arXiv subjects

E. Pian

Publications and source records attributed to E. Pian.

At least 19 recordsLinked to original sources

arXiv2026

ENGRAVE follow-up of a type IIb supernova spatially coincident with the sub-threshold gravitational wave trigger S250818k

The candidate gravitational wave (GW) event S250818k was one of only three non-retracted LIGO-Virgo-KAGRA public alerts issued during the fourth observing run of the network (O4) with a binary neutron star (BNS) merger classification probability exceeding one percent. This triggered a prompt search for a potential electromagnetic (EM) counterpart in the large localisation error region (949 deg$^2$ projected in the sky at 90% credible level). The transient SN2025ulz, discovered by the Zwicky Transient Facility (ZTF) during the search, attracted a great deal of attention due to a potential spatial and temporal coincidence, and due to its initial fast decay and featureless spectrum. Here, we report on the follow up of this transient by the Electromagnetic counterparts of gravitational wave sources at the Very Large Telescope (ENGRAVE) Collaboration. We conducted an extensive multi-wavelength observational campaign, which led to the spectral classification of the transient as a type IIb supernova (SN), indicating that it is unrelated to the candidate GW event. In this article, we describe our observing strategies, data reduction, and interpretation. All of our results confirm and strengthen our classification of the source, and also show that shock cooling tails associated with type IIb SNe are one of the most prominent contaminants in kilonova searches.

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arXiv2025

The broad-lined type Ic supernova 2020lao experienced an energetic explosion with no central-engine signatures

We present infant-phase observations of the SN Ic-BL 2020lao, including optical spectroscopy beginning 48 hrs after explosion. The explosion time was constrained by power-law fits to the rising TESS and ZTF light curves, with the first ZTF detection occurring 27 hrs after explosion. The optical light curves show a rapid rise lasting 8.8 days and a peak luminosity typical of SNe Ic-BL (Mr=-18.5 mag). Unlike some engine-driven SN Ic-BL events, the light curve of SN 2020lao shows no evidence of an optical afterglow or excess emission, and the absence of shock-cooling in the TESS and ZTF data constrains the progenitor to a Wolf-Rayet-like star with radius less than a few times the solar radius, ruling out any extended envelope. The spectra resemble those of the X-ray-flash-associated SN 2006aj but with higher expansion velocities. From Arnett-type fits to the bolometric light curve and measured FeII velocities, we infer a Ni mass of 0.2 solar masses, an ejecta mass of 3.2 solar masses, and a kinetic energy of about 23x10^51 erg, corresponding to a specific kinetic energy of 7x10^51 erg per solar mass. Spectral synthesis modeling broadly reproduces the photospheric spectra of SN 2020lao and suggests a specific kinetic energy of 5x10^51 erg per solar mass. SN 2020lao and SN 2006aj synthesized comparable amounts of Ni, yet SN 2020lao exhibits specific kinetic energy values 5-10 times larger. VLA and Swift/XRT non-detections reveal no afterglow emission, allowing limits on relativistic ejecta and dense circumstellar material. Given that SN 2020lao reaches a specific kinetic energy typical of engine-driven SNe Ic-BL, the lack of an optical excess with the non-detections in the radio and X-ray bands suggests that if a relativistic jet was launched, it was either viewed far off axis or choked before breakout. If there was no relativistic jet, SN 2020lao would be an extreme nonrelativistic SN Ic-BL.

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arXiv2025

VSTPOL: making the VST a large survey telescope for optical polarimetry

Since the start of operations in 2011, the VLT Survey Telescope (VST) has been one of the most efficient wide-field imagers in the optical bands. However, in the next years the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) will be a game-changer in this field. Hence, the timing is appropriate for specializing the VST with additions that can make it unique in well-defined scientific cases. VSTPOL is a project that aims to provide the addition of wide-field polarimetric capabilities to the VST telescope, making it the first large survey telescope for linear optical polarimetry. Actually, while there are quite a number of optical telescopes, the telescopes providing polarimetric instrumentation are just a few. The number of relatively large mirror polarimetric telescopes is small, although they would be specifically needed e.g. to support many science cases of the Cherenkov Telescope Array (CTA) that, in the southern hemisphere, is co-located with the VST. The VST telescope is equipped with a single instrument, the OmegaCAM wide-field imaging camera operating in the visible bands with a field of view of 1$^\circ\times1^\circ$. The polarimetric mode will be implemented through the insertion of a large rotatable polarizer installed on the field-corrector optics, which will be exchangeable with the non-polarimetric corrector optics. The limiting polarimetric systematic errors due to variable atmospheric conditions and instrumental polarization can be corrected down to a level of $\sim10^{-3}$ by leveraging the large amount of unpolarized stars within each field-of-view. By the user point of view, VSTPOL will be an additional mode for the VST wide-field imaging camera.

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arXiv2023

A Hubble Space Telescope Search for r-Process Nucleosynthesis in Gamma-ray Burst Supernovae

The existence of a secondary (in addition to compact object mergers) source of heavy element ($r$-process) nucleosynthesis, the core-collapse of rapidly-rotating and highly-magnetized massive stars, has been suggested by both simulations and indirect observational evidence. Here, we probe a predicted signature of $r$-process enrichment, a late-time ($\gtrsim 40$ days post-burst) distinct red color, in observations of GRB-supernovae (GRB-SNe) which are linked to these massive star progenitors. We present optical to near-IR color measurements of four GRB-SNe at $z \lesssim 0.4$, extending out to $> 500$ days post-burst, obtained with the Hubble Space Telescope and large-aperture ground-based telescopes. Comparison of our observations to models indicates that GRBs 030329, 100316D and 130427A are consistent with both no enrichment and producing $0.01 - 0.15 M_{\odot}$ of $r$-process material if there is a low amount of mixing between the inner $r$-process ejecta and outer SN layers. GRB 190829A is not consistent with any models with $r$-process enrichment $\geq 0.01 M_{\odot}$. Taken together the sample of GRB-SNe indicates color diversity at late times. Our derived yields from GRB-SNe may be underestimated due to $r$-process material hidden in the SN ejecta (potentially due to low mixing fractions) or the limits of current models in measuring $r$-process mass. We conclude with recommendations for future search strategies to observe and probe the full distribution of $r$-process produced by GRB-SNe.

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arXiv2023

Panning for gold, but finding helium: discovery of the ultra-stripped supernova SN2019wxt from gravitational-wave follow-up observations

We present the results from multi-wavelength observations of a transient discovered during the follow-up 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 SN2019wxt, a young transient in a galaxy whose sky position (in the 80\% GW contour) and distance ($\sim$150\,Mpc) were plausibly compatible with the localisation uncertainty of the GW event. Initially, the transient's tightly constrained age, its relatively faint peak magnitude ($M_i \sim -16.7$\,mag) and the $r-$band decline rate of $\sim 1$\,mag per 5\,days appeared suggestive of a compact binary merger. However, SN2019wxt 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 $\sim 0.1\,M_\odot$, with $^{56}$Ni comprising $\sim 20\%$ 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 progenitors that could give rise to the observed properties of SN2019wxt, and concluded that an ultra-stripped origin in a binary system is the most likely explanation. Disentangling electromagnetic counterparts to GW events from transients such as SN2019wxt is challenging: in a bid to characterise the level of contamination, we estimated the rate of events with properties comparable to those of SN2019wxt and found that $\sim 1$ 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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arXiv2023

The first JWST spectrum of a GRB afterglow: No bright supernova in observations of the brightest GRB of all time, GRB 221009A

We present JWST and Hubble Space Telescope (HST) observations of the afterglow of GRB 221009A, the brightest gamma-ray burst (GRB) ever observed. This includes the first mid-IR spectra of any GRB, obtained with JWST/NIRSPEC (0.6-5.5 micron) and MIRI (5-12 micron), 12 days after the burst. Assuming that the intrinsic spectral slope is a single power-law, with $F_ν \propto ν^{-β}$, we obtain $β\approx 0.35$, modified by substantial dust extinction with $A_V = 4.9$. This suggests extinction above the notional Galactic value, possibly due to patchy extinction within the Milky Way or dust in the GRB host galaxy. It further implies that the X-ray and optical/IR regimes are not on the same segment of the synchrotron spectrum of the afterglow. If the cooling break lies between the X-ray and optical/IR, then the temporal decay rates would only match a post jet-break model, with electron index $p<2$, and with the jet expanding into a uniform ISM medium. The shape of the JWST spectrum is near-identical in the optical/nIR to X-shooter spectroscopy obtained at 0.5 days and to later time observations with HST. The lack of spectral evolution suggests that any accompanying supernova (SN) is either substantially fainter or bluer than SN 1998bw, the proto-type GRB-SN. Our HST observations also reveal a disc-like host galaxy, viewed close to edge-on, that further complicates the isolation of any supernova component. The host galaxy appears rather typical amongst long-GRB hosts and suggests that the extreme properties of GRB 221009A are not directly tied to its galaxy-scale environment.

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arXiv2023

The brightest GRB ever detected: GRB 221009A as a highly luminous event at z = 0.151

Context: The extreme luminosity of gamma-ray bursts (GRBs) makes them powerful beacons for studies of the distant Universe. The most luminous bursts are typically detected at moderate/high redshift, where the volume for seeing such rare events is maximized and the star-formation activity is greater than at z = 0. For distant events, not all observations are feasible, such as at TeV energies. Aims: Here we present a spectroscopic redshift measurement for the exceptional GRB 221009A, the brightest GRB observed to date with emission extending well into the TeV regime. Methods: We used the X-shooter spectrograph at the ESO Very Large Telescope (VLT) to obtain simultaneous optical to near-IR spectroscopy of the burst afterglow 0.5 days after the explosion. Results: The spectra exhibit both absorption and emission lines from material in a host galaxy at z = 0.151. Thus GRB 221009A was a relatively nearby burst with a luminosity distance of 745 Mpc. Its host galaxy properties (star-formation rate and metallicity) are consistent with those of LGRB hosts at low redshift. This redshift measurement yields information on the energy of the burst. The inferred isotropic energy release, $E_{\rm iso} > 5 \times 10^{54}$ erg, lies at the high end of the distribution, making GRB 221009A one of the nearest and also most energetic GRBs observed to date. We estimate that such a combination (nearby as well as intrinsically bright) occurs between once every few decades to once per millennium.

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arXiv2022

A blast from the infant Universe: the very high-z GRB 210905A

We present a detailed follow-up of the very energetic GRB 210905A at a high redshift of z = 6.312 and its luminous X-ray and optical afterglow. We obtained a photometric and spectroscopic follow-up in the optical and near-infrared (NIR), covering both the prompt and afterglow emission from a few minutes up to 20 Ms after burst. With an isotropic gamma-ray energy release of Eiso = 1.27E54 erg, GRB 210905A lies in the top ~7% of gamma-ray bursts (GRBs) in terms of energy released. Its afterglow is among the most luminous ever observed. It starts with a shallow evolution that can be explained by energy injection, and it is followed by a steeper decay, while the spectral energy distribution is in agreement with slow cooling in a constant-density environment within the standard fireball theory. A jet break at ~ 46.2+-16.3 d (~6.3 d rest-frame) has been observed in the X-ray light curve; however, it is hidden in the H band due to the contribution from the likely host galaxy, the fourth GRB host at z > 6 known to date. We derived a half-opening angle of 8.4+-1.0 degrees, which is the highest ever measured for a z>6 burst, but within the range covered by closer events. The resulting collimation-corrected gamma-ray energy release of 1E52 erg is also among the highest ever measured. The moderately large half-opening angle argues against recent claims of an inverse dependence of the half-opening angle on the redshift. The total jet energy is likely too large to be sustained by a standard magnetar, and it suggests that the central engine of this burst was a newly formed black hole. Despite the outstanding energetics and luminosity of both GRB 210905A and its afterglow, we demonstrate that they are consistent with those of less distant bursts, indicating that the powering mechanisms and progenitors do not evolve significantly with redshift.

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arXiv2022

Unusual gas structure in an otherwise normal spiral galaxy hosting GRB 171205A / SN 2017iuk

We study the structure of atomic hydrogen (HI) in the host galaxy of GRB 171205A / SN 2017iuk at z=0.037 through HI 21cm emission line observations with the Karl G. Jansky Very Large Array. These observations reveal unusual morphology and kinematics of the HI in this otherwise apparently normal galaxy. High column density, cold HI is absent from an extended North-South region passing by the optical centre of the galaxy, but instead is extended towards the South, on both sides of the galaxy. Moreover, the HI kinematics do not show a continuous change along the major axis of the galaxy as expected in a classical rotating disk. We explore several scenarios to explain the HI structure and kinematics in the galaxy: feedback from a central starburst and/or an active galactic nucleus, ram pressure stripping, accretion, and tidal interaction from a companion galaxy. All of these options are ruled out. The most viable remaining explanation is the penetrating passage of a satellite through the disk only a few Myr ago, redistributing the HI in the GRB host without yet affecting its stellar distribution. It can also lead to the rapid formation of peculiar stars due to a violent induced shock. The location of GRB 171205A in the vicinity of the distorted area suggests that its progenitor star(s) originated in extreme conditions that share the same origin as the peculiarities in HI. This could explain the atypical location of GRB 171205A in its host galaxy.

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arXiv2022

The peculiar short-duration GRB 200826A and its supernova

Gamma-ray bursts (GRBs) are classified as long and short events. Long GRBs (LGRBs) are associated with the end states of very massive stars, while short GRBs (SGRBs) are linked to the merger of compact objects. GRB 200826A was a peculiar event, because by definition it was a SGRB, with a rest-frame duration of ~ 0.5 s. However, this event was energetic and soft, which is consistent with LGRBs. The relatively low redshift (z = 0.7486) motivated a comprehensive, multi-wavelength follow-up campaign to characterize its host, search for a possible associated supernova (SN), and thus understand the origin of this burst. To this aim we obtained a combination of deep near-infrared (NIR) and optical imaging together with spectroscopy. Our analysis reveals an optical and NIR bump in the light curve whose luminosity and evolution is in agreement with several LGRB-SNe. Analysis of the prompt GRB shows that this event follows the $E_{\rm p,i}-E_{\rm iso}$ relation found for LGRBs. The host galaxy is a low-mass star-forming galaxy, typical for LGRBs, but with one of the highest star-formation rates (SFR), especially with respect to its mass ($\log M_\ast/M_\odot = 8.6$, SFR $\sim 4.0 \,M_\odot$/yr). We conclude that GRB 200826A is a typical collapsar event in the low tail of the duration distribution of LGRBs. These findings support theoretical predictions that events produced by collapsars can be as short as 0.5 s in the host frame and further confirm that duration alone is not an efficient discriminator for the progenitor class of a GRB.

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arXiv2021

The supernova of the MAGIC GRB190114C

We observed GRB190114C (redshift z = 0.4245), the first GRB ever detected at TeV energies, at optical and near-infrared wavelengths with several ground-based telescopes and the Hubble Space Telescope, with the primary goal of studying its underlying supernova, SN2019jrj. The monitoring spanned the time interval between 1.3 and 370 days after the burst, in the observer frame. We find that the afterglow emission can be modelled with a forward shock propagating in a uniform medium modified by time-variable extinction along the line of sight. A jet break could be present after 7 rest-frame days, and accordingly the maximum luminosity of the underlying SN ranges between that of stripped-envelope corecollapse supernovae (SNe) of intermediate luminosity, and that of the luminous GRB-associated SN2013dx. The observed spectral absorption lines of SN2019jrj are not as broad as in classical GRB-SNe, and are rather more similar to those of less-luminous core-collapse SNe. Taking the broad-lined stripped-envelope core-collapse SN2004aw as an analogue, we tentatively derive the basic physical properties of SN2019jrj. We discuss the possibility that a fraction of the TeV emission of this source might have had a hadronic origin and estimate the expected high-energy neutrino detection level with IceCube.

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arXiv2021

Modelling of SN2013dx associated with the low-redshift GRB130702A points to diversity in GRB/SN properties

The properties of the broad-lined type Ic supernova (SN) 2013dx, associated with the long gamma-ray burst GRB130702A at a redshift z = 0.145, are derived via spectral modelling. SN2013dx was similar in luminosity to other GRB/SNe, with a derived value of the mass of 56Nickel ejected in the explosion of ~0.4 Msun. However, its spectral properties suggest a smaller explosion kinetic energy. Radiation transport models were used to derive a plausible mass and density distribution of the SN ejecta in a one-dimensional approximation. While the mass ejected in the explosion that is obtained from the modelling (Mej ~ 9 Msun) is similar to that of all other well-studied GRB/SNe, the kinetic energy is significantly smaller (KE ~ 10^{52}erg). This leads to a smaller KE/Mej ratio, ~ 10^{51} erg/Msun, which is reflected in the narrower appearance of the spectral lines. While the low KE does not represent a problem for the scenario in which magnetar energy aids powering the explosion and the nucleosynthesis, it is nevertheless highly unusual. SNe Ic with similar KE have never been seen in coincidence with a GRB, and no well-observed GRB/SN has shown similarly low KE and KE/Mej.

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arXiv2021

Time Domain Astronomy with the THESEUS Satellite

THESEUS is a medium size space mission of the European Space Agency, currently under evaluation for a possible launch in 2032. Its main objectives are to investigate the early Universe through the observation of gamma-ray bursts and to study the gravitational waves electromagnetic counterparts and neutrino events. On the other hand, its instruments, which include a wide field of view X-ray (0.3-5 keV) telescope based on lobster-eye focusing optics and a gamma-ray spectrometer with imaging capabilities in the 2-150 keV range, are also ideal for carrying out unprecedented studies in time domain astrophysics. In addition, the presence onboard of a 70 cm near infrared telescope will allow simultaneous multi-wavelegth studies. Here we present the THESEUS capabilities for studying the time variability of different classes of sources in parallel to, and without affecting, the gamma-ray bursts hunt.

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arXiv2020

10.4m GTC observations of the nearby VHE-detected GRB 190829A/SN 2019oyw

Aims. GRB 190829A (z = 0.0785), detected by Fermi and Swift with two emission episodes separated by a quiescent gap of ~40 s, was also observed by the H.E.S.S. telescopes at Very-High Energy (VHE). We present the 10.4m GTC observations of the afterglow of GRB 190829A and underlying supernova and compare it against a similar GRB 180728A and discuss the implications on underlying physical mechanisms producing these two GRBs. Methods. We present multi-band photometric data along with spectroscopic follow-up observations taken with the 10.4m GTC telescope. Together with the data from the prompt emission, the 10.4m GTC data are used to understand the emission mechanisms and possible progenitor. Results. A detailed analysis of multi-band data of the afterglow demands cooling frequency to pass between the optical and X-ray bands at early epochs and dominant with underlying SN 2019oyw later on. Conclusions. Prompt emission temporal properties of GRB 190829A and GRB 180728A are similar, however the two pulses seem different in the spectral domain. We found that the supernova (SN) 2019oyw associated with GRB 190829A, powered by Ni decay, is of Type Ic-BL and that the spectroscopic/photometric properties of this SN is consistent with those observed for SN 1998bw but evolved comparatively early.

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arXiv2020

Local starburst conditions and formation of GRB 980425 / SN 1998bw within a collisional ring

We present the first spatially resolved study of molecular gas in the vicinity of a Gamma Ray Burst, using CO(2-1) emission line observations with the Atacama Large Millimetre Array (ALMA) at ~50 pc scales. The host galaxy of GRB 980425 contains a ring of high column density HI gas which is likely to have formed due to a collision between the GRB host and its companion galaxy, within which the GRB is located. We detect eleven molecular gas clumps in the galaxy, seven of which are within the gas ring. The clump closest to the GRB position is at a projected separation of ~280 pc. Although it is plausible that the GRB progenitor was ejected from clusters formed in this clump, we argue that the in situ formation of the GRB progenitor is the most likely scenario. We measure the molecular gas masses of the clumps and find them to be sufficient for forming massive star clusters. The molecular gas depletion times of the clumps show a variation of ~2 dex, comparable with the large variation in depletion times found in starburst galaxies in the nearby Universe. This demonstrates the presence of starburst modes of star formation on local scales in the galaxy, even while the galaxy as a whole cannot be categorised as a starburst based on its global properties. Our findings suggest that the progenitor of GRB 9802425 was originated in a young massive star cluster formed in the starburst mode of star formation.

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arXiv2020

A Supernova Candidate at z=0.092 in XMM-Newton Archival Data

During a search for X-ray transients in the XMM-Newton archive within the EXTraS project, we discovered a new X-ray source that is detected only during a ~5 min interval of a ~21 h-long observation performed on 2011 June 21 (EXMM 023135.0-603743, probability of a random Poissonian fluctuation: ~$1.4\times10^{-27}$). With dedicated follow-up observations, we found that its position is consistent with a star-forming galaxy (SFR = 1-2 $M_\odot$ yr$^{-1}$) at redshift $z=0.092\pm0.003$ ($d=435\pm15$ Mpc). At this redshift, the energy released during the transient event was $2.8\times10^{46}$ erg in the 0.3-10 keV energy band (in the source rest frame). The luminosity of the transient, together with its spectral and timing properties, make EXMM 023135.0-603743 a gripping analog to the X-ray transient associated to SN 2008D, which was discovered during a Swift/XRT observation of the nearby ($d=27$ Mpc) supernova-rich galaxy NGC 2770. We interpret the XMM-Newton event as a supernova shock break-out or an early cocoon, and show that our serendipitous discovery is compatible with the rate of core-collapse supernovae derived from optical observations and much higher than that of tidal disruption events.

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arXiv2020

A comparison between short GRB afterglows and AT2017gfo: shedding light on kilonovae properties

Multi-messenger astronomy received a great boost following the discovery of kilonova AT2017gfo, the optical counterpart of the gravitational wave source GW170817 associated with the short gamma-ray burst GRB 170817A. AT2017gfo was the first kilonova that could be extensively monitored in time both photometrically and spectroscopically. Previously, only few candidates have been observed against the glare of short GRB afterglows. In this work, we aim to search the fingerprints of AT2017gfo-like kilonova emissions in the optical/NIR light curves of 39 short GRBs with known redshift. For the first time, our results allow us to study separately the range of luminosity of the blue and red components of AT2017gfo-like kilonovae in short GRBs. In particular, the red component is similar in luminosity to AT2017gfo, while the blue kilonova can be more than 10 times brighter. Finally, we find further evidence to support all the claimed kilonova detections and we exclude an AT2017gfo-like kilonova in GRBs 050509B and 061201.

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arXiv2019

Short GRB 160821B: a reverse shock, a refreshed shock, and a well-sampled kilonova

We report our identification of the optical afterglow and host galaxy of the short-duration gamma-ray burst GRB 160821B. The spectroscopic redshift of the host is $z=0.162$, making it one of the lowest redshift sGRBs identified by Swift. Our intensive follow-up campaign using a range of ground-based facilities as well as HST, XMM and Swift, shows evidence for a late-time excess of optical and near-infrared emission in addition to a complex afterglow. The afterglow light-curve at X-ray frequencies reveals a narrow jet, $θ_j\sim1.9^{+0.10}_{-0.03}$ deg, that is refreshed at $>1$ day post-burst by a slower outflow with significantly more energy than the initial outflow that produced the main GRB. Observations of the 5 GHz radio afterglow shows a reverse shock into a mildly magnetised shell. The optical and near-infrared excess is fainter than AT2017gfo associated with GW170817, and is well explained by a kilonova with dynamic ejecta mass $M_{\rm dyn}=(1.0\pm0.6)\times10^{-3}$ M$_{\odot}$ and a secular (postmerger) ejecta mass with $M_{\rm pm}=(1.0\pm0.6)\times10^{-2}$ M$_\odot$, consistent with a binary neutron star merger resulting in a short-lived massive neutron star. This optical and near-infrared dataset provides the best-sampled kilonova light-curve without a gravitational wave trigger to date.

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