Swift/UVOT calibration

The calibration has been improved over the years, with the zeropoints of Poole et al. 2008 being updated in Breeveld et al. 2011.  As of 2010-11-30 these new zeropoints have been included in the Swift CALDB used with the HEASARC software (ie uvotsource and uvotmaghist).  A sensitivity loss of about 1% per year has also been discovered and is incorporated into the HEASARC software as of 2010-6-30. Many papers reference only the Poole et al. 2008 calibration, but we hope the new zeropoints and sensitivity correction are being implemented.

Filter Zero Point Error 

V 17.89  0.01
B 19.11  0.02
U 18.34  0.02
UVW1 17.44  0.03
UVM2 16.85  0.03
UVW2 17.38  0.03
White 20.29  0.04

 The site here also gives the AB zeropoints, but I don't list them here to avoid confusion.

The official filter curves are given in the CALDB  but in a fits format designed for certain x-ray analysis tools.  I have placed ascii version of the curves on this page.  The curves are the effective area of the filters in units of cm^2, ie multiplying the area of the telescope mirror with the transmission functions of the filters, reflectivity of the mirrors, and corrections to match the inflight observations.

Ultra Super Super Explosions

I tried to add another superlative to the title:  Ultraviolet Observations of Super-Chandrasekhar Mass Type Ia Supernova Candidates with Swift UVOT.  It would have been fair, since these objects are extreme, but seemed cheesy.  Nevertheless, Ultra-Super-Super Explosions seems an adequate short title.

Bright Stars

Swift/UVOT has brightness limits to protect the detector.  The limit is approximately V~ 5 but depends on the spectral type and which filter(s) you want to use.  A 20' radius is checked to allow for different position angles and uncertainty in the spacecraft pointing.  Based on the color and optical brightness, the count rate in each filter is predicted.  If the predicted count rate is higher than the limit for a requested filter, that filter is skipped in the observing sequence and only the requested filters that are deemed safe are observed.  There is a bright star checker at the following link, but it only tells you which bright stars are in the field--it doesn't actually tell you which filters can and can't be observed.   Rough limits for bright stars, as well as the allowed angles from the sun, earth, moon, and planets, are posted at http://swift.gsfc.nasa.gov/analysis/uvot_digest/numbers.html

Because the white filter and the grisms let in photons from the widest wavelength range, they have the strictest limits.  The UV filters, on the other hand, are the least likely to be prohibited.  Note, these are the brightness limits of stars which can be safely observed.  The bright limits at which you can do photometry are much fainter (though different analysis using the halos or the readout streaks can push brighter than normal aperture photometry, see  http://adsabs.harvard.edu/abs/2013MNRAS.436.1684P  ).

In addition to the strict safety limits, bright stars can also interfere with the data analysis.  Stars of moderate brightness (I should quantify this better, but the star below is about 9th mag in V) will have a filled in halo with a 20 arcsec diameter.  A fainter halo is visible around it, with a radius of 140 arcsec, which gets brighter for brighter stars and will mess up the photometry of faint sources.  Bright sources in the halo may still be recoverable by choosing a background region of similar brightness. 

 

 [ details of the above image which I first sent out in reference to SN2007C ]

So you know the issues involved, I've attached an
image from the field of GRB060729.  (it is a UVW1
image so that we can see the effect in the UV of
bright stars--the effect is worse in the optical for
us, but the usefullness of the UV data is our main
concern.)  It shows the 20" radius bright core around
a 9th mag star and the 2' radius halo around an 8th mag star
(the halo is faintly visible around the fainter star
but not a dominant problem).  So being within 20" of a
star brighter than 10th or so would be nearly
impossible.  The larger halo would be hard to subtract
correctly but not a show stopper if the SN is
interesting and bright enough.  And if there are stars
brighter than 7th or so within 20' of the target
pointing we can't observe at all.  (That's something
the Swift people will check since it depends on the
spectral type, number of sources, and such, but just
so people are aware.)

Supernova SN2014J in M82

Here are my before and after images of the new Supernova in M82.  The picture with the supernova looks grainier because it has much less exposure time. 

NASA just had a press release : http://www.nasa.gov/content/goddard/nasa-spacecraft-take-aim-at-nearby-supernova/#.UuMU6PbnZjM

Swift/UVOT photometry version 13.2

Swift has observed hundreds of supernovae over the last nine years, including dozens of very well sampled light curves published in many different papers (see post to Swift SN publication list here).  The calibration has been improved over the years, with the zeropoints of Poole et al. 2008 being updated in Breeveld et al. 2011.  As of 2010-11-30 these new zeropoints have been included in the Swift CALDB used with the HEASARC software (ie uvotsource and uvotmaghist).  A sensitivity loss of about 1% per year has also been discovered and is incorporated into the HEASARC software as of 2010-6-30.  Combining these effects could lead to a 0.1 magnitude error when comparing new and old photometry.

To allow an accurate comparison between new SNe and those published earlier in the mission, I have redone the photometry for using the latest calibration.  The photometry for the SNe Ia published in Brown et al. 2009 and 2012 have been redone and posted here.  The original references are given in the files, and I will be redoing other SNe as time allows.  I have also posted the reduction code here.  Please report any bugs, suspicious data, or deviations from ground photometry (for B and V, the Swift u band is quite different from ground-based u' or U) in the comments below or to uv dot supernova at gmail dot com.

A set of CC SN photometry from Pritchard et al. 2013 on the same system (but slightly different analysis) is posted here.

Notes:

The coincidence-loss saturation limit is currently set at 0.98 counts per frame.  This should be more rigorously tested.

There is something wrong with the three sigma upper limits, as the SN2006mr photometry lists magnitudes that are only 1-2 sigma detections.  I think the photometry is right, but there is an inconsistency in how the photometric errors are calculated and how the upper limits are calculated.  It might be the inclusion of the galaxy subtraction errors in the limit, as SN2006mr had the largest underlying galaxy count rate.

Swift Supernovae in the Future (2014-2018) Brainstorm List

Swift Supernovae in the Future (2014-2018) Brainstorm List

This post is intended as a brainstorming list to dump new ideas about what Swift should be doing in regards to supernovae over the next four years.  We don't need a list of all the wonderful science you could do, but how different target selection or observations can add to the science we are doing.  Adding something here doesn't necessarily mean you think we should do it, but that it could be done.  Adding something does not obligate you to do anything about it now, but we may ask for more details or ideas on implementation.  Suggestions do not need to be strongly justified on this page, but sometimes a brief clarification is useful.  Obviously we cannot do everything, but by airing these ideas we can probably find more synergistic observations which can benefit multiple projects.  Please add your ideas in the comments.

##############################

SNe in the Hubble Flow -- a large fraction of the Swift SNe are at very low redshifts (z<0.02) where the peculiar velocity contribution to the absolute magnitude error is greater than 0.1 mag (in the absence of other distance measurements).  More SNe Ia could be targeted in the range 0.02 < z < 0.03 or so, and UV bright SNe II even farther out.  [ Peter B]

Early grism observations -- the updated grism software includes calibration for positions across much of the field.  A slew in place, while desirable in the planned schedules, is no longer necessary, so grism observations can be uploaded to Swift as immediate targets for earlier spectra.  [Peter B]

[ April 22, 2014 Update -- I only listed a few ideas so that others could contribute without having most ideas already listed.  Since there hasn't been much response, I'll go ahead and list the other ideas I have. ]

V grism observations of SNe Ia -- We've focused mostly on the UV grism since the V grism only extends a little past what can be done from the ground (to about 2900 Angstroms).  But there are interesting features there that might explain the difference between NUV-red and NUV-blue SNe, and using the V grism would allow for SNe to be targeted at higher redshifts than the UV grism.

Reddened SNe -- Now that we've got a large sample of low reddened SNe to compare with we should also target mildly reddened SNe (E(B-V) <~ 0.5) to understand the wavelength dependence of the extinction law(s?) applicable to SNe

V grism observations of reddened SNe?  We can't get much mid-UV flux anyway, but V grism observations could probe the extinction law down to 2900 Angstroms

UV light curves of SNe Ib/c -- we still don't have many good light curves of SNe Ib/c, especially in the mid-UV (uvm2 band).   There aren't many close enough to detect in short observations (2 ks) but we could take longer exposures to get light curves of SNe Ib/c a little farther away.

Distance-limited sample of SNe IIP -- IIn

Target all SLSN candidates brighter than g/B/V ~ 20 -- some might not be UV bright but we could get a feel for what fraction are and what the brightness distribution is in the UV

Swift Supernova Publications

I am trying to create a master list of all publications presenting or making significant use of Swift Supernova data.  Right now I am sticking to refereed (or arXiv papers in the refereeing process) papers but may add ATELs and IAU Circulars at some point -- there are quite a few.  Please alert me to any I am missing.  To minimize clutter I will delete comments after adding the reference.


https://docs.google.com/document/d/1V1WVF9--sWfTfV_4vjoXQ-KtzFNl13WVpUbDZEf4JS0/edit?usp=sharing

As of September 2015 I count 107 publications from 63 different authors.