R.Terrier

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Nature, March 18 2004
Point source contribution to the soft ?-ray
Galactic diffuse emission
R.Terrier APC, Paris SAp CEA/Saclay
F. Lebrun, A. Bazzano, G. Bélanger, A. Bird, L.
Bouchet, A. Dean, M. del Santo, A. Goldwurm, N.
Lund, H. Morand, A. Parmar, J. Paul, J.-P.
Roques, V. Schönfelder, A.W. Strong, P. Ubertini,
R. Walter, C. Winkler
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Sources detected with ISGRI during GCDE
Use all GCDE 2 ToO data (2000 scw) Maps in
the 20-40, 40-60, 60-120, 120-220 keV energy
bands Filtered to remove residual background
non-uniformities significance computed from local
fluctuations in the image Find excesses in 20-60
keV map 91 sources above
6? Fit source position in 20-60 keV map Determine
flux in each energy band Look for possible
counterparts Look in X, ?, radio sources as well
as pulsars, SNRs catalogs
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Sources detected with ISGRI during GCDE
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40 LMXB
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7 HMXB
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SNR, pulsars, SGR, CV
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AGNs
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sources of unknown type
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New sources
15 new sources
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Total of 91 detected sources
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Statistics (as in Nature paper). 40 LMXB 7
HMXB 2 pulsars 1 ms pulsar 2 SNR 1 SGR
1 Seyfert 1 --------------- 54 identified 11
unknown at other wavelengths 26
new -------------- 91 total
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Using IBIS as a collimator

• IBIS imagery suppresses structures larger than
  the PSF (12')
• but large FOV (19 FWHM) Use IBIS as a
  collimator experiment
• Compare measured count rate to expected count
  rate from detected sources
• Cosmic-ray induced BKG (especially time varying
  BKG)
• Isotropic constant component (Cosmic diffuse
  BKG internal BKG)
• Point sources diffuse Galactic emission
• Need to
• Correct for isotropic and time varying
  backgrounds
• Estimate precisely source count rate in the
  detector
• Residuals
• Should be distributed along Galactic plane
  
  additional constraint
• Give the maximal level of interstellar Galactic
  emission

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On background subtraction
Strong BKG variations need to be corrected
for High energy (gt500 keV) count rate
dominated by CR induced events (60 c/s) ?-ray
events lt0.3 c/s for 1 Crab
Correct for BKG variation using HE information
Determine correlation with gt500 keV count
rate using high latitude observations bgt30 (no
sources, no Galactic emission) Uncertainties in
this relation limits the accuracy of flux
correction Allows to evaluate isotropic
constant BKG
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Estimating the sources count rate

• To estimate the total point sources flux
• through 'ISGRI collimator'
• For each 2.2ks pointing
• For all detected sources
• Use the estimated count rate
• based on imaging
• Correct for acceptance absorption
• using the source position in the FOV
• Sum all sources count rates
• Normalize total flux using crab observations to
  correct for inaccuracies in imaging flux
  calibration
• Use Crab observations from rev 102-103
• Systematics of 10 are introduced by this
  calibration

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Longitude profiles
blt5
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Longitude profiles
blt5
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Fitting the Galactic emission

• To further constraint the diffuse emission in the
  residuals
• Assume Galactic interstellar emission
  distributed as
• 5 FWHM gaussian along Galactic plane
• In central regions (llt20 blt25)
• Subtract source count rate from corrected count
  rate
• Fit the latitude distribution of residuals with
  Galactic emission profile
• ?cor(b) - ?s(b) ??GB(b) ?

Isotropic component
Width dominated by FOV
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Latitude profiles
llt20
Sco-X1 large angle acceptance pb
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Latitude profiles
llt20
Shielding transparency larger
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Remaining diffuse flux
20 40 keV 8.6 2.9 c/s 14 4.5 of
total Galactic emission 40 60 keV 0.2 1.1
c/s lt 19 (3?) 60 -120 keV 0.7 1.3
c/s lt 27 (3?) 120 220 keV -0.3 0.6 c/s
lt 52 (3?)
sources
Consistent with SPI. SPI more sensitive at high
energies
diffuse
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Summary future prospects

• Galactic emission is dominated by point sources
• Fraction 14 unaccounted for by detected point
  sources ( 20- 40 keV)
• No significant residual Galactic emission
  detected by IBIS at higher energies
• But IBIS Constraint not very
  strong above 100 keV
• Need to reduce the systematics
• Use a better angular response for ISGRI
• Take into account source variability
• Need to understand better the connection with low
  energy diffuse process Study of the 15-20 keV
  band in progress
• logN-logS studies to evaluate undetected sources
  contribution
• High energy studies using simultaneously IBIS
  SPI
• Precise determination of truly diffuse emission
  in the reach of INTEGRAL

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