# Homework Help: Astrophysics Photometry Exercise Coursework

1. Apr 22, 2012

### Monarch

hi guys

I'm a 2nd year undergrad doing astrophysics, and I was set this piece of coursework a couple of weeks ago, and it's due in a few weeks, so I'm getting started. Problem is, I don't know where to go with it.

We've basically been given a load of data of measured fluxes in the U, B and V bands from objects in a (5x5 arcminute) field of view (using UBV Harris filters). These objects were identified using an algorithm, and basically, within this field of view is a high mass X-ray binary, and I need to identify the optical counterpart of the X-ray source, determine its magnitude and distance etc...I'm stuck on the first bit though.

Note: All fluxes have been debiased, flat fielded and normalized for 1s exposure.

1. The problem statement, all variables and given/known data

There are 23 objects, but I'll just give a sample of the data, for what I'm unsure on, it's enough;

Object UCount BCount VCount
1------9179----71222---107370
2------1471----21903---42817

(Uncertainties on each result are ±100

Because of the presence of circumstellar material in HMXRB, they have optical colours that are different to the majority of stars. In particular, due to emission below the Balmer discontinuity, they often have an excess flux in the U band compared with that expected from a star of their effective temperature. The U − B colour measures the strength of the Balmer discontinuity, and the B − V colour is a good indicator of stellar temperature. Therefore by plotting these quantities against each other we can often identify the anomalous counterpart.

To do this you must first convert the measured counts into magnitudes. Observations of
standards were obtained at a similar airmass to the field, and therefore no airmass correction is necessary. These standards give the following calibration corrections for the Harris photometric system:

U - 25.32
B - 27.45
V - 26.85

I'm basically stuck on converting the counts into magnitudes because my plot of U-B vs B-V seems a bit random.

2. Relevant equations

I'm used m = ZP - 2.5*log(f) in each band

ZP = zero point, and would be 25.32 in the U band, 27.45 in the V band etc... and f is the flux count in each band listed above, giving m, apparent magnitude, in each respective band.

3. The attempt at a solution

Using that equation basically gives me;

Object UCount BCount VCount--- UMAG ---BMAG ---VMAG
1------9179----71222---107370--15.413--15.318--14.273
2------1471----21903---42817---17.401--16.599--15.271

They seem to be relatively believable numbers, but I'm not entirely sure if that's the right way to get the magnitudes. Also, doing the plot gives a pretty nondescript graph with relatively randomly placed points.

I hope this makes sense, I think I've included everything :)

Thanks