Astrophysics Photometry Exercise Coursework

[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.

Homework Statement

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.

Homework 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.

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--15.271They 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

 

[mark726]

Dear fellow student,

I am an astrophysicist and I can help you with your coursework. It seems like you are on the right track with your approach to convert the counts into magnitudes using the zero point and flux count in each band. However, I noticed that you did not take into account the uncertainties in your calculations. When converting the counts into magnitudes, you should also take into account the uncertainties by using the formula: m = ZP - 2.5*log(f) ± 2.5*log(1+∆f/f), where ∆f is the uncertainty in the flux count. This will give you a range of possible magnitudes for each object, which can help you in your plot.

Additionally, when plotting U-B vs B-V, make sure to plot the colors, not just the magnitudes. The U-B color can be calculated by subtracting the U magnitude from the B magnitude, and the B-V color can be calculated by subtracting the B magnitude from the V magnitude. This will give you a clearer picture of the anomalous counterparts.

Finally, it is important to note that the presence of circumstellar material in HMXRB can cause some variations in the colors and magnitudes, so it is possible that your plot may not show a clear trend. In this case, it might be helpful to consult with your professor or other resources to confirm your findings.

I hope this helps and good luck with your coursework!