THE Chandra GALACTIC PLANE DEEP FIELD
8.3 Chandra and INTEGRAL
coupled devices (CCD) to convert X-rays into a number of electrons directly proportional to the photon energy, and reads them out in a fixed pattern which preserves the incoming X-ray photon position.
Both the instruments (ACIS and HRC) provide an imaging detector (I) as well as a spec-troscopy detector (S). The latter is designed to serve as a readout for the photons dispersed by the transmission gratings that, if desired, can be inserted into the optical path. The focal plane instrument layout is shown in Fig. 8.2. The ISIM moves in both the X-axis (focus) and the Z-axis.
Behind the mirrors, there are two transmission grating spectrometers that can be optionally used. One is optimised for low energies (Low Energy Transmisison Grating, LETG, 0.08-0.2 keV) and the other for high energies (High Energy Transmission Grating, HETG, 0.4-10.0 keV). Only one grating at a time can be used. Spectral resolving power (E/ΔE) in the range 100 to over 1000 can be achieved with good efficiency. These produce spectra dispersed in space at the focal plane. Either the ACIS array or the HRC can be used to record data. In Fig. 8.2 the Y-axis is parallel to the dispersion direction of the gratings, hence the elongated form of the -S arrays that are designed to serve as a readout for the HETG (ACIS-S) and LETG (HRC-S).
The main characteristics of the different components of theChandraObservatory are given in Table 8.1. We refer to e.g. http://cxc.harvard.edu/ and the documents therein for more detailed information on this mission.
8.3 Chandra and INTEGRAL
Chandra and INTEGRALare two fundamental missions for high energy astrophysics currently operating. They are a big step forward in imaging and spectral performances with respect to the previous missions observing in similar energy bands.
The INTEGRAL angular resolution of 12 seems poor compared to the ∼<1 of Chandra.
But the energy range in which such a resolution is achieved makes the difference. Soft X-rays (∼<10 keV) can be focused by grazing-incidence mirrors, hard X-rays andγ-rays would go through such mirrors and in order to stop, i.e. observe, them alternative methods are to be used like the coded mask technology which makes imaging a very difficult task43.
A good source location accuracy, roughly ∼(angular resolution)/(source significance), is not important just for the sake of precision. It is fundamental to understand the physical processes involved in an object. In fact it allows to attribute the emission in different energy ranges, as given from different missions, to the correct source, especially in the case of crowded regions (e.g. the already discussed case of GX 5-1). It also enables follow-ups at different energy ranges essential to understand the nature of newly detected sources: INTEGRALwith its few arcminute position location accuracy is sharp enough to allow for a follow-up observation in the Chandra field of view (of the order of tens of arcminutes). Then Chandra, with its sub-arcsec position accuracy will in turn restrict the number of optical candidates allowing follow-up observations at optical or radio wavelengths.
In the frame of the monitoring program described in this thesis we observe sources that have been extensively studied and identified in soft X-rays. What we focus on is the poorly known hard
43Note that the terms "soft-X" and "hard-X" rays are used differently in this chapter with respect to the INTEGRAL related ones. In fact, in the case of Chandra (energy range 0.08-10 keV) a 2–10 keV emission is considered "hard". ForINTEGRALstandards (similarly toBeppoSAX andRXTE) a hard emission is∼>20 keV and∼<10 keV is normally considered soft. Care was taken to make clear from the context what band is actually being discussed.
Table 8.1: Main characteristics of theChandra instruments and gratings HRMA
Focal length 10.066 m
Plate scale 48.8μ/
PSF FWHM (with detector) 0.5 ACIS
I-array (ACIS-I) 4 CCDs used for imaging
S-array (ACIS-S) 6 CCDs used for either imaging or as a grating readout
CCD format 1024 by 1024 pixels
Pixel size 24.0μ(0.492 ±0.0001 )
ACIS-I array size 16.9 by 16.9
ACIS-S array size 8.3 by 50.6
Point-source sensitivity 4 X 10−15 ergs/cm2/s in 104 s (0.4 to 6.0 keV) HRC
HRC-I and HRC-S CsI-coated microchannel plate pairs
HRC-I field of view 30X30
HRC-S field of view 6 X 99
Spatial resolution, FWHM ∼20μ,∼0.4
Energy range 0.08 - 10.0 keV
Spectral resolution (E/ΔE) ∼1 at 1 keV
Time resolution 16 μs
Point-source sensitivity 9 X 10−16erg/cm2/s (3σ in 3 X 105 s) HETGS(HETG and ACIS-S)
HETGS range 0.4 - 10.0 keV
High Energy Grating (HEG) range 0.8 - 10.0 keV Medium Energy Grating (MEG) range 0.4 - 5.0 keV HEG resolving power (E/ΔE) 1000 at 1 keV
MEG resolving power 660 at 0.826 keV
LETG
HRC-S energy range 0.07 - 7.29 keV
ACIS-S energy range 0.20 - 8.86 keV
Resolving power (E/ΔE) ∼>1000
8.3. Chandraand INTEGRAL 123
X-ray emission. But when INTEGRALdiscovers a new source things are the other way round.
We come to know the source from its hard-X ray part of the spectrum first and then we try to look for it in softer energy bands. The importance of observing such sources in the soft X-ray domain with Chandra relies also in the fact that the measurement of the spectral continuum in the soft X-rays (∼<10 keV) gives information on the physics of the components that are expected as e.g. thermal emission from accretion discs in LMXRBs and from the hot surface of weakly magnetised neutron stars. It also allows one to determine the Galactic absorption towards the source (which might constrain the source distance), to detect the presence of absorbing material intrinsic to the source (e.g., for wind accreters, where short term NH variations can be observed).
For bright sources, discrete features in the soft X-ray spectrum provide further information on the matter surrounding the source, including its temperature and ionisation state.
It is in this frame that we have submitted two proposals44 to investigate new bright INTE-GRALsources withChandra. In order to obtain a good quality spectrum we have chosen to use theChandra high energy grating, HETGS (HETG and ACIS-S) on a bright, ∼50 mCrab, source in ISGRI 20–40 keV band. The proposals have been awarded 20 ksecs each but, unfortunately, at the time of writing we have not had the opportunity to trigger the Chandra observations.
44Chandra AO5 and AO6, Proposal number: 05400648 and 06400243, PI: A. Paizis.
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