NOT Stockholm Supernovae

Jesper Sollerman

Department of Astronomy, Stockholm University, Sweden
The Oskar Klein Centre, Albanova, Stockholm University
Stockholm Observatory

Abstract

This proceeding contribution is a short summary of the invited talk about
observational supernova science at Stockholm University that has been
conducted at the Nordic Optical Telescope over the past 25 years, and some
expectations for the future.

Keywords: Stockholm, Supernovae, Sollerman, Transients, Nordic Optical
Telescope

##  1 Introduction

In the NOT workshop in June 2022, I delivered an invited talk about supernova
research conducted at the Nordic Optical Telescope (NOT) from Stockholm
University. It was titled The Nordic Optical Telescope and Stockholm
Supernovae 1994-2024 (my personal view). This talk was given in a semi-
historical context from my own first observing runs in 1994 to future plans
for supernova classifications of LSST supernovae. The talk is available on the
NOT WWW, and here I simply give a summary of a few of the points given in that
presentation. I have been running a supernova program at the NOT as service
mode / Target-of-Opportunity for the past 10+ years, most of them as Large
Programmes. The talk was not intended to be comprehensive, rather nostalgic.

##  2 Early days

I have been doing supernova observations for over 30 years and am a keen user
of the NOT. It was indeed the first observing runs at this telescope that made
me aim for an observational astronomy career, even though all other supernova
researchers at Stockholm observatory at that time were doing modeling.

I remember well my first observing runs as amazingly fascinating, although in
hindsight maybe not that efficient. Filling the nitrogen dewars yourself while
integrating on a supernova, using the offline PC to find coordinates for
guide-stars to track on, screwdriver in the pocket to fight with filter
wheels. The NOT in the 1990s was a great telescope on a fantastic site, but
the instrumentation and automation that enabled efficient observing, including
service mode and Target-of-Opportunity, came later. I argued in my talk that
this was at least a good way to learn the skills of observing, even if the
scattered epochs of supernova observations that emerged from a typical visitor
run in the 1990s is a far cry from what modern supernova science requires. The
training aspect later motivated me to run two summer schools at the NOT with
Nordic PhD students (2003 and 2006). Other contributions in this conference
proceeding focus more on the important educational aspects of the telescope.

In my talk I also briefly reviewed the development of supernova surveys. In
the early days, a great number of nearby supernovae were in fact discovered by
dedicated amateur astronomers, and professional astronomers were slow to take
up the supernova survey strategies developed by Fritz Zwicky in the middle of
the last century. With the High-z Supernova Search team (Tonry et al., 2003)
where I participated to search for Type Ia supernovae for cosmology in the
late 1990s, the technique was adopted with modern CCDs to image the sky
several nights apart and search for transients in the difference image
subtractions. With subsequent pre-planned visitor runs on larger telescopes we
could then classify the newly discovered SNe. ESSENCE (Miknaitis et al., 2007)
followed a similar scheme, whereas the SDSS-II supernova search (Frieman et
al., 2008) employed a dedicated telescope with a rolling search. This
automatically gave well-covered multi-band, high-cadence supernova light
curves. iPTF and its successor ZTF (Graham et al., 2019; Bellm et al., 2019)
have added increasingly larger fields, higher cadence and automation and are
bringing transient science to the new era that the Vera Rubin Observatory will
cement.

This development and evolution in survey capacity has not only hugely
increased the number of SNe detected with time, but also the quality of the
observations. More SNe means that we find rarer objects, and there is often a
lot of physics to deduce from such outliers. Higher cadence means we find the
SNe earlier, and the untargeted search strategy gives a less biased view of
the underlying populations, which has also revealed new classes of objects.

##  3 Recent past

I have not done the exercise to count the number of refereed papers that have
been published based on the supernova programmes at the NOT led from
Stockholm. There must be $\gtrsim 100$ such papers, and deciding which ones
have been most important of course depends on who you ask. Some handful of PhD
students have based their supernova PhD-theses on NOT data, most of them
supervised by myself on the core-collapse side of the supernova spectrum, or
by Ariel Goobar for the thermonuclear ones. A handful of publications have
made it into high-impact journals like Nature and Science over the years111For
the 2 dozens of papers in Nature/Science journals that I have co-authored over
the past 20 years, about a third include data from the NOT, including the
birth of the field of Flash spectroscopy for supernovae (Gal-Yam et al., 2014;
Yaron et al., 2017), and the new Type Icn supernova class (Gal-Yam et al.,
2022)..

The reason NOT is so good for us doing transient astronomy is that it is
flexible, simple, accessible and often fast. Transient astronomy is about
getting the data when needed, and within international collaborations we can
complement or even rival larger telescopes by being smart on when and how to
get the spectra. This has allowed us to participate in many interesting
investigations, but also to take the lead in a number of projects.

Figure 1 illustrates this by showing a person-gallery of Stockholm scientists
that have all led papers based on data from the NOT, including work on (this
is from left to right, from top to bottom in Fig. 1): Helium-rich Type Ia-CSM
supernova with radio detections (Kool et al., 2022), oxygen abundances in Type
II supernovae (Jerkstrand et al., 2014), the stripped-envelope supernova
iPTF13bvn (Fremling et al., 2016), superluminous supernovae from ZTF (Lunnan
et al., 2020) the magnetar-driven iPTF15dtg (Taddia et al., 2019), and the
enigmatic zombie-supernova iPTF14hls (Arcavi et al., 2017; Sollerman et al.,
2019).

Figure 1: To pick a single slide from my presentation I would select this one.
These are all astronomers that first-authored a refereed publication including
observations from the NOT while they were working at Stockholm University. See
text for further information on some of these articles.

A stripped-envelope SN with a H-shell (Tartaglia et al., 2021), Type Ia
supernova distances (Johansson et al., 2021), the candidate pair-instability
supernova SN 2018ibb (Schulze and et al., 2022), or hydrogen deficient Type
Ibn supernovae (Karamehmetoglu et al., 2021), and a Type Ic SN metamorphing
into a Type IIn (Chen et al., 2018), or the bumpy light-curve of Type IIn
iPTF13z (Nyholm et al., 2017).

The PhD-thesis papers on the now canonical Type IIb SN 2011dh (Ergon et al.,
2014, 2015), the low-luminosity supernova SN 2020cxd (Yang et al., 2021), the
almost superluminous SN 2012aa (Roy et al., 2016), the multiply imaged
gravitationally lensed Type Ia supernova iPTF16geu (Goobar et al., 2017), or
work on circumstellar interaction supernovae like in the monumental paper on
SN 2010jl where NOT data were complemented with spectra from the Hubble Space
Telescope (Fransson et al., 2014), or analysis of Type Ia SN spectra (Nordin
et al., 2011), the power of supernova siblings (Biswas et al., 2022), or
extinction law studies for Type Ia supernovae (Amanullah et al., 2015), and
the full sample of stripped-envelope Type Ic SNe from PTF (Barbarino et al.,
2021).

Bumpy superluminous supernovae (SLSNe) of Type I (West et al., 2022) and of
SLSNe Type II (Kangas et al., 2022), constraints on hydrogen in Type Ia SNe
(Lundqvist et al., 2015) and a very detailed study of a normal Type Ia
(Stanishev et al., 2007). The NOT spectroscopic follow-up of SDSS-II SNe Ia
(Östman et al., 2011) and long-lived interaction powered Type IIn supernovae
(Stritzinger et al., 2012), an oxygen-rich stripped envelope supernova
(Schweyer et al., 2022) and finally the Crab supernova remnant and its famous
pulsar (Sandberg and Sollerman, 2009).

There is a strong legacy not only in the scientific data and these published
results, but also in the number of researchers that have benefited from this
long-term collaboration. The relative diversity in the topics mentioned above
also illustrates that an observing programme for transient sources needs
flexibility to allow seizing the opportunities as they arise in the universe.
We must both be ready to work on systematic and complete samples and to
quickly jump on whatever new explosion that occurs.

That Nordic transient astronomy has flourished with the help of the NOT was
also obvious from the many presentations given during this workshop. In the
distant past, the proposal I lead was the only one for supernovae at the NOT,
while today there is a plethora of projects from for example Århus, Åbo,
Copenhagen and also several from outside the Nordic countries (like Spain and
Italy). Many of these Nordic supernova nodes are now led by astronomers that
once did an observational supernova postdoc with us in Stockholm. NOT has
clearly been a key component for allowing astronomers in the Nordic countries
to do competitive transient science on the international scene.

##  4 Nowadays

The current main focus of my own SN research is the Zwicky Transient Facility
(ZTF). With a refurbished Schmidt telescope (P48) on classical Palomar, a
super-sized 47 square-degree detector, and a very low-resolution dedicated
follow-up spectrograph (SEDM on P60), I can discover a handful of supernovae
every night, immediately get a spectrum and classify them. Since 2018 we have
trawled the Northern sky on a nightly basis and within the Bright Transient
Survey (BTS) (Fremling et al., 2020; Perley et al., 2020) we have so far
classified over 6000 SNe. I have discovered just as many myself, which
highlights the new era we are in222Fritz Zwicky himself was for a long time
the record holder in discovering SNe, with 120 SNe over a period of $\sim 50$
years. Hard work at a cold telescope, daytime blinking of the plates and
endurance that contrasts with my own clicking on the computer over breakfast
coffee.. The light curves of these supernovae are directly made available to
the public via the Bright Transient Survey explorer and the classification
spectra are immediately uploaded to the Transient Name Server (TNS).

In contrast to the early days of supernova science, this rolling, untargeted
and systematic system more or less automatically provides three ($gri$)
photometric band light curves for supernovae and other transients. The new era
thus combines the survey strategies once developed in the supernova cosmology
framework with the rapid follow-up capacities inherited from the fast-shooting
$\gamma$-ray burst community.

An illustrative example of how the scope of the transient astronomy has
expanded could be to think of supernova 1987A, which was the main topic of
research for the supernova group in Stockholm when I started my PhD. It got
the designation ‘A’ since it was the first supernova discovered in that year,
on February 24 1987. This means that the first 54 days that year did not have
a single supernova. In contrast, this year, 2022, 340 classified SNe had
already been reported to the TNS up to February 24 (about 6 per day), so a SN
discovered that day got a more complicated name such as SN 2022dek. Of these
340 SNe, ZTF follows some 68% and discovered around 20%. But the classified
supernovae are only a small fraction of the reported transients; there were in
fact over 3000 transients reported to TNS in the first 55 days of the year
2022, so only 11% were classified supernovae. One third of the classifications
were done with the SEDM on the P60, for which I am the main responsible.

The name of the game is now instead to recognize which of all the new objects
are interesting enough for a dedicated follow-up campaign. We have seldom used
the NOT for pure classifications, most objects are simply not interesting
enough to waste valuable NOT time. Today the SEDM on the P60 is the
classification work horse, and we trigger NOT mainly for objects that we
already know are going to be of interest (for some specific publication). Once
I have allocated a target to SEDM, the robotic execution is automatic, the
reductions are automatic (Rigault et al., 2019) and if it is a typical SN Ia
it will automatically be classified (Fremling et al., 2021) and uploaded to
the TNS. Once NOT confirms the interest of a particular object, many other,
often larger, telescopes are typically triggered for further investigations.
This is a good food-chain for transient astronomy where every telescope kicks
in where (and only where) it is best needed.

##  5 Future

I strongly believe NOT will continue to be competitive due to its size,
location and flexibility. At Stockholm University, half a dozen of scientists
have joined the Vera Rubin Observatory to conduct the Large Synoptic Survey
Telescope (LSST) survey over the coming 10 years (starting 2024). This will
find tens of thousands of transients and will again transform time domain
astronomy. We have offered to provide $\sim 300$ hours of NOT telescope time
over three years to the LSST project for classifications of supernovae, and
for this we, of course, hope to use the new Nordic Transient Explorer (NTE)
with its improved sensitivity and extended wavelength range. There will be no
lack of targets for future supernova astronomers, the challenge will be to
cope with the flood of data and cleverly select the most interesting targets
for follow-up.

## Acknowledgements

Based on observations made with the Nordic Optical Telescope, owned in
collaboration by the University of Turku and Aarhus University, and operated
jointly by Aarhus University, the University of Turku and the University of
Oslo, representing Denmark, Finland and Norway, the University of Iceland and
Stockholm University at the Observatorio del Roque de los Muchachos, La Palma,
Spain, of the Instituto de Astrofisica de Canarias. The data presented here
were obtained [in part] with ALFOSC, which is provided by the Instituto de
Astrofisica de Andalucia (IAA) under a joint agreement with the University of
Copenhagen and NOT. In my talk I ended with a slide saying: Thanks a lot NOT!
It has been a real pleasure. Send any comments on this to<EMAIL_ADDRESS>Jesper Sollerman is professor at the Department of Astronomy at Stockholm
University and affiliated to the Oskar Klein Centre. He was the Swedish
representative of NOTSA and is the Stockholm representative in the new NOT
board. He has regularly been using the NOT since 1994.

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