Observation by Robin Leadbeater: SN 2021J - Teasing out the spectrum

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Robin Leadbeater


Robin Leadbeater


2021 Mar 18 - 23:11


2021 Mar 30 - 10:27



  • ALPY 600 spectrograph
  • ATIK 428 camera
  • Celestron C11

9x600 sec


Wigton Cumbria

Target name

SN 2021J in NGC 4414


SN 2021J - Teasing out the spectrum

About this image

Discovered by the Zwicky Transient Facility on New Years Day 2021, SN 2021J  in NGC 4414 was a bright one as supernovae go, reaching mag 12.5 at the end of January. Here though about 50 days later it has faded to mag 15.3 and its proximity to the galaxy core makes it difficult to avoid severe contamination in the spectrum from the brighter host galaxy. 

Fortunately there are historic spectra of NGC 4414 available in the literature and accessible in digital form via the NASA Extragalactic Database. These can be used to subtract the galaxy spectrum from the blend revealing the spectrum of the supernova.(Done by matching the intensity of the narrow galactic absorption lines.  In this case only ~20% of the light in the spectrum is from the supernova.)

Comparison with the spectra of other supernovae using the supernova identification program SNID confirms it still matches a type Ia in agreement with the original classification made at discovery, even though  the spectrum has evolved significantly.

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