| 1. |
- Agudo, I., et al.
(författare)
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Panning for gold, but finding helium: Discovery of the ultra-stripped supernova SN 2019wxt from gravitational-wave follow-up observations
- 2023
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 675
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Tidskriftsartikel (refereegranskat)abstract
- We present the results from multi-wavelength observations of a transient discovered during an intensive follow-up campaign of S191213g, a gravitational wave (GW) event reported by the LIGO-Virgo Collaboration as a possible binary neutron star merger in a low latency search. This search yielded SN 2019wxt, a young transient in a galaxy whose sky position (in the 80% GW contour) and distance (∼150 Mpc) were plausibly compatible with the localisation uncertainty of the GW event. Initially, the transienta's tightly constrained age, its relatively faint peak magnitude (Mi ∼ -16.7 mag), and the r-band decline rate of ∼1 mag per 5 days appeared suggestive of a compact binary merger. However, SN 2019wxt spectroscopically resembled a type Ib supernova, and analysis of the optical-near-infrared evolution rapidly led to the conclusion that while it could not be associated with S191213g, it nevertheless represented an extreme outcome of stellar evolution. By modelling the light curve, we estimated an ejecta mass of only ∼0.1 M·, with 56Ni comprising ∼20% of this. We were broadly able to reproduce its spectral evolution with a composition dominated by helium and oxygen, with trace amounts of calcium. We considered various progenitor channels that could give rise to the observed properties of SN 2019wxt and concluded that an ultra-stripped origin in a binary system is the most likely explanation. Disentangling genuine electromagnetic counterparts to GW events from transients such as SN 2019wxt soon after discovery is challenging: in a bid to characterise this level of contamination, we estimated the rate of events with a volumetric rate density comparable to that of SN 2019wxt and found that around one such event per week can occur within the typical GW localisation area of O4 alerts out to a luminosity distance of 500 Mpc, beyond which it would become fainter than the typical depth of current electromagnetic follow-up campaigns.
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| 2. |
- Ackley, K., et al.
(författare)
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Observational constraints on the optical and near-infrared emission from the neutron star-black hole binary merger candidate S190814bv
- 2020
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 643
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Tidskriftsartikel (refereegranskat)abstract
- Context. Gravitational wave (GW) astronomy has rapidly reached maturity, becoming a fundamental observing window for modern astrophysics. The coalescences of a few tens of black hole (BH) binaries have been detected, while the number of events possibly including a neutron star (NS) is still limited to a few. On 2019 August 14, the LIGO and Virgo interferometers detected a high-significance event labelled S190814bv. A preliminary analysis of the GW data suggests that the event was likely due to the merger of a compact binary system formed by a BH and a NS.Aims. In this paper, we present our extensive search campaign aimed at uncovering the potential optical and near infrared electromagnetic counterpart of S190814bv. We found no convincing electromagnetic counterpart in our data. We therefore use our non-detection to place limits on the properties of the putative outflows that could have been produced by the binary during and after the merger.Methods. Thanks to the three-detector observation of S190814bv, and given the characteristics of the signal, the LIGO and Virgo Collaborations delivered a relatively narrow localisation in low latency - a 50% (90%) credible area of 5 deg(2) (23 deg(2)) - despite the relatively large distance of 26752 Mpc. ElectromagNetic counterparts of GRAvitational wave sources at the VEry Large Telescope collaboration members carried out an intensive multi-epoch, multi-instrument observational campaign to identify the possible optical and near infrared counterpart of the event. In addition, the ATLAS, GOTO, GRAWITA-VST, Pan-STARRS, and VINROUGE projects also carried out a search on this event. In this paper, we describe the combined observational campaign of these groups.Results. Our observations allow us to place limits on the presence of any counterpart and discuss the implications for the kilonova (KN), which was possibly generated by this NS-BH merger, and for the strategy of future searches. The typical depth of our wide-field observations, which cover most of the projected sky localisation probability (up to 99.8%, depending on the night and filter considered), is r similar to 22 (resp. K similar to 21) in the optical (resp. near infrared). We reach deeper limits in a subset of our galaxy-targeted observations, which cover a total similar to 50% of the galaxy-mass-weighted localisation probability. Altogether, our observations allow us to exclude a KN with large ejecta mass M greater than or similar to 0.1 M-circle dot to a high (> 90%) confidence, and we can exclude much smaller masses in a sub-sample of our observations. This disfavours the tidal disruption of the neutron star during the merger.Conclusions. Despite the sensitive instruments involved in the campaign, given the distance of S190814bv, we could not reach sufficiently deep limits to constrain a KN comparable in luminosity to AT 2017gfo on a large fraction of the localisation probability. This suggests that future (likely common) events at a few hundred megaparsecs will be detected only by large facilities with both a high sensitivity and large field of view. Galaxy-targeted observations can reach the needed depth over a relevant portion of the localisation probability with a smaller investment of resources, but the number of galaxies to be targeted in order to get a fairly complete coverage is large, even in the case of a localisation as good as that of this event.
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| 3. |
- Charalampopoulos, P., et al.
(författare)
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A detailed spectroscopic study of tidal disruption events
- 2022
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 659
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Tidskriftsartikel (refereegranskat)abstract
- Spectroscopically, tidal disruption events (TDEs) are characterized by broad (similar to 10(4) km s(-1)) emission lines and show a large diversity as well as different line profiles. After carefully and consistently performing a series of data reduction tasks including host galaxy light subtraction, we present here the first detailed, spectroscopic population study of 16 optical and UV TDEs. We study a number of emission lines prominent among TDEs including Hydrogen, Helium, and Bowen lines and we quantify their evolution with time in terms of line luminosities, velocity widths, and velocity offsets. We report a time lag between the peaks of the optical light curves and the peak luminosity of H alpha spanning between similar to 7 and 45 days. If interpreted as light echoes, these lags correspond to distances of similar to 2 - 12 x 10(16) cm, which are one to two orders of magnitudes larger than the estimated blackbody radii (R-BB) of the same TDEs and we discuss the possible origin of this surprisingly large discrepancy. We also report time lags for the peak luminosity of the He I 5876 angstrom line, which are smaller than the ones of H alpha for H TDEs and similar or larger for N III Bowen TDEs. We report that N III Bowen TDEs have lower H alpha velocity widths compared to the rest of the TDEs in our sample and we also find that a strong X-ray to optical ratio might imply weakening of the line widths. Furthermore, we study the evolution of line luminosities and ratios with respect to their radii (R-BB) and temperatures (T-BB). We find a linear relationship between H alpha luminosity and the R-BB (L-line proportional to R-BB) and potentially an inverse power-law relation with T-BB (L-line proportional to T-BB(-beta)), leading to weaker H alpha emission for T-BB >= 25 000 K. The He II/He I ratio becomes large at the same temperatures, possibly pointing to an ionization effect. The He II/H alpha ratio becomes larger as the photospheric radius recedes, implying a stratified photosphere where Helium lies deeper than Hydrogen. We suggest that the large diversity of the spectroscopic features seen in TDEs along with their X-ray properties can potentially be attributed to viewing angle effects.
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| 4. |
- Chen, T.-W., et al.
(författare)
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SN 2017ens : The Metamorphosis of a Luminous Broadlined Type Ic Supernova into an SN IIn
- 2018
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 867:2
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Tidskriftsartikel (refereegranskat)abstract
- We present observations of supernova (SN) 2017ens, discovered by the ATLAS survey and identified as a hot blue object through the GREAT program. The redshift z = 0.1086 implies a peak brightness of M-g = -21.1 mag, placing the object within the regime of superluminous supernovae. We observe a dramatic spectral evolution, from initially being blue and featureless, to later developing features similar to those of the broadlined Type Ic SN 1998bw, and finally showing 2000 km s(-1) wide H alpha and H beta emission. Relatively narrow Balmer emission (reminiscent of a SN IIn) is present at all times. We also detect coronal lines, indicative of a dense circumstellar medium. We constrain the progenitor wind velocity to similar to 50-60 km s(-1) based on P-Cygni profiles, which is far slower than those present in Wolf-Rayet stars. This may suggest that the progenitor passed through a luminous blue variable phase, or that the wind is instead from a binary companion red supergiant star. At late times we see the similar to 2000 km s(-1) wide H alpha emission persisting at high luminosity (similar to 3 x 10(40) erg s(-1)) for at least 100 day, perhaps indicative of additional mass loss at high velocities that could have been ejected by a pulsational pair instability.
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| 5. |
- De Cia, Annalisa, et al.
(författare)
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Light Curves of Hydrogen-poor Superluminous Supernovae from the Palomar Transient Factory
- 2018
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 860:2
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the light-curve properties of a sample of 26 spectroscopically confirmed hydrogen- poor superluminous supernovae (SLSNe-I) in the Palomar Transient Factory survey. These events are brighter than SNe Ib/c and SNe Ic-BL, on average, by about 4 and 2. mag, respectively. The peak absolute magnitudes of SLSNe-I in rest-frame g band span -22 less than or similar to M-g less than or similar to -20 mag, and these peaks are not powered by radioactive Ni-56, unless strong asymmetries are at play. The rise timescales are longer for SLSNe than for normal SNe Ib/c, by roughly 10 days, for events with similar decay times. Thus, SLSNe-I can be considered as a separate population based on photometric properties. After peak, SLSNe-I decay with a wide range of slopes, with no obvious gap between rapidly declining and slowly declining events. The latter events show more irregularities (bumps) in the light curves at all times. At late times, the SLSN-I light curves slow down and cluster around the 56Co radioactive decay rate. Powering the late-time light curves with radioactive decay would require between 1 and 10M(circle dot) of Ni masses. Alternatively, a simple magnetar model can reasonably fit the majority of SLSNe-I light curves, with four exceptions, and can mimic the radioactive decay of 56Co, up to similar to 400 days from explosion. The resulting spin values do not correlate with the host-galaxy metallicities. Finally, the analysis of our sample cannot strengthen the case for using SLSNe-I for cosmology.
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| 6. |
- Fiore, A., et al.
(författare)
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SN 2017gci : a nearby Type I Superluminous Supernova with a bumpy tail
- 2021
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 502:2, s. 2120-2139
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Tidskriftsartikel (refereegranskat)abstract
- We present and discuss the optical spectrophotometric observations of the nearby (z = 0.087) Type I superluminous supernova (SLSN I) SN 2017gci, whose peak K-corrected absolute magnitude reaches M-g = -21.5 mag. Its photometric and spectroscopic evolution includes features of both slow- and of fast-evolving SLSN I, thus favoring a continuum distribution between the two SLSN-I subclasses. In particular, similarly to other SLSNe I, the multiband light curves (LCs) of SN 2017gci show two re-brightenings at about 103 and 142 d after the maximum light. Interestingly, this broadly agrees with a broad emission feature emerging around 6520 angstrom after similar to 51 d from the maximum light, which is followed by a sharp knee in the LC. If we interpret this feature as H alpha, this could support the fact that the bumps are the signature of late interactions of the ejecta with a (hydrogen-rich) circumstellar material. Then we fitted magnetar- and CSM-interaction-powered synthetic LCs on to the bolometric one of SN 2017gci. In the magnetar case, the fit suggests a polar magnetic field B-p similar or equal to 6 x 10(14) G, an initial period of the magnetar P-initial similar or equal to 2.8 ms, an ejecta mass M-ejecta similar or equal to 9M(circle dot) and an ejecta opacity kappa similar or equal to 0.08 cm(2) g(-1). A CSM-interaction scenario would imply a CSM mass similar or equal to 5 M-circle dot and an ejecta mass similar or equal to 12M(circle dot). Finally, the nebular spectrum of phase + 187 d was modeled, deriving a mass of similar or equal to 10 M-circle dot for the ejecta. Our models suggest that either a magnetar or CSM interaction might be the power sources for SN 2017gci and that its progenitor was a massive (40 M-circle dot) star.
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| 7. |
- Fraser, M., et al.
(författare)
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SN 2009md : another faint supernova from a low-mass progenitor
- 2011
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 417, s. 1417-1433
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Tidskriftsartikel (refereegranskat)abstract
- We present adaptive optics imaging of the core-collapse supernova (SN) 2009md, which we use together with archival Hubble Space Telescope data to identify a coincident progenitor candidate. We find the progenitor to have an absolute magnitude of V=-4.63+0.3-0.4 mag and a colour of V-I= 2.29+0.25-0.39 mag, corresponding to a progenitor luminosity of log L/L&sun;˜ 4.54 ± 0.19 dex. Using the stellar evolution code STARS, we find this to be consistent with a red supergiant progenitor with M= 8.5+6.5-1.5 M&sun;. The photometric and spectroscopic evolution of SN 2009md is similar to that of the class of sub-luminous Type IIP SNe; in this paper we compare the evolution of SN 2009md primarily to that of the sub-luminous SN 2005cs. We estimate the mass of 56Ni ejected in the explosion to be (5.4 ± 1.3) × 10-3 M&sun; from the luminosity on the radioactive tail, which is in agreement with the low 56Ni masses estimated for other sub-luminous Type IIP SNe. From the light curve and spectra, we show the SN explosion had a lower energy and ejecta mass than the normal Type IIP SN 1999em. We discuss problems with stellar evolutionary models, and the discrepancy between low observed progenitor luminosities (log L/L&sun;˜4.3-5 dex) and model luminosities after the second dredge-up for stars in this mass range, and consider an enhanced carbon burning rate as a possible solution. In conclusion, SN 2009md is a faint SN arising from the collapse of a progenitor close to the lower mass limit for core collapse. This is now the third discovery of a low-mass progenitor star producing a low-energy explosion and low 56Ni ejected mass, which indicates that such events arise from the lowest end of the mass range that produces a core-collapse SN (7-8 M&sun;).
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| 8. |
- Gal-Yam, A., et al.
(författare)
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A WC/WO star exploding within an expanding carbon-oxygen-neon nebula
- 2022
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 601:7892, s. 201-204
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Tidskriftsartikel (refereegranskat)abstract
- The final fate of massive stars, and the nature of the compact remnants they leave behind (black holes and neutron stars), are open questions in astrophysics. Many massive stars are stripped of their outer hydrogen envelopes as they evolve. Such Wolf-Rayet stars(1) emit strong and rapidly expanding winds with speeds greater than 1,000 kilometres per second. A fraction of this population is also helium-depleted, with spectra dominated by highly ionized emission lines of carbon and oxygen (types WC/WO). Evidence indicates that the most commonly observed supernova explosions that lack hydrogen and helium (types Ib/Ic) cannot result from massive WC/WO stars(2,3), leading some to suggest that most such stars collapse directly into black holes without a visible supernova explosion(4). Here we report observations of SN 2019hgp, beginning about a day after the explosion. Its short rise time and rapid decline place it among an emerging population of rapidly evolving transients(5-8). Spectroscopy reveals a rich set of emission lines indicating that the explosion occurred within a nebula composed of carbon, oxygen and neon. Narrow absorption features show that this material is expanding at high velocities (greater than 1,500 kilometres per second), requiring a compact progenitor. Our observations are consistent with an explosion of a massive WC/WO star, and suggest that massive Wolf-Rayet stars may be the progenitors of some rapidly evolving transients.
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| 9. |
- Gutiérrez, C. P., et al.
(författare)
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Type II supernovae in low-luminosity host galaxies
- 2018
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 479:3, s. 3232-3253
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Tidskriftsartikel (refereegranskat)abstract
- We present an analysis of a new sample of type II core-collapse supernovae (SNe II) occurring within low-luminosity galaxies, comparing these with a sample of events in brighter hosts. Our analysis is performed comparing SN II spectral and photometric parameters and estimating the influence of metallicity (inferred from host luminosity differences) on SN II transient properties. We measure the SN absolute magnitude at maximum, the light-curve plateau duration, the optically thick duration, and the plateau decline rate in the V band, together with expansion velocities and pseudo-equivalent-widths (pEWs) of several absorption lines in the SN spectra. For the SN host galaxies, we estimate the absolute magnitude and the stellar mass, a proxy for the metallicity of the host galaxy. SNe II exploding in low-luminosity galaxies display weaker pEWs of Fe II lambda 5018, confirming the theoretical prediction that metal lines in SN II spectra should correlate with metallicity. We also find that SNe II in low-luminosity hosts have generally slower declining light curves and display weaker absorption lines. We find no relationship between the plateau duration or the expansion velocities with SN environment, suggesting that the hydrogen envelope mass and the explosion energy are not correlated with the metallicity of the host galaxy. This result supports recent predictions that mass-loss for red supergiants is independent of metallicity.
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| 10. |
- Kromer, Markus, et al.
(författare)
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The peculiar Type Ia supernova iPTF14atg : Chandrasekhar-mass explosion or violent merger?
- 2016
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 459:4, s. 4428-4439
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Tidskriftsartikel (refereegranskat)abstract
- iPTF14atg, a subluminous peculiar Type Ia supernova (SN Ia) similar to SN 2002es, is the first SN Ia for which a strong UV flash was observed in the early-time light curves. This has been interpreted as evidence for a single-degenerate (SD) progenitor system, where such a signal is expected from interactions between the SN ejecta and the non-degenerate companion star. Here, we compare synthetic observables of multidimensional state-of-the-art explosion models for different progenitor scenarios to the light curves and spectra of iPTF14atg. From our models, we have difficulties explaining the spectral evolution of iPTF14atg within the SD progenitor channel. In contrast, we find that a violent merger of two carbon-oxygen white dwarfs with 0.9 and 0.76 M-aS (TM), respectively, provides an excellent match to the spectral evolution of iPTF14atg from 10 d before to several weeks after maximum light. Our merger model does not naturally explain the initial UV flash of iPTF14atg. We discuss several possibilities like interactions of the SN ejecta with the circumstellar medium and surface radioactivity from an He-ignited merger that may be able to account for the early UV emission in violent merger models.
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