Master thesis - UBC Physics & Astronomy

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84 M. Alexandersen : Master thesis sizes that a host galaxy should be seen as, using tan θ = r/d, (27) θ ≈ r/d, (28) which applies for small angles, where r is the radial size of the host galaxy, and d is the distance to it. Galaxies usually have radii in the range 0.5 − 50 kpc. The closest GRB ever observed is GRB980425 at redshift 0.0085 (Tinney et al. 1998), corresponding to a distance of 36 Mpc. So if the host galaxy is large, with r = 50 kpc, the angular size of the galaxy would be θ = 290 arcsec, so θ 2 ≈ 84000 arcsec 2 . So this is the largest possible angular size of a GRB host galaxy that I could encounter. If I set theta2min to this however, it would cut out most of the useful part of the image, and any sign of most haloes. Of course, most GRB host galaxies are much further from us than GRB080425, and few of them are of 50 kpc radius. A more typical galaxy of r = 5 kpc at d = 36 Mpc would have θ = 29 arcsec, or θ 2 = 836 arcsec 2 . A large galaxy (50 kpc) at a more typical redshift of 1.0, corresponding to d = 2.5 Gpc, would have θ = 4 arcsec, so much smaller than XRT’s point spread function. So in most cases, the angular size of the afterglow will be mainly due to the instruments PSF, and not the physical size of the host galaxy. theta2max This boundary I had originally not intended to impose, and simply allow the use of all the events right to the edge of the CCD. However, I found that my background calculation for the DDD did not function well with this. This is most likely because my determination of the CCD edges are poor. So I had to find a limit which in most cases is mostly inside the CCD edges, but without cutting too much off, as the high θ 2 values contribute the small distances in the DDD. I know that the CCD is 1416 arcsec across (Capalbi et al.), and that the location error radius of BAT is 240 arcsec, so GRB position should always be within 468 arcsec of the edge. So, I thought maybe to set
M. Alexandersen : Master thesis 85 theta2max = (468 arcsec) 2 ≈ 220000 arcsec 2 . However, I discovered that in some settings, the full CCD is not used, in worst case making it only 1132.8 arcsec across, thereby making the minimum distance from the outer edge of the error circle to the edge of the CCD only 326.4 arcsec. However, if this were adopted, so theta2max ≈ 100000 arcsec 2 , then counts from dust closer than 125 pc would begin to be excluded before 16200 s after the burst. This value should preferably be lower, so I had to accept that my data, and the synthetic background, were cut partly by a maximum radius, and partly by the edge of the CCD. I tried 300000 arcsec 2 and 250000 arcsec 2 , which were both good compromises between cutting before the edge of the CCD, and not cutting too much. See Fig. 25. These both gave significantly better result than the run with theta2max = 600000 that I tried, and the runs without an upper limit during the development. In the end I became convinced that the 250000 arcsec 2 was the better choice. This gives that at 16200 s after the burst, this radius corresponds to 54 pc, so that all distances larger than this are represented for all times. timeintv Since the halo is most visible at early times, and some observations last much longer than any halo would be visible, the data is filtered to only include the first timeintv of the observation. So timeintv is not the maximum time after burst, but the maximum observation time to include. I had during programming found that 10 ks was not enough, as many GRBs would then not have enough counts to be useful. I also knew from my experimental reduction stage, that for GRB05724, 15 ks was good. I tried 20 ks and 15 ks here, and concluded that for the test GRBs, 15 ks were better, and the last 5. ks mostly added noise. Eventually I got the idea to use 16.2 ks, as this corresponds to three orbits of Swift, so that the observation should always include three full orbits, no matter when within an orbit the observation starts. I checked that this did not alter the results significantly from using 15 ks. This was particularly because only a few of the observations actually had exposure in the period 15000−16200 s after
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An almost automated search for dust
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Master thesis - UBC Physics & Astronomy

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