V5668 Sgr (Nova Sgr 2015 No. 2) campaign

Thanks to the support from ground observations as well as ongoing monitoring by the Swift UVOT the nova was successfully observed by the STIS imaging spectrograph on board the Hubble Space Telescope. 

The nova is currently just a bit too bright for normal observations with Swift. It is too bright for normal aperture photometry as the image of the nova itself is too bright to be measured. However, the image is readout while the telescope keeps feeding photons to the detector while it is shifting the image off the CCD to the readout area. This means that for such a bright source the UVOT shows line through the source from those photons collected during the readout of the CCD, and that has been calibrated. This was used to get uv magnitudes to monitor the uv. However, if the nova had become any brighter, we would not have been able to follow that, so for safety reasons we needed ground based data as well.  

Fortunately, all went as expected, and the nova kept its brightness, and actually showed a small 0.2 mag decrease in the uv over the past two weeks.  

The optical nova spectrum has also been observed at R=28,000 with the Chiron on the SMARTS telescope at CTOI.  We have now a spectroscopic snapshot of the state of the nova after the dust formation was done.  The question of how the enormously strong dust signature has affected the gas we observe can now be tackled. For we would expect that when carbon is captured in dust, there will be less left in the gas, or perhaps silicon is affected more.  

The HST STIS spectrum also shows interstellar line absorption. The most prominent is that from the primary line from Hydrogen, the Lyman-alpha line at 1215.67 Angstrom. It can be used to infer how many neutral Hydrogen atoms are present between us and the nova. It is a huge number: 1.4x10^21 per cm squared. That large number is mostly so because of the large distance, because the space density is pretty low. (how much exactly is not known exactly as we do not have a good measurement of the distance).  I include a plot of the spectral region around the Hydrogen Ly-alpha line to show you what we got. The plot shows the brightness as an energy flux per wavelength bins on a logarithmic scale as the lines are so bright. 

We also see bright lines of C III and N V. Notice the three peaks in the N V line, which are evidence for some kind of structure in the outflow.  Not all lines show the same structure, so hopefully we can learn about the structure in the outflow from those differences. 

With thanks again to all who helped,    

      Paul Kuin (Mullard Space Science Laboratory of the University College London)