INTEGRAL IBIS/ISGRI energy calibration in OSA 10
I. Caballero, J. A. Zurita Heras, F. Mattana, S. Soldi, P. Laurent, F. Lebrun, L. Natalucci, M. Fiocchi, C. Ferrigno, R. Rohlfs
aa r X i v : . [ a s t r o - ph . I M ] A p r INTEGRAL IBIS/ISGRI energy calibration in OSA 10
I. Caballero ∗ a , J. A. Zurita Heras b , F. Mattana b , S. Soldi a , b , P. Laurent b , F. Lebrun b ,L. Natalucci c , M. Fiocchi c , C. Ferrigno d , R. Rohlfs d a Laboratoire AIM, CEA/IRFU, CNRS/INSU, Université Paris Diderot, CEA DSM/IRFU/SAp,91191 Gif-sur-Yvette, France b APC, Astroparticule et Cosmologie, UMR 7164, Université Paris Diderot, Sorbonne Paris-Cité,CNRS/IN2P3, Observatoire de Paris, 10, rue A. Domon et L. Duquet, 75205 Cedex 13, France c INAF-Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, I-00133Roma, Italy d ISDC Data Center for Astrophysics, University of Geneva, chemin d’Écogia, 16, 1290 Versoix,SwitzerlandE-mail: [email protected]
We present the new energy calibration of the ISGRI detector onboard INTEGRAL, that has beenimplemented in the Offline Scientific Analysis (OSA) version 10. With the previous OSA 9version, a clear departure from stability of both W and Na background lines was observed afterMJD ∼ ∼ >
6% in OSA 9),and the energy reconstruction at low energies is more stable compared to previous OSA versions.We extracted Crab light curves with ISGRI in different energy bands using all available data sincethe beginning of the mission, and found a very good agreement with the currently operationalhard X-ray instruments Swift/BAT and Fermi/GBM.
An INTEGRAL view of the high-energy sky (the first 10 years)" 9th INTEGRAL Workshop and celebrationof the 10th anniversary of the launch,October 15-19, 2012Bibliotheque Nationale de France, Paris, France ∗ Speaker. c (cid:13) Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlikeLicence. http://pos.sissa.it/
NTEGRAL IBIS/ISGRI energy calibration in OSA 10
I. Caballero
1. IBIS/ISGRI energy calibration
ISGRI [1] is the low energy detector of the IBIS imager [2] on-board INTEGRAL [3], and ismade of 128x128 cadmium telluride (CdTe) pixels grouped in 8 Modular Detector Units (MDU).The scientific data analysis is performed with the Offline Scientific Analysis software packageOSA, delivered by the ISDC . The ISGRI spectral gain decreases with time. In the OSA 9 ver-sion, the description of the gain drift was based on the Radiation Environment Monitor (IREM)counters [4] integrated over time, to take into account the solar flares. The gain is followed us-ing the radioactive sodium ( Na) and tungsten (W) fluorescence lines located at 511 and 58.8297keV , respectively. As shown in Fig. 1, the energy reconstruction used in OSA 9 is not valid sinceIJD ∼ ∼ L i ne [ k e V ] OSA9OSA10 F W H M [ k e V ] L i ne [ k e V ] OSA9OSA10 F W H M [ k e V ] Figure 1:
Left: evolution of the W fluorescence line position (top) and FWHM (bottom) obtained with OSA 9 (black)and OSA 10 (red). Right: Evolution of the Na line position (top) and FWHM (bottom) obtained with OSA 9 (black)and OSA 10 (red). The dashed horizontal lines in the upper panels represent the nominal positions of the W and Nalines.
The temperature and voltage dependence of the gains and offsets of the events rise time andpulse height was evaluated on ground and in flight [7]. In previous OSA versions, the temperatureoffset of each module with regard to the average was assumed to be constant, i.e., a stable thermalmap. This assumption turned out to be wrong. A more accurate temperature correction has beenintroduced in OSA 10, in which the temperatures from the ISGRI thermal probes in each detector’smodule are used, instead of assuming a constant D T . The mean temperature of the ISGRI modulesvaries by ∼ ◦ C, and the maximum temperature difference between the modules is about 3 ◦ C. ISDC Data Centre for Astrophysics, This energy is the mean obtained between the K a (59.3 keV) and K a (57.98 keV) lines [5]. Using the ratioI( a )/I( a )=0.57 from [6], the mean energy is 58.8297 keV. INTEGRAL Julian Date, IJD=MJD-51544. NTEGRAL IBIS/ISGRI energy calibration in OSA 10
I. Caballero
In OSA 10, the gain drift is measured using the W and Na background lines between rev-olutions 42 and 1106. To increase the statistics, particularly important in the Na region, thebackground lines are measured in bins of 15 revolutions. The pulse height gain and offset are thendescribed as a function of the pulse rise time and the time. Fig. 2 (left) shows the pulse height gainfor different rise time intervals as a function of time. For energies below ∼
50 keV, charge loss isnegligible. We assume that the pulse height offset is constant and that the gain evolution does notdepend on the rise time, and the gain is modeled with a linear function of time. For energies above ∼
50 keV, the pulse height gain and offset are modeled both as a function of the rise time and time.After the correction, both background lines show a remarkably stable behavior, as shown in Fig.1, with a relative energy variation below 1% around the nominal position (>6% in OSA 9). TheFWHM of the W line increases by a factor 2 between revolutions 39 and 1142 (instead of 3 withOSA 9), indicating a better energy reconstruction.
2. Low threshold correction
The low threshold (LT) position is corrected with the new energy calibration (Fig. 2, right).The LT is stable in channel units. Since OSA 10, the LT shape follows the evolution of the spectralresolution, instead of being fixed at the W line resolution at the beginning of the mission as inprevious OSA versions. Therefore, its resolution corresponds to the W line resolution, and alsoevolves with time. The accuracy achieved is around 1%. The jumps in Fig. 2 (right) correspond todifferent uploaded LT settings over the whole mission duration. After ten years in orbit, the lowerthreshold is still below 23 keV.
200 400 600 800 1000 1200revolution1.21.41.61.82.02.2 G a i n [ c hanne l / k e V ] [15−30][33−34][37−38][41−42][53−55][69−71][83−90]law1
200 400 600 800 1000INTEGRAL revolution1416182022 Lo w T h r e s ho l d [ k e V ] Figure 2:
Left: Pulse height gain evolution for several rise time intervals (labeled in the figure) as a function of therevolution number. The linear fits used to describe the gain for the different rise time intervals are overplotted. Theevolution of the gain for energies below ∼
50 keV, for which a constant offset is assumed (see text), is also plotted,labeled as “law 1”. Right: Low threshold position with OSA 10.
3. Spectral analysis
Crab and background spectra extracted with OSA 9 and OSA 10 for a sample of revolutionsare shown in Figs. 3 and 4. By comparing the left and right panels of Fig. 3, the OSA 10 correction3
NTEGRAL IBIS/ISGRI energy calibration in OSA 10
I. Caballero . r m a li z ed c oun t s s − k e V − Energy (keV)black: 239, red: 666, green: 835, blue: 1019Crab OSA 9 . r m a li z ed c oun t s s − k e V − Energy (keV)black: 239, red: 666, green: 835, blue: 1019Crab OSA 10
Figure 3:
Crab spectra for a sample of revolutions (239, 666, 835, and 1019) extracted with OSA 9 (left) and OSA 10(right).
100 100020 50 200 500 . r m a li z ed c oun t s s − k e V − Energy (keV)black: 239, red: 666, green: 835, blue: 1019background OSA 9
100 100020 50 200 500 . r m a li z ed c oun t s s − k e V − Energy (keV)black: 239, red: 666, green: 835, blue: 1019background OSA 10
Figure 4:
Background spectra for a sample of revolutions (239, 666, 835, and 1019) extracted with OSA 9 (left) andOSA 10 (right).
Table 1:
Best fit parameters for the ISGRI Crab observation in revolution 839. 0.3% systematic errors areincluded. The quoted errors are at 90% confidence level. G G E break [keV] norm c /d.o.f.[photons keV − cm − s − @ 1 keV ]2 . ± .
003 2 . ± .
03 94 + − . ± .
4. Cross-calibration
Using the results of the imaging extraction, we built Crab light curves in the three energy4
NTEGRAL IBIS/ISGRI energy calibration in OSA 10
I. Caballero no r m a li z ed c oun t s s − k e V − data and folded model c Energy (keV)
Figure 5:
ISGRI Crab spectrum for revolution 839 (top) and residuals of best fit model (bottom) extracted with OSA 10. bands 25–50, 50–100, and 100–200 keV. Figure 6 shows the comparison between the ISGRI countrate and that of other currently operational hard X-ray instruments, Swift/BAT [9] and Fermi/GBM[10]. For each instrument, the light curve has been renormalized to the average count rate measuredduring the period MJD=[54690, 54790], as in [11]. Note that the Fermi/GBM light curve in thehighest energy band refers to the range 100–300 keV. The Fermi/GBM light curves are from theGBM Occultation Project . For the Swift/BAT instrument, two different sets of light curves are re-ported, the 15–50 keV one being provided by the Swift/BAT Hard X-ray Transient Monitor pages ,and the 25–50, 50–100, and 100–200 keV ones obtained from the Swift/BAT 58-months Hard X-ray survey . In general, there is a very good agreement between the light curves measured withthe different instruments. Small differences are observed in the 25-50 keV band for MJD<54000between ISGRI and BAT, and at E>100keV at MJD>55600 between ISGRI and GBM.
5. Conclusions
A new energy correction has been implemented in OSA 10. The new calibration significantlyimproves the energy reconstruction. It includes a new description of the events gain and offset as afunction of time and the events rise time, a more accurate temperature correction per ISGRI module,and a varying shape of the low threshold, corrected for the degradation of the spectral resolution.The background lines positions are remarkably stable. Very good agreement is obtained betweenthe ISGRI long term Crab light curves and those obtained by other currently operational hard X-rayobservatories, Swift/BAT and Fermi/GBM.The limitations of the current energy calibration and known issues that the user should beaware of are kept up to date in the IBIS Analysis User Manual, in the section “Known Limitations”,available at the ISDC. http://heastro.phys.lsu.edu/gbm/ http://swift.gsfc.nasa.gov/docs/swift/results/transients/ http://swift.gsfc.nasa.gov/docs/swift/results/bs58mon/index.php NTEGRAL IBIS/ISGRI energy calibration in OSA 10
I. Caballero
Figure 6:
INTEGRAL/ISGRI, Swift/BAT, and Fermi/GBM Crab light curves over the period of the INTEGRALobservations. Each light curve has been renormalized to its average value measured during the period MJD=[54690,54790].
Acknowledgments
The present work is partly based on observations with INTEGRAL, an ESA project with in-struments and science data center (ISDC) funded by ESA members states (especially the PI coun-tries: Denmark, France, Germany, Italy, Switzerland, Spain, Czech Republic and Poland, and withthe participation of Russia and the USA). ISGRI has been realized by CEA-Saclay/DAPNIA withthe support of the French Space Agency CNES. IC, JZH, and SS acknowledge financial supportfrom CNES. FM acknowledges financial support from CNES and Pôle Emploi.
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