arXiv subjects

T. Kangas

Publications and source records attributed to T. Kangas.

At least 37 records · Page 2Linked to original sources

arXiv2024

Time-varying double-peaked emission lines following the sudden ignition of the dormant galactic nucleus AT2017bcc

We present a pan-chromatic study of AT2017bcc, a nuclear transient that was discovered in 2017 within the skymap of a reported burst-like gravitational wave candidate, G274296. It was initially classified as a superluminous supernova, and then reclassified as a candidate tidal disruption event. Its optical light curve has since shown ongoing variability with a structure function consistent with that of an active galactic nucleus, however earlier data shows no variability for at least 10 years prior to the outburst in 2017. The spectrum shows complex profiles in the broad Balmer lines: a central component with a broad blue wing, and a boxy component with time-variable blue and red shoulders. The H$α$ emission profile is well modelled using a circular accretion disc component, and a blue-shifted double Gaussian which may indicate a partially obscured outflow. Weak narrow lines, together with the previously flat light curve, suggest that this object represents a dormant galactic nucleus which has recently been re-activated. Our time-series modelling of the Balmer lines suggests that this is connected to a disturbance in the disc morphology, and we speculate this could involve a sudden violent event such as a tidal disruption event involving the central supermassive black hole, though this cannot be confirmed, and given an estimated black hole mass of $\gtrsim10^7-10^8$ M$_\odot$ instabilities in an existing disc may be more likely. Although we find that the redshifts of AT2017bcc ($z=0.13$) and G274296 ($z>0.42$) are inconsistent, this event adds to the growing diversity of both nuclear transients and multi-messenger contaminants.

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arXiv2024

Deep JWST/NIRCam imaging of Supernova 1987A

JWST/NIRCam obtained high angular-resolution (0.05-0.1''), deep near-infrared 1--5 micron imaging of Supernova (SN) 1987A taken 35 years after the explosion. In the NIRCam images, we identify: 1) faint H2 crescents, which are emissions located between the ejecta and the equatorial ring, 2) a bar, which is a substructure of the ejecta, and 3) the bright 3-5 micron continuum emission exterior to the equatorial ring. The emission of the remnant in the NIRCam 1-2.3 micron images is mostly due to line emission, which is mostly emitted in the ejecta and in the hot spots within the equatorial ring. In contrast, the NIRCam 3-5 micron images are dominated by continuum emission. In the ejecta, the continuum is due to dust, obscuring the centre of the ejecta. In contrast, in the ring and exterior to the ring, synchrotron emission contributes a substantial fraction to the continuum. Dust emission contributes to the continuum at outer spots and diffuse emission exterior to the ring, but little within the ring. This shows that dust cooling and destruction time scales are shorter than the synchrotron cooling time scale, and the time scale of hydrogen recombination in the ring is even longer than the synchrotron cooling time scale. With the advent of high sensitivity and high angular resolution images provided by JWST/NIRCam, our observations of SN 1987A demonstrate that NIRCam opens up a window to study particle-acceleration and shock physics in unprecedented details, probed by near-infrared synchrotron emission, building a precise picture of how a SN evolves.

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arXiv2024

SN 2020zbf: A fast-rising hydrogen-poor superluminous supernova with strong carbon lines

SN\,2020zbf is a hydrogen-poor superluminous supernova (SLSN) at $z = 0.1947$ that shows conspicuous \ion{C}{II} features at early times, in contrast to the majority of H-poor SLSNe. Its peak magnitude is $M_{\rm g}$ = $-21.2$~mag and its rise time ($\lesssim 26.4$ days from first light) places SN\,2020zbf among the fastest rising type I SLSNe. We used spectra taken from ultraviolet (UV) to near-infrared wavelengths to identify spectral features. We paid particular attention to the \ion{C}{II} lines as they present distinctive characteristics when compared to other events. We also analyzed UV and optical photometric data and modeled the light curves considering three different powering mechanisms: radioactive decay of $^{56}$Ni, magnetar spin-down, and circumstellar medium (CSM) interaction. The spectra of SN\,2020zbf match the model spectra of a C-rich low-mass magnetar-powered supernova model well. This is consistent with our light curve modeling, which supports a magnetar-powered event with an ejecta mass $M_{\rm ej}$ = 1.5~$\rm M_\odot$. However, we cannot discard the CSM-interaction model as it may also reproduce the observed features. The interaction with H-poor, carbon-oxygen CSM near peak light could explain the presence of \ion{C}{II} emission lines. A short plateau in the light curve around 35 -- 45 days after peak, in combination with the presence of an emission line at 6580~Å,\ can also be interpreted as being due to a late interaction with an extended H-rich CSM. Both the magnetar and CSM-interaction models of SN\,2020zbf indicate that the progenitor mass at the time of explosion is between 2 and 5~$\rm M_\odot$. Modeling the spectral energy distribution of the host galaxy reveals a host mass of 10$^{8.7}$~$\rm M_\odot$, a star formation rate of 0.24$^{+0.41}_{-0.12}$~$\rm M_\odot$~yr$^{-1}$, and a metallicity of $\sim$ 0.4~$\rm Z_\odot$.

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arXiv2023

The broad-lined Type-Ic supernova SN 2022xxf with extraordinary two-humped light curves

We report on our study of supernova (SN) 2022xxf based on observations obtained during the first four months of its evolution. The light curves (LCs) display two humps of similar maximum brightness separated by 75 days, unprecedented for a broad-lined (BL) Type Ic supernova (SN IcBL). SN 2022xxf is the most nearby SN IcBL to date (in NGC 3705, $z = 0.0037$, at a distance of about 20 Mpc). Optical and near-infrared photometry and spectroscopy are used to identify the energy source powering the LC. Nearly 50 epochs of high signal-to-noise-ratio spectroscopy were obtained within 130 days, comprising an unparalleled dataset for a SN IcBL, and one of the best-sampled SN datasets to date. The global spectral appearance and evolution of SN 2022xxf points to typical SN Ic/IcBL, with broad features (up to $\sim14000$ km s$^{-1}$) and a gradual transition from the photospheric to the nebular phase. However, narrow emission lines (corresponding to $\sim1000-2500$ km s$^{-1}$) are present in the spectra from the time of the second rise, suggesting slower-moving circumstellar material (CSM). These lines are subtle, in comparison to the typical strong narrow lines of CSM-interacting SNe, for example, Type IIn, Ibn, and Icn, but some are readily noticeable at late times such as in Mg I $λ$5170 and [O I] $λ$5577. Unusually, the near-infrared spectra show narrow line peaks in a number of features formed by ions of O and Mg. We infer the presence of CSM that is free of H and He. We propose that the radiative energy from the ejecta-CSM interaction is a plausible explanation for the second LC hump. This interaction scenario is supported by the color evolution, which progresses to the blue as the light curve evolves along the second hump, and the slow second rise and subsequent rapid LC drop. (Abstract abridged)

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arXiv2023

Near-infrared evolution of the equatorial ring of SN 1987A

We use adaptive-optics imaging and integral field spectroscopy from the Very Large Telescope, together with images from the \emph{Hubble Space Telescope}, to study the near-infrared (NIR) evolution of the equatorial ring (ER) of SN~1987A. We study the NIR line and continuum flux and morphology over time in order to lay the groundwork for \emph{James Webb Space Telescope} observations of the system. We also study the differences in the interacting ring structure and flux between optical, NIR and other wavelengths, and between line and continuum emission, to constrain the underlying physical processes. Mostly the evolution is similar in the NIR and optical. The morphology of the ER has been skewed toward the west side (with roughly 2/3 of the NIR emission originating there) since around 2010. A steady decline in the ER flux, broadly similar to the MIR and the optical, is ongoing since roughly this time as well. The expansion velocity of the ER hotspots in the NIR is fully consistent with the optical. However, continuum emission forms roughly 70 per cent of the NIR luminosity, and is relatively stronger outside the hotspot-defined extent of the ER than the optical emission or NIR line emission since 2012--2013, suggesting a faster-expanding continuum component. We find that this outer NIR emission can have a significant synchrotron contribution. Even if emission from hot ($\sim$2000~K) dust is dominant within the ER, the mass of this dust must be vanishingly small (a few $\times10^{-12}$~M$_\odot$) compared to the total dust mass in the ER ($\gtrsim10^{-5}$~M$_\odot$) to account for the observed $HKs$ flux. The NIR continuum emission, however, expands slower than the more diffuse 180-K dust emission that dominates in the MIR, indicating a different source, and the same hot dust component cannot account for the $J$-band emission.

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arXiv2022

SN2020qlb: A hydrogen-poor superluminous supernova with well-characterized light curve undulations

SN\,2020qlb (ZTF20abobpcb) is a hydrogen-poor superluminous supernova (SLSN-I) that is among the most luminous (maximum M$_{g} = -22.25$ mag) and that has one of the longest rise times (77 days from explosion to maximum). We estimate the total radiated energy to be $>2.1\times10^{51}$ 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. We discuss the potential power source of this immense explosion as well as the mechanisms behind its observed light curve undulations. 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]$λ$4363, auroral line, allowing for a direct metallicity measurement. We rule out the Arnett $^{56}$Ni 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 $32\pm6$ 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.

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arXiv2022

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

During the Zwicky Transient Facility (ZTF) Phase-I operation, 78 hydrogen-poor superluminous supernovae (SLSNe-I) were discovered in less than three years, making up the largest sample from a single survey. This paper (Paper I) presents the data, including the optical/ultraviolet 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 the ZTF in the $g,r,i$ 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) $z_{\rm med} = 0.265^{+0.143}_{-0.135}$. The peak luminosity covers $-22.8\,{\rm mag} \leq M_{g,\rm peak} \leq -19.8$\,mag, with a median value of $-21.48^{+1.13}_{-0.61}$\,mag. Their light curves evolve slowly with the mean rest-frame rise time of $t_{\rm rise} = 41.9\pm17.8$\,days. The luminosity and time scale distributions suggest that low luminosity SLSNe-I with peak luminosity $\sim -20$\,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 ultraviolet photometry, we construct bolometric luminosity and derive a bolometric correction relation generally applicable for converting $g,r$-band photometry to bolometric luminosity for SLSNe-I.

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arXiv2022

The Hydrogen-Poor Superluminous Supernovae from the Zwicky Transient Facility Phase-I Survey: II. Light Curve Modeling and Characterization of Undulations

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 $^{56}$Ni decay models. This implies that LCs alone can not unambiguously constrain the physical power sources for a 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 favor 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 1$σ$ errors) being as $1.7\%^{+1.5\%}_{-0.7\%}\,\rm E_{\rm rad,total}$ and $28.8^{+14.4}_{-9.1}$\,days, respectively. Our analysis of the undulation time scales suggests that intrinsic temporal variations of the central engine can explain half of the undulating events, while CSM interaction can account for the majority of the sample. Finally, all of the well-observed He-rich SLSNe-Ib have either 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.

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arXiv2021

Core-collapse supernova subtypes in luminous infrared galaxies

We present the discovery and follow-up observations of two CCSNe that occurred in the luminous infrared galaxy (LIRG), NGC3256. The first, SN2018ec, was discovered using the ESO HAWK-I/GRAAL adaptive optics seeing enhancer, and was classified as a Type Ic with a host galaxy extinction of $A_V=2.1^{+0.3}_{-0.1}$ mag. The second, AT2018cux, was discovered during the course of follow-up observations of SN2018ec, and is consistent with a sub-luminous Type IIP classification with an $A_V=2.1 \pm 0.4$ mag of host extinction. A third CCSN, PSNJ10275082-4354034 in NGC3256, has previously been reported in 2014, and we recovered the source in late time archival HST imaging. Based on template light-curve fitting, we favour a Type IIn classification for it with modest host galaxy extinction of $A_V=0.3^{+0.4}_{-0.3}$ mag. We also extend our study with follow-up data of the recent Type IIb SN2019lqo and Type Ib SN2020fkb that occurred in the LIRG system Arp299 with host extinctions of $A_V=2.1^{+0.1}_{-0.3}$ and $A_V=0.4^{+0.1}_{-0.2}$ mag, respectively. Motivated by the above, we inspected, for the first time, a sample of 29 CCSNe located within a projected distance of 2.5 kpc from the host galaxy nuclei in a sample of 16 LIRGs. We find that, if star formation within these galaxies is modelled assuming a global starburst episode and normal IMF, there is evidence of a correlation between the starburst age and the CCSN subtype. We infer that the two subgroups of 14 H-poor (Type IIb/Ib/Ic/Ibn) and 15 H-rich (Type II/IIn) CCSNe have different underlying progenitor age distributions, with the H-poor progenitors being younger at 3$σ$ significance. However, we do note that the available sample sizes of CCSNe and host LIRGs are so far small, and the statistical comparisons between subgroups do not take into account possible systematic or model errors related to the estimated starburst ages. (abridged)

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arXiv2020

SN 2016gsd: An unusually luminous and linear type II supernova with high velocities

We present observations of the unusually luminous Type II supernova (SN) 2016gsd. With a peak absolute magnitude of V = $-$19.95 $\pm$ 0.08, this object is one of the brightest Type II SNe, and lies in the gap of magnitudes between the majority of Type II SNe and the superluminous SNe. Its light curve shows little evidence of the expected drop from the optically thick phase to the radioactively powered tail. The velocities derived from the absorption in H$α$ are also unusually high with the blue edge tracing the fastest moving gas initially at 20000 km s$^{-1}$, and then declining approximately linearly to 15000 km s$^{-1}$ over $\sim$100 d. The dwarf host galaxy of the SN indicates a low-metallicity progenitor which may also contribute to the weakness of the metal lines in its spectra. We examine SN 2016gsd with reference to similarly luminous, linear Type II SNe such as SNe 1979C and 1998S, and discuss the interpretation of its observational characteristics. We compare the observations with a model produced by the JEKYLL code and find that a massive star with a depleted and inflated hydrogen envelope struggles to reproduce the high luminosity and extreme linearity of SN 2016gsd. Instead, we suggest that the influence of interaction between the SN ejecta and circumstellar material can explain the majority of the observed properties of the SN. The high velocities and strong H$α$ absorption present throughout the evolution of the SN may imply a circumstellar medium configured in an asymmetric geometry.

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arXiv2020

The long-lived Type IIn SN 2015da: Infrared echoes and strong interaction within an extended massive shell

In this paper we report the results of the first $\sim$four years of spectroscopic and photometric monitoring of the Type IIn supernova SN 2015da (also known as PSN J13522411+3941286, or iPTF16tu). The supernova exploded in the nearby spiral galaxy NGC 5337 in a relatively highly extinguished environment. The transient showed prominent narrow Balmer lines in emission at all times and a slow rise to maximum in all bands. In addition, early observations performed by amateur astronomers give a very well-constrained explosion epoch. The observables are consistent with continuous interaction between the supernova ejecta and a dense and extended H-rich circumstellar medium. The presence of such an extended and dense medium is difficult to reconcile with standard stellar evolution models, since the metallicity at the position of SN 2015da seems to be slightly subsolar. Interaction is likely the mechanism powering the light curve, as confirmed by the analysis of the pseudo bolometric light curve, which gives a total radiated energy $\gtrsim10^{51}\,\rm{erg}$. Modeling the light curve in the context of a supernova shock breakout through a dense circumstellar medium allowed us to infer the mass of the prexisting gas to be $\simeq8\,\rm{M_{\odot}}$, with an extreme mass-loss rate for the progenitor star $\simeq0.6\,\rm{M_{\odot}}\,\rm{yr^{-1}}$, suggesting that most of the circumstellar gas was produced during multiple eruptive events. Near- and mid-infrared observations reveal a flux excess in these domains, similar to those observed in SN 2010jl and other interacting transients, likely due to preexisting radiatively heated dust surrounding the supernova. By modeling the infrared excess, we infer a mass $\gtrsim0.4\times10^{-3}\,\rm{M_{\odot}}$ for the dust.

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arXiv2018

The optical afterglow of GW170817 at one year post-merger

We present observations of the optical afterglow of GRB\,170817A, made by the {\it Hubble Space Telescope}, between February and August 2018, up to one year after the neutron star merger, GW170817. The afterglow shows a rapid decline beyond $170$~days, and confirms the jet origin for the observed outflow, in contrast to more slowly declining expectations for `failed-jet' scenarios. We show here that the broadband (radio, optical, X-ray) afterglow is consistent with a structured outflow where an ultra-relativistic jet, with Lorentz factor $Γ\gtrsim100$, forms a narrow core ($\sim5^\circ$) and is surrounded by a wider angular component that extends to $\sim15^\circ$, which is itself relativistic ($Γ\gtrsim5$). For a two-component model of this structure, the late-time optical decline, where $F \propto t^{-α}$, is $α=2.20\pm0.18$, and for a Gaussian structure the decline is $α=2.45\pm0.23$. We find the Gaussian model to be consistent with both the early $\sim10$ days and late $\gtrsim290$ days data. The agreement of the optical light curve with the evolution of the broadband spectral energy distribution and its continued decline indicates that the optical flux is arising primarily from the afterglow and not any underlying host system. This provides the deepest limits on any host stellar cluster, with a luminosity $\lesssim 4000 L_\odot~(M_{\rm F606W}\gtrsim-4.3)$.

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arXiv2018

The optical afterglow of the short gamma-ray burst associated with GW170817

The binary neutron star merger GW170817 was the first multi-messenger event observed in both gravitational and electromagnetic waves. The electromagnetic signal began approximately 2 seconds post-merger with a weak, short burst of gamma-rays, which was followed over the next hours and days by the ultraviolet, optical and near-infrared emission from a radioactively- powered kilonova. Later, non-thermal rising X-ray and radio emission was observed. The low luminosity of the gamma-rays and the rising non-thermal flux from the source at late times could indicate that we are outside the opening angle of the beamed relativistic jet. Alternatively, the emission could be arising from a cocoon of material formed from the interaction between a jet and the merger ejecta. Here we present late-time optical detections and deep near-infrared limits on the emission from GW170817 at 110 days post-merger. Our new observations are at odds with expectations of late-time emission from kilonova models, being too bright and blue. Instead, the emission arises from the interaction between the relativistic ejecta of GW170817 and the interstellar medium. We show that this emission matches the expectations of a Gaussian structured relativistic jet, which would have launched a high luminosity short GRB to an aligned observer. However, other jet structure or cocoon models can also match current data - the future evolution of the afterglow will directly distinguish the origin of the emission.

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arXiv2018

AT 2017be - a new member of the class of Intermediate-Luminosity Red Transients

We report the results of our spectrophotometric monitoring campaign for AT~2017be in NGC~2537. Its lightcurve reveals a fast rise to an optical maximum, followed by a plateau lasting about 30 days, and finally a fast decline. Its absolute peak magnitude ($M_{r}$ $\simeq$ $-$12 $\rm{mag}$) is fainter than that of core-collapse supernovae, and is consistent with those of supernova impostors and other Intermediate-Luminosity Optical Transients. The quasi-bolometric lightcurve peaks at $\sim$ 2 $\times$ 10$^{40}$ erg s$^{-1}$, and the late-time photometry allows us to constrain an ejected $^{56}$Ni mass of $\sim$ 8 $\times$ 10$^{-4}$\msun. The spectra of AT~2017be show minor evolution over the observational period, a relatively blue continuum showing at early phases, which becomes redder with time. A prominent H$α$ emission line always dominates over other Balmer lines. Weak Fe {\sc ii} features, Ca~{\sc ii} H$\&$K and the Ca {\sc ii} NIR triplet are also visible, while P-Cygni absorption troughs are found in a high resolution spectrum. In addition, the [Ca~{\sc ii}] $λ$7291,7324 doublet is visible in all spectra. This feature is typical of Intermediate-Luminosity Red Transients (ILRTs), similar to SN~2008S. The relatively shallow archival Spitzer data are not particularly constraining. On the other hand, a non-detection in deeper near-infrared HST images disfavours a massive Luminous Blue Variable eruption as the origin for AT~2017be. As has been suggested for other ILRTs, we propose that AT~2017be is a candidate for a weak electron-capture supernova explosion of a super-asymptotic giant branch star, still embedded in a thick dusty envelope.

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arXiv2018

A dust-enshrouded tidal disruption event with a resolved radio jet in a galaxy merger

Tidal disruption events (TDEs) are transient flares produced when a star is ripped apart by the gravitational field of a supermassive black hole (SMBH). We have observed a transient source in the western nucleus of the merging galaxy pair Arp 299 that radiated >1.5x10^52 erg in the infrared and radio, but was not luminous at optical or X-ray wavelengths. We interpret this as a TDE with much of its emission re-radiated at infrared wavelengths by dust. Efficient reprocessing by dense gas and dust may explain the difference between theoretical predictions and observed luminosities of TDEs. The radio observations resolve an expanding and decelerating jet, probing the jet formation and evolution around a SMBH.

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arXiv2018

SNhunt151: an explosive event inside a dense cocoon

SNhunt151 was initially classified as a supernova (SN) impostor (nonterminal outburst of a massive star). It exhibited a slow increase in luminosity, lasting about 450 d, followed by a major brightening that reaches M_V ~ -18 mag. No source is detected to M_V > -13 mag in archival images at the position of SNhunt151 before the slow rise. Low-to-mid-resolution optical spectra obtained during the pronounced brightening show very little evolution, being dominated at all times by multicomponent Balmer emission lines, a signature of interaction between the material ejected in the new outburst and the pre-existing circumstellar medium. We also analyzed mid-infrared images from the Spitzer Space Telescope, detecting a source at the transient position in 2014 and 2015. Overall, SNhunt151 is spectroscopically a Type IIn SN, somewhat similar to SN2009ip. However, there are also some differences, such as a slow pre-discovery rise, a relatively broad light-curve peak showing a longer rise time (~ 50 d) and a slower decline, along with a negligible change in the temperature around the peak (T < 10^4 K). We suggest that SNhunt151 is the result of an outburst, or a SN explosion, within a dense circumstellar nebula, similar to those embedding some luminous blue variables like Eta Carinae and originating from past mass-loss events.

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arXiv2017

A population of highly energetic transient events in the centres of active galaxies

Recent all-sky surveys have led to the discovery of new types of transients. These include stars disrupted by the central supermassive black hole, and supernovae that are 10-100 times more energetic than typical ones. However, the nature of even more energetic transients that apparently occur in the innermost regions of their host galaxies is hotly debated. Here we report the discovery of the most energetic of these to date: PS1-10adi, with a total radiated energy of ~ 2.3 x 10^52 erg. The slow evolution of its light curve and persistently narrow spectral lines over ~3 yr are inconsistent with known types of recurring black hole variability. The observed properties imply powering by shock interaction between expanding material and large quantities of surrounding dense matter. Plausible sources of this expanding material are a star that has been tidally disrupted by the central black hole, or a supernova. Both could satisfy the energy budget. For the former, we would be forced to invoke a new and hitherto unseen variant of a tidally disrupted star, while a supernova origin relies principally on environmental effects resulting from its nuclear location. Remarkably, we also discovered that PS1-10adi is not an isolated case. We therefore surmise that this new population of transients has previously been overlooked due to incorrect association with underlying central black hole activity.

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arXiv2017

The environment of the binary neutron star merger GW170817

We present Hubble Space Telescope and Chandra imaging, combined with Very Large Telescope MUSE integral field spectroscopy of the counterpart and host galaxy of the first binary neutron star merger detected via gravitational wave emission by LIGO & Virgo, GW170817. The host galaxy, NGC 4993, is an S0 galaxy at z=0.009783. There is evidence for large, face-on spiral shells in continuum imaging, and edge-on spiral features visible in nebular emission lines. This suggests that NGC 4993 has undergone a relatively recent (<1 Gyr) ``dry'' merger. This merger may provide the fuel for a weak active nucleus seen in Chandra imaging. At the location of the counterpart, HST imaging implies there is no globular or young stellar cluster, with a limit of a few thousand solar masses for any young system. The population in the vicinity is predominantly old with <1% of any light arising from a population with ages <500 Myr. Both the host galaxy properties and those of the transient location are consistent with the distributions seen for short-duration gamma-ray bursts, although the source position lies well within the effective radius (r_e ~ 3 kpc), providing an r_e-normalized offset that is closer than ~90% of short GRBs. For the long delay time implied by the stellar population, this suggests that the kick velocity was significantly less than the galaxy escape velocity. We do not see any narrow host galaxy interstellar medium features within the counterpart spectrum, implying low extinction, and that the binary may lie in front of the bulk of the host galaxy.

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