To be done..

• Model burster population more accurately and
  determine possible trigger rates
• Model spectra and predict resulting constraints
• Refine requirements