arXiv subjects

A. Somero

Publications and source records attributed to A. Somero.

12 recordsLinked to original sources

arXiv2026

SN 2017ati: A luminous type IIb explosion from a massive progenitor

We present optical photometric and spectroscopic observations of the Type~IIb supernova (SN)~2017ati. It reached the maximum light at about 27~d after the explosion and the light curve shows a broad, luminous peak with an absolute $r$-band magnitude of $M_{r} = -18.48 \pm 0.16$~mag. At about 50~d after maximum light, SN~2017ati exhibits a decline rate close to that expected from the $^{56}$Co $\rightarrow$ $^{56}$Fe radioactive decay, at 0.98 mag per 100 days, as usually observed in SNe IIb. However, it remains systematically brighter at late times by about 1--2~mag, exceeding the usual upper luminosity range of this class. As a result, modelling the light curve of SN~2017ati with a standard $^{56}$Ni decay scenario requires a large nickel mass of up to $\sim0.37\,M_{\odot}$ and still fails to reproduce the early-time light curve adequately. In contrast, incorporating additional energy input from a magnetar yields a significantly improved fit to the light curve of SN~2017ati, which would reduce the nickel mass to $\sim0.21\,M_{\odot}$, still close to the upper end of the range typically inferred for SNe~IIb. Comparing the fitted results of SN~2017ati with the known sample of SNe~IIb indicates that its luminosity evolution is best explained by a combination of neutron star spin-down energy and radioactive nickel deposition. From late-time nebular spectra of SN~2017ati, the luminosity of the [\Oi]~$λ\lambda6300,6364$ doublet implies an oxygen mass of $\sim1.82-3.34\,M_{\odot}$, and the combination of a [\Caii]/[\Oi] flux ratio of $\sim0.5$ with nebular spectral model comparisons favours a progenitor zero-age main-sequence mass of $\geq17\,M_{\odot}$.

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arXiv2022

V838 Mon: A slow waking up of Sleeping Beauty?

Context. V838 Monocerotis is a peculiar binary that underwent an immense stellar explosion in 2002, leaving behind an expanding cool supergiant and a hot B3V companion. Five years after the outburst, the B3V companion disappeared from view, and so far did not recover. Aims. We investigate the changes in the light curve and spectral features Methods. A monitoring campaign has been performed during the past 13 years with the Nordic Optical Telescope to obtain optical photometric and spectroscopic data. The data sets are used to analyse the temporal evolution of the spectral features and the spectral energy distribution, and to characterize the object. Results. Our photometric data show a steady brightening in all bands during the past 13 years, which is particularly prominent in the blue. This rise is also reflected in the spectra, showing a gradual relative increase in the continuum flux at shorter wavelengths. In addition, a slow brightening of the Ha emission line starting in 2015 was detected. These changes might imply that the B3V companion is slowly reappearing. During the same time interval, our analysis reveals a considerable change in the observed colours of the object along with a steady decrease in the strength and width of molecular absorption bands in our low-resolution spectra. These changes suggest a rising temperature of the cool supergiant along with a weakening of its wind, most likely combined with a slow recovery of the secondary due to the evaporation of the dust and accretion of the material from the shell in which the hot companion is embedded. From our medium-resolution spectra, we find that the heliocentric radial velocity of the atomic absorption line of TiI 6556.06 A has been stable for more than a decade. We propose that TiI lines are tracing the velocity of the red supergiant in V838 Mon, and not representing the infalling matter as previously stated.

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arXiv2022

The nuclear transient AT 2017gge: a tidal disruption event in a dusty and gas-rich environment and the awakening of a dormant SMBH

We present the results from a dense multi-wavelength (optical/UV, near-infrared (IR), and X-ray) follow-up campaign of the nuclear transient AT2017gge, covering a total of 1698 days from the transient's discovery. The bolometric lightcurve, the black body temperature and radius, the broad H and He I $λ$5876 emission lines and their evolution with time, are all consistent with a tidal disruption event (TDE) nature. A soft X-ray flare is detected with a delay of $\sim$200 days with respect to the optical/UV peak and it is rapidly followed by the emergence of a broad He II $λ$4686 and by a number of long-lasting high ionization coronal emission lines. This indicate a clear connection between a TDE flare and the appearance of extreme coronal line emission (ECLEs). An IR echo, resulting from dust re-radiation of the optical/UV TDE light is observed after the X-ray flare and the associated near-IR spectra show a transient broad feature in correspondence of the He I $λ$10830 and, for the first time in a TDE, a transient high-ionization coronal NIR line (the [Fe XIII] $λ$10798) is also detected. The data are well explained by a scenario in which a TDE occurs in a gas and dust rich environment and its optical/UV, soft X-ray, and IR emission have different origins and locations. The optical emission may be produced by stellar debris stream collisions prior to the accretion disk formation, which is instead responsible for the soft X-ray flare, emitted after the end of the circularization process.

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arXiv2021

Intermediate-luminosity red transients: Spectrophotometric properties and connection to electron-capture supernova explosions

We present the spectroscopic and photometric study of five intermediate-luminosity red transients (ILRTs), namely AT 2010dn, AT 2012jc, AT 2013la, AT 2013lb, and AT 2018aes. They share common observational properties and belong to a family of objects similar to the prototypical ILRT SN~2008S. These events have a rise time that is less than 15 days and absolute peak magnitudes of between $-11.5$ and $-14.5$ mag. Their pseudo-bolometric light curves peak in the range $0.5$ - $9.0 \times10^{40}~\mathrm{erg~s}^{-1}$ and their total radiated energies are on the order of (0.3 - 3) $\times$~10$^{47}$~erg. After maximum brightness, the light curves show a monotonic decline or a plateau, resembling those of faint supernovae IIL or IIP, respectively. At late phases, the light curves flatten, roughly following the slope of the $^{56}$Co decay. If the late-time power source is indeed radioactive decay, these transients produce $^{56}$Ni masses on the order of $10^{-4}$ to $10^{-3}$~\msun. The spectral energy distribution of our ILRT sample, extending from the optical to the mid-infrared (MIR) domain, reveals a clear IR excess soon after explosion and non-negligible MIR emission at very late phases. The spectra show prominent H lines in emission with a typical velocity of a few hundred km~s$^{-1}$, along with Ca~II features. In particular, the [Ca~II] $λ$7291,7324 doublet is visible at all times, which is a characteristic feature for this family of transients. The identified progenitor of SN~2008S, which is luminous in archival Spitzer MIR images, suggests an intermediate-mass precursor star embedded in a dusty cocoon. We propose the explosion of a super-asymptotic giant branch star forming an electron-capture supernova as a plausible explanation for these events.

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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

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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arXiv2017

Gaia16apd -- a link between fast-and slowly-declining type I superluminous supernovae

We present ultraviolet, optical and infrared photometry and optical spectroscopy of the type Ic superluminous supernova (SLSN) Gaia16apd (= SN 2016eay), covering its evolution from 26 d before the $g$-band peak to 234.1 d after the peak. Gaia16apd was followed as a part of the NOT Unbiased Transient Survey (NUTS). It is one of the closest SLSNe known ($z = 0.102\pm0.001$), with detailed optical and ultraviolet (UV) observations covering the peak. Gaia16apd is a spectroscopically typical type Ic SLSN, exhibiting the characteristic blue early spectra with O II absorption, and reaches a peak $M_{g} = -21.8 \pm 0.1$ mag. However, photometrically it exhibits an evolution intermediate between the fast- and slowly-declining type Ic SLSNe, with an early evolution closer to the fast-declining events. Together with LSQ12dlf, another SLSN with similar properties, it demonstrates a possible continuum between fast- and slowly-declining events. It is unusually UV-bright even for a SLSN, reaching a non-$K$-corrected $M_{uvm2} \simeq -23.3$ mag, the only other type Ic SLSN with similar UV brightness being SN 2010gx. Assuming that Gaia16apd was powered by magnetar spin-down, we derive a period of $P = 1.9\pm0.2$ ms and a magnetic field of $B = 1.9\pm0.2 \times 10^{14}$ G for the magnetar. The estimated ejecta mass is between 8 and 16 $\mathrm{M}_{\odot}$ and the kinetic energy between 1.3 and $2.5 \times 10^{52}$ erg, depending on opacity and assuming that the entire ejecta is swept up into a thin shell. Despite the early photometric differences, the spectra at late times are similar to slowly-declining type Ic SLSNe, implying that the two subclasses originate from similar progenitors.

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arXiv2016

High-resolution optical spectroscopy of RS Ophiuchi during 2008 -- 2009

RS Ophiuchi is a symbiotic variable and a recurrent nova. We have monitored it with the Nordic Optical Telescope and obtained 30 high resolution (R=46 000) optical spectra over one orbital cycle during quiescence. To our knowledge this is the best-sampled high resolution spectroscopic dataset of RS Oph over one orbital period. We do not detect any direct signatures of an accretion disc such as double peaked emission lines, but many line profiles are complex consisting of superimposed emission and absorption components. We measure the spin of the red giant and conclude that it is tidally locked to the binary orbit. We observe Na I absorption features, probably arising from the circumbinary medium, that has been shaped by previous recurrent nova outbursts. We do not detect any intrinsic polarisation in the optical wavelengths.

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arXiv2012

Phase-resolved optical and X-ray spectroscopy of low-mass X-ray binary X1822-371

(Abridged) X1822-371 is the prototypical accretion disc corona X-ray source, a low-mass X-ray binary viewed at very high inclination, thereby allowing the disc structure and extended disc coronal regions to be visible. We study the structure of the accretion disc in X1822-371 by modelling the phase-resolved spectra both in optical and X-ray regime. We analyse high time resolution optical ESO/VLT spectra of X1822-371 to study the variability in the emission line profiles. In addition, we use data from XMM-Newton space observatory to study phase-resolved as well as high resolution X-ray spectra. We apply the Doppler tomography technique to reconstruct a map of the optical emission distribution in the system. We fit multi-component models to the X-ray spectra. We find that our results from both the optical and X-ray analysis can be explained with a model where the accretion disc has a thick rim in the region where the accretion stream impacts the disc. The behaviour of the H_beta line complex implies that some of the accreting matter creates an outburst around the accretion stream impact location and that the resulting outflow of matter moves both away from the accretion disc and towards the centre of the disc. Such behaviour can be explained by an almost isotropic outflow of matter from the accretion stream impact region. The optical emission lines of HeII 4686 and 5411 show double peaked profiles, typical for an accretion disc at high inclination. However, their velocities are slower than expected for an accretion disc in a system like X1822-371. This, combined with the fact that the HeII emission lines do not get eclipsed during the partial eclipse in the continuum, suggests that the line emission does not originate in the orbital plane and is more likely to come from above the accretion disc, for example the accretion disc wind.

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arXiv2011

First Kepler results on compact pulsators VI. Targets in the final half of the survey phase

We present results from the final six months of a survey to search for pulsations in white dwarfs and hot subdwarf stars with the Kepler spacecraft. Spectroscopic observations are used to separate the objects into accurate classes, and we explore the physical parameters of the subdwarf B (sdB) stars and white dwarfs in the sample. From the Kepler photometry and our spectroscopic data, we find that the sample contains 5 new pulsators of the V1093 Her type, one AM CVn type cataclysmic variable, and a number of other binary systems. This completes the survey for compact pulsators with Kepler. No V361 Hya type of short-period pulsating sdB stars were found in this half, leaving us with a total of one single multiperiodic V361 Hya and 13 V1093 Her pulsators for the full survey. Except for the sdB pulsators, no other clearly pulsating hot subdwarfs or white dwarfs were found, although a few low-amplitude candidates still remain. The most interesting targets discovered in this survey will be observed throughout the remainder of the Kepler Mission, providing the most long-term photometric datasets ever made on such compact, evolved stars. Asteroseismic investigations of these datasets will be invaluable in revealing the interior structure of these stars, and will boost our understanding of their evolutionary history.

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arXiv2008

A planetary nebula around nova V458 Vul undergoing flash ionization

Nova V458 Vul erupted on 2007 August 8th and reached a visual magnitude of 8.1 a few days later. H$α$ images obtained six weeks before the outburst as part of the IPHAS galactic plane survey reveal an 18th magnitude progenitor surrounded by an extended nebula. Subsequent images and spectroscopy of the nebula reveal an inner nebular knot increasing rapidly in brightness due to flash ionization by the nova event. We derive a distance of 13 kpc based on light travel time considerations, which is supported by two other distance estimation methods. The nebula has an ionized mass of 0.2 M$_{\odot}$ and a low expansion velocity: this rules it out as ejecta from a previous nova eruption, and is consistent with it being a ~14,000 year old planetary nebula, probably the product of a prior common envelope (CE) phase of evolution of the binary system. The large derived distance means that the mass of the erupting WD component of the binary is high. We identify two possible evolutionary scenarios, in at least one of which the system is massive enough to produce a Type Ia supernova on merging.

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