I am currently teaching the undergraduate Cosmology course, AS5003, which is given in the second half of the fall semester.

I am always interested in chatting with potential master or bachelor students who want to do a degree project on supernovae - feel free to contact me or stop by my office!

I am broadly interested in unusual transients, and what they tell us both about massive star evolution and about star formation in extreme environments. Much of my work focuses on “superluminous” supernovae, a rare class of transients 10-100 times brighter than ordinary core-collapse and Type Ia supernovae. A typical superluminous supernova radiates more energy than the entire kinetic energy of a canonical core-collapse explosion (10^51 erg), and therefore cannot be easily explained by the same physical mechanisms. It is still debated what powers these extreme luminosities, with candidates including tapping the rotational energy of a newborn neutron star with a strong magnetic field (“magnetar”), a pair-instability explosion of an extremely massive star, or interaction between the supernova ejecta and dense circumstellar material.

I am an observational astronomer, and have worked with ground-based optical and infrared data from VLT, Magellan, MMT, Gemini and Keck, as well as UV, optical and infrared data from the Hubble Space Telescope, Spitzer Space Telescope, and Swift.

Understanding Superluminous Supernovae

Studying the supernova explosions themselves allows us to characterize the energies, timescales and velocities involved, and compare to the various physical models proposed to explain superluminous supernovae. I have worked on several studies of individual supernovae, as well as a compilation of objects from the Pan-STARRS Medium Deep Survey. I am currently working with the Zwicky Transient Facility, which is turning out to be a fantastic experiment for finding large numbers of superluminous supernovae and exploring their diversity. I explain some of our early results in this AAS Journal Author chat on Youtube; this work was also part of my Marie Curie project "SUPERS".

As part of a late-time spectroscopy campaign, we serendipitously discovered the presence of a fast-moving circumstellar shell around the superluminous supernova iPTF16eh through light echo emission from the Mg II resonance lines. This is exciting both because it provides conclusive evidence that some superluminous supernovae experience significant mass-loss episodes close to explosion, and because the shell properties were best explained by a pulsational pair-instability mass ejection. This regime of stellar evolution is also of great importance for understanding the black hole populations probed by gravitational wave experiments like LIGO/Virgo, and few observational constraints exist. You can read more details in the blog post I wrote for Nature Astronomy Community. I am currently leading a campaign using ESO's Very Large Telescope to constrain how common this phenomenon is.

In addition to finding nearby SLSNe that allows for late-time follow-up, ZTF (as well as its predecessor PTF) is also great for finding all kinds of rare transients. Recently, I worked with Caltech student Lindsey Whitesides to analyze an exciting object falling somewhere in between a superluminous supernova and a typical gamma-ray burst supernova, iPTF16asu.

Read more here:

SN 2020qlb: A hydrogen-poor superluminous supernova with well-characterized light curve undulations

Four (Super)Luminous Supernovae from the First Months of the ZTF Survey

A UV resonance line echo from a shell around a hydrogen-poor superluminous supernova

iPTF16asu: A Luminous, Rapidly-Evolving, and High-Velocity Supernova

Superluminous Supernovae from the Pan-STARRS1 Medium Deep Survey

Hydrogen-poor Superluminous Supernovae With Late-time H-alpha Emission: Three Events From the Intermediate Palomar Transient Factory

PS1-14bj: A Hydrogen-Poor Superluminous Supernova With a Long Rise and Slow Decay

PS1-10bzj: A Fast, Hydrogen-poor Superluminous Supernova in a Metal-poor Host Galaxy

Host Galaxy Environments of Superluminous Supernovae

Since SLSNe are so rare, we don’t expect any to occur close enough for a direct progenitor detection. However, studying the galaxy-scale environments gives us a handle on the stellar populations they come from, and thus an indirect probe of the progenitor population. A systematic sample study of the host galaxies of SLSNe reveal that they show a strong preference for low-mass, low-metallicity dwarf galaxy hosts. Many of these dwarf galaxies are also star-bursting, with properties that would be classified as “extreme emission line galaxies” or “green peas” in a galaxy survey. One interpretation of these findings is that superluminous supernova explosions require a low metallicity progenitor, or at least that the rate is suppressed at higher metallicities. It has also been suggested that the intensity of star formation plays a role - this can be tested with resolved HST UV imaging through measuring the star formation at the supernova sites.

One can also use SLSNe as a tool to study the galaxies they occur in, through host galaxy absorption lines in the supernova spectrum. Similar techniques are used with long gamma-ray burst afterglows. In Pan-STARRS we used this technique successfully to study the ISM of a galaxy at z=1.566.

Read more here:

Zooming In on the Progenitors of Superluminous Supernovae With the HST

Hydrogen-poor Superluminous Supernovae and Long-duration Gamma-Ray Bursts Have Similar Host Galaxies

Ultraluminous Supernovae as a New Probe of the Interstellar Medium in Distant Galaxies

Calcium-Rich Gap Transients

Calcium-rich gap transients are another rare class of explosions, which unlike superluminous supernovae are fainter and faster-evolving than ordinary supernovae, and predominantly found in old galaxy environments. This suggests that the progenitor is a binary system, but the origin of these transients is not well understood. With Mansi Kasliwal at Caltech, I took a detailed look at two of these transients discovered by PTF, as well as a broader look at the host galaxy environments of Ca-rich gap transients in general. Read more here:

Two New Calcium-rich Gap Transients in Group and Cluster Environments

Other Projects

In my first two years of graduate school, I worked with Prof. Lars Hernquist and Dr. Anna Frebel on various aspects of near-field cosmology and galactic archeology. Read about it here:

The Effects of Patchy Reionization on Satellite Galaxies of the Milky Way

The 300 km s-1 Stellar Stream near Segue 1: Insights from High-resolution Spectroscopy of Its Brightest Star

I wrote my senior thesis with Prof. J. Richard Gott, III at Princeton, exploring the extent to which the genus statistic could be used as a probe of the equation of state of dark energy. Read more here:

Using the topology of large-scale structure to constrain dark energy

A selection from Stockholm University publication database

• SN 2020qlb: A hydrogen-poor superluminous supernova with well-characterized light curve undulations

  2023. Stuart L. West (et al.). Astronomy and Astrophysics 670

  Article

  Context. SN 2020qlb (ZTF20abobpcb) is a hydrogen-poor superluminous supernova (SLSN-I) that is among the most luminous (maximum Mg = −22.25 mag) and that has one of the longest rise times (77 days from explosion to maximum). We estimate the total radiated energy to be > 2.1 × 10⁵¹ erg. SN 2020qlb has a well-sampled light curve that exhibits clear near and post peak undulations, a phenomenon seen in other SLSNe, whose physical origin is still unknown.

  Aims. We discuss the potential power source of this immense explosion as well as the mechanisms behind its observed light curve undulations.

  Methods. We analyze photospheric spectra and compare them to other SLSNe-I. We constructed the bolometric light curve using photometry from a large data set of observations from the Zwicky Transient Facility (ZTF), Liverpool Telescope (LT), and Neil Gehrels Swift Observatory and compare it with radioactive, circumstellar interaction and magnetar models. Model residuals and light curve polynomial fit residuals are analyzed to estimate the undulation timescale and amplitude. We also determine host galaxy properties based on imaging and spectroscopy data, including a detection of the [O III]λ4363, auroral line, allowing for a direct metallicity measurement.

  Results. We rule out the Arnett ⁵⁶Ni decay model for SN 2020qlb’s light curve due to unphysical parameter results. Our most favored power source is the magnetic dipole spin-down energy deposition of a magnetar. Two to three near peak oscillations, intriguingly similar to those of SN 2015bn, were found in the magnetar model residuals with a timescale of 32 ± 6 days and an amplitude of 6% of peak luminosity. We rule out centrally located undulation sources due to timescale considerations; and we favor the result of ejecta interactions with circumstellar material (CSM) density fluctuations as the source of the undulations.

  Read more about SN 2020qlb

• The Hydrogen-poor Superluminous Supernovae from the Zwicky Transient Facility Phase I Survey. I. Light Curves and Measurements

  2023. Z. H. Chen (et al.). Astrophysical Journal 943 (1)

  Article

  During the Zwicky Transient Facility (ZTF) Phase I operations, 78 hydrogen-poor superluminous supernovae (SLSNe-I) were discovered in less than 3 yr, constituting the largest sample from a single survey. This paper (Paper I) presents the data, including the optical/UV light curves and classification spectra, while Paper II in this series will focus on the detailed analysis of the light curves and modeling. Our photometry is primarily taken by ZTF in the g, r, and i bands, and with additional data from other ground-based facilities and Swift. The events of our sample cover a redshift range of z = 0.06 − 0.67, with a median and 1σ error (16% and 84% percentiles) of z_med=0.265. The peak luminosity covers −22.8 mag ≤ M_g,peak ≤ −19.8 mag, with a median value of -21.48. The light curves evolve slowly with a mean rest-frame rise time of t_rise = 41.9 ± 17.8 days. The luminosity and timescale distributions suggest that low-luminosity SLSNe-I with a peak luminosity ∼−20 mag or extremely fast-rising events (<10 days) exist, but are rare. We confirm previous findings that slowly rising SLSNe-I also tend to fade slowly. The rest-frame color and temperature evolution show large scatters, suggesting that the SLSN-I population may have diverse spectral energy distributions. The peak rest-frame color shows a moderate correlation with the peak absolute magnitude, i.e., brighter SLSNe-I tend to have bluer colors. With optical and UV photometry, we construct the bolometric luminosity and derive a bolometric correction relation that is generally applicable for converting g, r-band photometry to the bolometric luminosity for SLSNe-I.

  Read more about The Hydrogen-poor Superluminous Supernovae from the Zwicky Transient Facility Phase I Survey. I. Light Curves and Measurements

• The Hydrogen-poor Superluminous Supernovae from the Zwicky Transient Facility Phase I Survey. II. Light-curve Modeling and Characterization of Undulations

  2023. Z. H. Chen (et al.). Astrophysical Journal 943 (1)

  Article

  We present analysis of the light curves (LCs) of 77 hydrogen-poor superluminous supernovae (SLSNe I) discovered during the Zwicky Transient Facility Phase I operation. We find that the majority (67%) of the sample can be fit equally well by both magnetar and ejecta–circumstellar medium (CSM) interaction plus ⁵⁶Ni decay models. This implies that LCs alone cannot unambiguously constrain the physical power sources for an SLSN I. However, 23% of the sample show inverted V-shape, steep-declining LCs or features of long rise and fast post-peak decay, which are better described by the CSM+Ni model. The remaining 10% of the sample favors the magnetar model. Moreover, our analysis shows that the LC undulations are quite common, with a fraction of 18%–44% in our gold sample. Among those strongly undulating events, about 62% of them are found to be CSM-favored, implying that the undulations tend to occur in the CSM-favored events. Undulations show a wide range in energy and duration, with median values (and 1σ errors) being as  and  days, respectively. Our analysis of the undulation timescales suggests that intrinsic temporal variations of the central engine can explain half of the undulating events, while CSM interaction (CSI) can account for the majority of the sample. Finally, all of the well-observed He-rich SLSNe Ib either have strongly undulating LCs or the LCs are much better fit by the CSM+Ni model. These observations imply that their progenitor stars have not had enough time to lose all of the He-envelopes before supernova explosions, and H-poor CSM are likely to present in these events.

  Read more about The Hydrogen-poor Superluminous Supernovae from the Zwicky Transient Facility Phase I Survey. II. Light-curve Modeling and Characterization of Undulations

• Four (Super)luminous Supernovae from the First Months of the ZTF Survey

  2020. Ragnhild Lunnan (et al.). Astrophysical Journal 901 (1)

  Article

  We present photometry and spectroscopy of four hydrogen-poor luminous supernovae discovered during the 2-month long science commissioning and early operations of the Zwicky Transient Facility (ZTF) survey. Three of these objects, SN 2018bym (ZTF18aapgrxo), SN 2018avk (ZTF18aaisyyp), and SN 2018bgv (ZTF18aavrmcg), resemble typical SLSN-I spectroscopically, while SN 2018don (ZTF18aajqcue) may be an object similar to SN 2007bi experiencing considerable host galaxy reddening, or an intrinsically long-lived, luminous, and red SN Ic. We analyze the light curves, spectra, and host galaxy properties of these four objects and put them in context of the population of SLSN-I. SN 2018bgv stands out as the fastest-rising SLSN-I observed to date, with a rest-frame g-band rise time of just 10 days from explosion to peak—if it is powered by magnetar spin-down, the implied ejecta mass is only 1 M _⊙. SN 2018don also displays unusual properties—in addition to its red colors and comparatively massive host galaxy, the light curve undergoes some of the strongest light-curve undulations postpeak seen in an SLSN-I, which we speculate may be due to interaction with circumstellar material. We discuss the promises and challenges of finding SLSNe in large-scale surveys like ZTF given the observed diversity in the population.

  Read more about Four (Super)luminous Supernovae from the First Months of the ZTF Survey

• Helium-rich Superluminous Supernovae from the Zwicky Transient Facility

  2020. Lin Yan (et al.). Astrophysical Journal Letters 902 (1)

  Article

  Helium is expected to be present in the massive ejecta of some hydrogen-poor superluminous supernovae (SLSN-I). However, until now only one event has been identified with He features in its photospheric spectra (PTF10hgi). We present the discovery of a new He-rich SLSN-I, ZTF19aawfbtg (SN2019hge), atz = 0.0866. This event has more than 10 optical spectra at phases from -41 to +103 days relative to the peak, most of which match well with that of PTF10hgi. Confirmation comes from a near-IR spectrum taken at +34 days, revealing Heifeatures with P-Cygni profiles at 1.083 and 2.058 mu m. Using the optical spectra of PTF10hgi and SN2019hge as templates, we examined 70 other SLSNe-I discovered by Zwicky Transient Facility in the first two years of operation and found five additional SLSNe-I with distinct He-features. The excitation of Heiatoms in normal core-collapse supernovae requires nonthermal radiation, as proposed by previous studies. These He-rich events cannot be explained by the traditional(56)Ni mixing model because of their blue spectra, high peak luminosities, and long rise timescales. Magnetar models offer a possible solution since pulsar winds naturally generate high-energy particles, potential sources of nonthermal excitation. An alternative model is the interaction between the ejecta and dense H-poor circumstellar material, which may be supported by observed undulations in the light curves. These six SLSNe-Ib have relatively low-peak luminosities (rest frameM(g) = -20.06 0.16).

  Read more about Helium-rich Superluminous Supernovae from the Zwicky Transient Facility

• A UV resonance line echo from a shell around a hydrogen-poor superluminous supernova

  2018. Ragnhild Lunnan (et al.). Nature Astronomy 2 (11), 887-895

  Article

  Hydrogen-poor superluminous supernovae (SLSN-I) are a class of rare and energetic explosions that have been discovered in untargeted transient surveys in the past decade(1,2). The progenitor stars and the physical mechanism behind their large radiated energies (about 1O(51) erg or 1O(44) J) are both debated, with one class of models primarily requiring a large rotational energy(3,4) and the other requiring very massive progenitors that either convert kinetic energy into radiation through interaction with circumstellar material (CSM)(5-8 )or engender an explosion caused by pair-instability (loss of photon pressure due to particle-antiparticle production)(9,10). Observing the structure of the CSM around SLSN-I offers a powerful test of some scenarios, although direct observations are scarce(11,)(12). Here, we present a series of spectroscopic observations of the SLSN-I iPTF16eh, which reveal both absorption and time- and frequency-variable emission in the Mg n resonance doublet. We show that these observations are naturally explained as a resonance scattering light echo from a circumstellar shell. Modelling the evolution of the emission, we infer a shell radius of 0.1 pc and velocity of 3,300 km s(-1), implying that the shell was ejected three decades before the supernova explosion. These properties match theoretical predictions of shell ejections occurring because of pulsational pair-instability and imply that the progenitor had a helium core mass of about 50-55 M-circle dot, corresponding to an initial mass of about 115 M-circle dot.

  Read more about A UV resonance line echo from a shell around a hydrogen-poor superluminous supernova

• Hydrogen-poor Superluminous Supernovae from the Pan-STARRS1 Medium Deep Survey

  2018. Ragnhild Lunnan (et al.). Astrophysical Journal 852 (2)

  Article

  We present light curves and classification spectra of 17 hydrogen-poor superluminous supernovae (SLSNe) from the Pan-STARRS1 Medium Deep Survey (PS1 MDS). Our sample contains all objects from the PS1. MDS sample with spectroscopic classification that are similar to either of the prototypes SN 2005ap or SN 2007bi, without an explicit limit on luminosity. With a redshift range 0.3 < z < 1.6, PS1. MDS is the first SLSN sample primarily probing the high-redshift population; our multifilter PS1 light curves probe the rest-frame UV emission, and hence the peak of the spectral energy distribution. We measure the temperature evolution and construct bolometric light curves, and find peak luminosities of (0.5-5) x 10(44) erg s(-1) and lower limits on the total radiated energies of (0.3-2) x 10(51) erg. The light curve shapes are diverse, with both rise and decline times spanning a factor of similar to 5 and several examples of double-peaked light curves. When correcting for the flux-limited nature of our survey, we find a median peak luminosity at 4000 angstrom of M-4000 = -21.1 mag and a spread of sigma = 0.7 mag.

  Read more about Hydrogen-poor Superluminous Supernovae from the Pan-STARRS1 Medium Deep Survey

Show all publications by Ragnhild Lunnan at Stockholm University