ESA Science & Technology - Be an INTEGRAL astronomer: Task 1

In order to make a meaningful estimate of the X-ray emission from each of the objects under investigation you will first need to look at the Crab Nebula.

The Crab Nebula is a steady, bright X-ray source. It is frequently used as a calibration source.

(A calibration source in this context is an astronomical source that emits X-rays at a predictable level. By comparing observations of that source that have been made with an instrument over months or years astronomers can check that the instrument response is constant. If the observations are not constant it may indicate that the instrument response has changed. In this case astronomers can correct their measurements by taking account of this change.)

For the Crab Nebula use the data available to derive an average count rate flux, and the error on this average.

An instrument detector measures a count rate; the data files provided contain the count rate as recorded by the IBIS/ISGRI detector. The physical unit for the average flux is sometimes referred to as ‘1 Crab’.

To calculate the energy flux for other sources, first divide the average source count rate by the average Crab Nebula count rate. This tells you what the energy flux of your source is compared to the Crab Nebula.

Before you proceed to the next task ensure you have followed the guidelines carefully. Once you have done this follow the link in the menu on the right back to 'Your Mission' to continue to Task 2.

For your interest

If the behaviour of the detector is well known the observed count rate can be converted into the photon or energy flux in order to obtain a more physical picture on the emitting behaviour of the source.

The energy flux, often expressed in ph/cm2/s, is a measure of the energy emitted from a source as received by a detector. In this form it gives the number of photons (ph) from a particular source, that hit a centimetre squared (cm2) section of an instrument’s detector per second (s). In general the efficiency of a detector is not 100%, which means that not every photon that enters the detector is recorded. What a detector actually measures is a count rate which is often expressed in counts per second (due to the efficiency being less than 100%, the number of counts registered is less than the number of photons).

The physical unit for the average flux is sometimes referred to as ‘1 Crab’. In the IBIS/ISGRI energy band used (18-40 keV) this is equivalent to 0.2 ph/cm2/s (so 1 photon arrives at a 1 centimetre squared section of the detector every 5 seconds).

It is possible to convert from the average count rate flux to the energy flux. You already know what the energy flux of 1 Crab is so the conversion to energy flux from the source is now straightforward. (This is a very simplified approach. In reality, to derive the energy flux of a source is a more complex process.)