Transcript X-ray Bursters with INTEGRAL

X-ray Bursters with ESTREMO
• Looking for burst lines: investigating the NS EOS
• Superbursts as peculiar thermonuclear flashes
• XrB population study (not a main driver)
• XrB spectroscopy (not a main driver)
-> requirements for advanced XrB studies
ESTREMO meeting, IASF-BO 05/05/2006
M. Cocchi, J.J.M. In ‘t Zand
Cottam, Paerels & Mendez: absorption lines in the burst spectra of EXO 0748-676
(28 events average, XMM/RGS)
ESTREMO meeting, IASF-BO 05/05/2006
M. Cocchi, J.J.M. In ‘t Zand
Lines -> redshift -> determination of M/R ratio -> guess on the NS EOS
Fe XXV, XXVI (n=2-3) and O VIII (n=1-2) with redshift z=0.35
-> in agreement with normal nuclear matter (some exotic models
excluded)
M = 1.4-1.8 Msun, R = 9-12 km
Better / updated NS atmosphere models (taking into account also burst
effects) are expected
ESTREMO meeting, IASF-BO 05/05/2006
M. Cocchi, J.J.M. In ‘t Zand
Looking for Bursts lines:
1) normal bursts:
look at the longer bursts (eg GS 1826-238, KS 1731-2609, H 1608-52,
4U 1812-12 - the latter 2 being Eddington-limited bursters)
GS1826-238 is the best candidate
+ easy trigger
+ most frequent bursts (1/3h)
+ pretty long events (~120 s)
+ GS 1826 shows quasi-periodic bursts (other pointings possible between
consecutive events)
- need for fast slew (<20 s not to miss the first phase)
- need for low energy sensitivity in WFGC (~4 keV)
ESTREMO meeting, IASF-BO 05/05/2006
M. Cocchi, J.J.M. In ‘t Zand
Looking for Bursts lines:
2) superbursts:
+ very long bursts (hours, excellent statistics)
+ no real need for quick repointing (a few minutes is ok)
+ WFGC low energy threshold can be higher (5 keV) as more
statistics available and integrating on larger timebins
+ less WFGC sensitivity needed
- tricky trigger, possibly manual (high persistent intensity sources)
- rare events (1-2 per year for 5-10% Edd sources)
- LEO gaps interfering with the burst light curve and affecting count
statistics
ESTREMO meeting, IASF-BO 05/05/2006
M. Cocchi, J.J.M. In ‘t Zand
Requirements on the WFGC low energy threshold for burst detection
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For bursts with BB kT<2.5 keV, >99% of photon flux is below 20 keV
For kT>2.0 keV, you get >85% of photons with LET=2 keV, >50% for LET=5
keV and >10% for LET=10 keV (so going down to 2 keV from 5 keV leads to an
improvement of 35% only)
For a 1 Crab (2-10 keV) 2.0 keV burst I5-20 keV = 0.9 c/s/cm2
For a 1 Crab (2-10 keV) 2.5 keV burst I5-20 keV = 2.3 c/s/cm2
In general, Aeff = 5 x sqrt(bkg) / sqrt(tri x delta-T) x I5-20 keV .
=> For a 2.5 keV burst, Aeff=70 cm2; for tri=0.5 Aeff= 100 cm2
Detect triggers 
• 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
ESTREMO meeting, IASF-BO 05/05/2006
M. Cocchi, J.J.M. In ‘t Zand
fallout science (depending on the pointing strategy):
1) XrB population study by WFGC:
burst recurrence time vs bolometric luminosity, burning regimes
big FOV, best sensitivity and energy response for a 40x40 ever.
=> Improving the nice results of BeppoSAX-WFC. Nothing like that
since BeppoSAX!
2) Spectroscopy of bursters: refinement of spectral models (e.g.
Compton tails & breaks, double Compton – see Thompson et al.)
ESTREMO meeting, IASF-BO 05/05/2006
M. Cocchi, J.J.M. In ‘t Zand
Burst Spectroscopy: 3 main spectral stases with INTEGRAL
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Simple Comptonized emission fits quite well the 20-200 keV spectra of
INTEGRAL bursters
Fitted electron kT’s range from ~3 to ~20 keV, possibly clustered in 2-3
ranges of values, likely related to the source state:
– kTe ~ 3 keV, bursters in soft state, no evidence for hard tail, emission
drops at < 50 keV (4U 1735-444, 4U 1820-303, possibly H 1702-429)
– kTe ~ 6-10 keV, bursters in a sort of intermediate state, emergence of
a hard Compton tail at 50-100 keV (GX 3+1, SLX 1744-300, GX 354-0,
H 1705-440)
– kTe > 15 keV, bursters in very hard state, Compton hard tail extended
well above 100 keV, up to 200 keV (GS 1826-238, 4U 1812-12)
ESTREMO meeting, IASF-BO 05/05/2006
M. Cocchi, J.J.M. In ‘t Zand
kT = 28 ± 5 keV
t = 1.1 ± 0.2
c2(dof) = 1.3 (17)
f = 11.5 10-10 erg cm-2 s-1
ESTREMO meeting, IASF-BO 05/05/2006
kT = 12.1 ± 1.4 keV
t = 4.6 ± 0.6
c2 (dof) = 1.0 (21)
f = 8.7 10-10 erg cm-2 s-1
kT = 3.0 ± 0.1 keV
t = 17 ± 10
c2 (dof) = 1.0 (18)
f = 2.15 10-10 erg cm-2 s-1
M. Cocchi, J.J.M. In ‘t Zand
What XrB community wants (requirements for ESTREMO):
WFGC: - bandpass: Emin = 3-4 keV , ok if Emin < 5 keV
Emax up to 200 keV for bolometric studies
- spectral resolution 15-20% FWHM
- detector area: a few hundred cm2
- timing: 1 ms
- field of view: at least 40 deg fwzr
(to trade off with the X-ray BGD)
- source.location.accuracy. 1 arcmin
- sensitivity: 500 mCrab in 1 s (normal burst trigger)
much less for superbursts
- slew speed: 1-2 deg/s, if combined with dedicated observation programs (e.g. Galactic
Bulge)
NF telescope:
- bandpass: 0.3-10 keV
- spectral resolution: 3 eV @ 1 keV, even less for fast rotators
- area: as large as possible..
- timing 0.1 ms (ms oscillations in bursts)
- maximum countrate: 10,000 (pile up?)
- field of view: none
- angular resolution: none
ESTREMO meeting, IASF-BO 05/05/2006
M. Cocchi, J.J.M. In ‘t Zand