<p>I think there is a missing piece of information. The BAT is a coded mask telescope. The imaging is done by photons passing through a mask and falling onto an array of 32768 detectors. <a href="http://swift.gsfc.nasa.gov/about_swift/bat_desc.html" rel="nofollow">http://swift.gsfc.nasa.gov/about_swift/bat_desc.html</a></p>

<p>The "mask-weighted" light curve is produced after a complex ray tracing exercise using an estimate of the position of the source. Looking at some of the software specifications (eg <a href="https://heasarc.gsfc.nasa.gov/ftools/caldb/help/batmasktaglc.html" rel="nofollow">https://heasarc.gsfc.nasa.gov/ftools/caldb/help/batmasktaglc.html</a> ) it seems that the count rate is divided by the number of active detectors.</p>

<p>So in this case it looks like the detection rate was more like 200,000 counts/s in the peak 2ms of the event. There would be 400 detected counts, assuming there are no other corrections for sensitivity or vignetting. However,  I think there probably are - the mask must take out about 50% of these counts and there are gaps between the detectors. A background rate has also been subtracted. So overall I guess that the first point in the burst is due to around 100 <em>detected</em> photons, and so on.</p>

CC BY-SA 3.0
2016-10-02T08:15:50.633Z
2016-10-02T08:15:50.633Z
ProfRob
<p>I'm reading a paper about a recent x-ray burst from a suspected magnetar (<a href="https://arxiv.org/pdf/1607.04264v3.pdf" rel="nofollow noreferrer">A. Dai et al 2016</a>) where they show a light curve of a burst that lasted about 10ms. (Their figure 1). The value of count rate for the peak of the burst from the graph is $6\text{ counts/s}$ over a timespan of less than $0.01s$. This corresponds to about $6*0.01=0.06 \text{ counts}$ for that time period. What does this mean? Is it not counts of photons? What is 0.06 photons? Is there a step in the data analysis I'm missing?</p>

<p><a href="https://i.sstatic.net/S5QCa.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/S5QCa.png" alt="Swift/BAT mask-weighted light curves in different energy bands"></a>
(Fig. 1 Swift/BAT mask-weighted light curves in different energy bands)</p>

CC BY-SA 3.0
2016-10-03T12:38:54.087Z
An average rate in counts per second hardly needs to be integral!
Carl Witthoft
CC BY-SA 3.0
2016-10-13T19:52:08.650Z
@CarlWitthoft I don't follow, how do you mean?
zeitoon
CC BY-SA 3.0
2016-10-13T19:53:04.560Z
Try it:  take 100 random integer samples and calculate the mean value.  Odds are it'll be a non-integer value.
Carl Witthoft
CC BY-SA 3.0
2016-10-13T19:55:25.297Z
Oh yes, I understand that, but my problem with 0.06 counts isn't that it's non-integer but that it's less than 1. If less than a single photon was detected in some 2ms time bin what was actually detected? Rob's answer below helped me answer that. When you said "integral" I thought "sum over total time span" rather than "integer"
zeitoon
<photometry><photons><x-ray>
What does less than one count from an x-ray detector mean? (Swift BAT detector)
zeitoon
# What does less than one count from an x-ray detector mean? (Swift BAT detector)

(Asked by: username_0 on 2016-10-02)

I'm reading a paper about a recent x-ray burst from a suspected magnetar ([A. Dai et al 2016](https://arxiv.org/pdf/1607.04264v3.pdf)) where they show a light curve of a burst that lasted about 10ms. (Their figure 1). The value of count rate for the peak of the burst from the graph is $6\text{ counts/s}$ over a timespan of less than $0.01s$. This corresponds to about $6\*0.01=0.06 \text{ counts}$ for that time period. What does this mean? Is it not counts of photons? What is 0.06 photons? Is there a step in the data analysis I'm missing?

[![Swift/BAT mask-weighted light curves in different energy bands](https://i.sstatic.net/S5QCa.png)](https://i.sstatic.net/S5QCa.png)
(Fig. 1 Swift/BAT mask-weighted light curves in different energy bands)

**Tags:** photometry, photons, x-ray
> **Comments:**
> **username_1**: An average rate in counts per second hardly needs to be integral!  
> **username_0**: @CarlWitthoft I don't follow, how do you mean?  
> **username_1**: Try it:  take 100 random integer samples and calculate the mean value.  Odds are it'll be a non-integer value.  
> **username_0**: Oh yes, I understand that, but my problem with 0.06 counts isn't that it's non-integer but that it's less than 1. If less than a single photon was detected in some 2ms time bin what was actually detected? Rob's answer below helped me answer that. When you said "integral" I thought "sum over total time span" rather than "integer"  

---

## 1 answers

### Answer by username_2 on 2016-10-02. Score: 4 | Accepted answer

I think there is a missing piece of information. The BAT is a coded mask telescope. The imaging is done by photons passing through a mask and falling onto an array of 32768 detectors. <http://swift.gsfc.nasa.gov/about_swift/bat_desc.html>

The "mask-weighted" light curve is produced after a complex ray tracing exercise using an estimate of the position of the source. Looking at some of the software specifications (eg <https://heasarc.gsfc.nasa.gov/ftools/caldb/help/batmasktaglc.html> ) it seems that the count rate is divided by the number of active detectors.

So in this case it looks like the detection rate was more like 200,000 counts/s in the peak 2ms of the event. There would be 400 detected counts, assuming there are no other corrections for sensitivity or vignetting. However, I think there probably are - the mask must take out about 50% of these counts and there are gaps between the detectors. A background rate has also been subtracted. So overall I guess that the first point in the burst is due to around 100 *detected* photons, and so on.


---

