arXiv subjects

Alessandra Corsi

Publications and source records attributed to Alessandra Corsi.

At least 55 records · Page 3Linked to original sources

arXiv2020

Kilonova Luminosity Function Constraints based on Zwicky Transient Facility Searches for 13 Neutron Star Mergers

We present a systematic search for optical counterparts to 13 gravitational wave (GW) triggers involving at least one neutron star during LIGO/Virgo's third observing run. We searched binary neutron star (BNS) and neutron star black hole (NSBH) merger localizations with the Zwicky Transient Facility (ZTF) and undertook follow-up with the Global Relay of Observatories Watching Transients Happen (GROWTH) collaboration. The GW triggers had a median localization of 4480 deg^2, median distance of 267 Mpc and false alarm rates ranging from 1.5 to 1e-25 per yr. The ZTF coverage had a median enclosed probability of 39%, median depth of 20.8mag, and median response time of 1.5 hr. The O3 follow-up by the GROWTH team comprised 340 UVOIR photometric points, 64 OIR spectra, and 3 radio. We find no promising kilonova (radioactivity-powered counterpart) and we convert the upper limits to constrain the underlying kilonova luminosity function. Assuming that all kilonovae are at least as luminous as GW170817 at discovery (-16.1mag), we calculate our joint probability of detecting zero kilonovae is only 4.2%. If we assume that all kilonovae are brighter than -16.6mag (extrapolated peak magnitude of GW170817) and fade at 1 mag/day (similar to GW170817), the joint probability of zero detections is 7%. If we separate the NSBH and BNS populations, the joint probability of zero detections, assuming all kilonovae are brighter than -16.6mag, is 9.7% for NSBH and 7.9% for BNS mergers. Moreover, <57% (<89%) of putative kilonovae could be brighter than -16.6mag assuming flat (fading) evolution, at 90% confidence. If we further account for the online terrestrial probability for each GW trigger, we find that <68% of putative kilonovae could be brighter than -16.6mag. Comparing to model grids, we find that some kilonovae must have Mej < 0.03 Msun or Xlan>1e-4 or phi>30deg to be consistent with our limits. (Abridged)

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arXiv2020

Radio follow-up of a candidate gamma-ray transient in the sky localization area of GW170608

After the identification of a candidate $γ$-ray transient in the error region of the binary black hole (BBH) merger GW150914 by the \textit{Fermi} satellite, the question of whether BBH mergers can be associated to electromagnetic counterparts remains highly debated. Here, we present radio follow-up observations of GW170608, a BBH merger that occurred during the second observing run (O2) of the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO). Our radio follow up focused on a specific field contained in the GW170608 sky localization area, where a candidate high-energy transient was detected by the \textit{Fermi} Large Area Telescope (LAT). We make use of data collected at 1.4\,GHz with the Karl G. Jansky Very Large Array (VLA), as well as with the VLA Low-band Ionosphere and Transient Experiment (VLITE). Our analysis is sensitive to potential radio afterglows with luminosity densities $L_{\rm 1.4\,GHz}\gtrsim 6\times10^{28}$\,erg\,s$^{-1}$\,Hz$^{-1}$. In the most optimistic theoretical models, $\approx 20\%$ of BBH events occurring in massive hosts could be associated with outflows as radio luminous as this. Although we find no evidence for the presence of a radio counterpart associated with the LAT candidate in the GW170608 error region, our analysis demonstrates the feasibility of future radio follow-up observations of well localized BBHs. Comparing our radio upper-limits with theoretical expectations for the radio afterglows potentially associated with jets launched in BBH mergers, we find that for jets of energy $\approx 10^{49}$\,erg seen on-axis, only jet angles $θ_{jet}\gtrsim 40^{\circ}$ are compatible with the observations.

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arXiv2020

Multi-waveform cross-correlation search method for intermediate-duration gravitational waves from gamma-ray bursts

Gamma-ray Bursts (GRBs) are flashes of gamma-rays thought to originate from rare forms of massive star collapse (long GRBs), or from mergers of compact binaries (short GRBs) containing at least one neutron star (NS). The nature of the post-explosion / post-merger remnant (NS versus black hole, BH) remains highly debated. In ~50% of both long and short GRBs, the temporal evolution of the X-ray afterglow that follows the flash of gamma-rays is observed to "plateau" on timescales of 100-10000 s since explosion, possibly signaling the presence of energy injection from a long-lived, highly magnetized NS (magnetar). The Cross-Correlation Algorithm (CoCoA) proposed by [R. Coyne et. al., (2016)] aims to optimize searches for intermediate-duration (100-10000s) gravitational waves (GWs) from GRB remnants. In this work, we test CoCoA on real data collected with ground-based GW detectors. We further develop the detection statistics on which CoCoA is based to allow for multi-waveform searches spanning a physically-motivated parameter space, so as to account for uncertainties in the physical properties of GRB remnants.

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arXiv2020

The late-time afterglow evolution of long gamma-ray bursts GRB 160625B and GRB 160509A

We present post-jet-break \textit{HST}, VLA and \textit{Chandra} observations of the afterglow of the long $γ$-ray bursts GRB 160625B (between 69 and 209 days) and GRB 160509A (between 35 and 80 days). We calculate the post-jet-break decline rates of the light curves, and find the afterglow of GRB 160625B inconsistent with a simple $t^{-3/4}$ steepening over the break, expected from the geometric effect of the jet edge entering our line of sight. However, the favored optical post-break decline ($f_ν \propto t^{-1.96 \pm 0.07}$) is also inconsistent with the $f_ν \propto t^{-p}$ decline (where $p \approx 2.3$ from the pre-break light curve), which is expected from exponential lateral expansion of the jet; perhaps suggesting lateral expansion that only affects a fraction of the jet. The post-break decline of GRB 160509A is consistent with both the $t^{-3/4}$ steepening and with $f_ν \propto t^{-p}$. We also use {\sc boxfit} to fit afterglow models to both light curves and find both to be energetically consistent with a millisecond magnetar central engine, although the magnetar parameters need to be extreme (i.e. $E \sim 3 \times 10^{52}$ erg). Finally, the late-time radio light curves of both afterglows are not reproduced well by {\sc boxfit} and are inconsistent with predictions from the standard jet model; instead both are well represented by a single power law decline (roughly $f_ν \propto t^{-1}$) with no breaks. This requires a highly chromatic jet break ($t_{j,\mathrm{radio}} > 10 \times t_{j,\mathrm{optical}}$) and possibly a two-component jet for both bursts.

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arXiv2020

The Broad-lined Ic Supernova ZTF18aaqjovh (SN 2018bvw): An Optically-discovered Engine-driven Supernova Candidate with Luminous Radio Emission

We present ZTF18aaqjovh (SN 2018bvw), a high-velocity ("broad-lined") stripped-envelope (Type Ic) supernova (Ic-BL SN) discovered in the Zwicky Transient Facility one-day cadence survey. ZTF18aaqjovh shares a number of features in common with engine-driven explosions: the photospheric velocity and the shape of the optical light curve are very similar to that of the Type Ic-BL SN 1998bw, which was associated with a low-luminosity gamma-ray burst (LLGRB) and had relativistic ejecta. However, the radio luminosity of ZTF18aaqjovh is almost two orders of magnitude fainter than that of SN 1998bw at the same velocity phase, and the shock velocity is at most mildly relativistic (v=0.06-0.4c). A search of high-energy catalogs reveals no compelling GRB counterpart to ZTF18aaqjovh, and the limit on the prompt GRB luminosity of $L_{γ,\mathrm{iso}} \approx 1.6 \times 10^{48}$ erg/sec excludes a classical GRB but not an LLGRB. Altogether, ZTF18aaqjovh represents another transition event between engine-driven SNe associated with GRBs and "ordinary" Ic-BL SNe.

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arXiv2019

Evidence for Late-stage Eruptive Mass-loss in the Progenitor to SN2018gep, a Broad-lined Ic Supernova: Pre-explosion Emission and a Rapidly Rising Luminous Transient

We present detailed observations of ZTF18abukavn (SN2018gep), discovered in high-cadence data from the Zwicky Transient Facility as a rapidly rising ($1.4\pm0.1$ mag/hr) and luminous ($M_{g,\mathrm{peak}}=-20$ mag) transient. It is spectroscopically classified as a broad-lined stripped-envelope supernova (Ic-BL SN). The high peak luminosity ($L_{\mathrm{bol}} \gtrsim 3 \times 10^{44}$ erg $\mathrm{sec}^{-1}$), the short rise time ($t_{\mathrm{rise}}= 3$ days in $g$-band), and the blue colors at peak ($g-r\sim-0.4$) all resemble the high-redshift Ic-BL iPTF16asu, as well as several other unclassified fast transients. The early discovery of SN2018gep (within an hour of shock breakout) enabled an intensive spectroscopic campaign, including the highest-temperature ($T_{\mathrm{eff}}\gtrsim40,000$ K) spectra of a stripped-envelope SN. A retrospective search revealed luminous ($M_g \sim M_r \approx -14\,$mag) emission in the days to weeks before explosion, the first definitive detection of precursor emission for a Ic-BL. We find a limit on the isotropic gamma-ray energy release $E_\mathrm{γ,iso}<4.9 \times 10^{48}$ erg, a limit on X-ray emission $L_{\mathrm{X}} < 10^{40}\,$erg sec$^{-1}$, and a limit on radio emission $νL_ν\lesssim 10^{37}\,$erg sec$^{-1}$. Taken together, we find that the early ($<10\,$days) data are best explained by shock breakout in a massive shell of dense circumstellar material ($0.02\,M_\odot$) at large radii ($3 \times 10^{14}\,$cm) that was ejected in eruptive pre-explosion mass-loss episodes. The late-time ($>10$ days) light curve requires an additional energy source, which could be the radioactive decay of Ni-56.

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arXiv2019

An ASKAP search for a radio counterpart to the first high-significance neutron star-black hole merger LIGO/Virgo S190814bv

We present results from a search for a radio transient associated with the LIGO/Virgo source S190814bv, a likely neutron star-black hole (NSBH) merger, with the Australian Square Kilometre Array Pathfinder. We imaged a $30\,{\rm deg}^2$ field at $ΔT$=2, 9 and 33 days post-merger at a frequency of 944\,MHz, comparing them to reference images from the Rapid ASKAP Continuum Survey observed 110 days prior to the event. Each epoch of our observations covers $89\%$ of the LIGO/Virgo localisation region. We conducted an untargeted search for radio transients in this field, resulting in 21 candidates. For one of these, \object[AT2019osy]{AT2019osy}, we performed multi-wavelength follow-up and ultimately ruled out the association with S190814bv. All other candidates are likely unrelated variables, but we cannot conclusively rule them out. We discuss our results in the context of model predictions for radio emission from neutron star-black hole mergers and place constrains on the circum-merger density and inclination angle of the merger. This survey is simultaneously the first large-scale radio follow-up of an NSBH merger, and the most sensitive widefield radio transients search to-date.

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arXiv2019

An optimized radio follow-up strategy for stripped-envelope core-collapse supernovae

Several on-going or planned synoptic optical surveys are offering or will soon be offering an unprecedented opportunity for discovering larger samples of the rarest types of stripped-envelope core-collapse supernovae (SNe), such as those associated with relativistic jets, mildly-relativistic ejecta, or strong interaction with the circumstellar medium (CSM). Observations at radio wavelengths are a useful tool to probe the fastest moving ejecta, as well as denser circumstellar environments, and can thus help us identify the rarest type of core-collapse explosions. Here, we discuss how to set up an efficient radio follow-up program to detect and correctly identify radio-emitting stripped-envelope core-collapse explosions. We use a method similar to the one described in \citealt{Carbone2018}, and determine the optimal timing of GHz radio observations assuming a sensitivity comparable to that of the Karl G. Jansky Very Large Array. The optimization is done so as to ensure that the collected radio observations can identify the type of explosion powering the radio counterpart by using the smallest possible amount of telescope time. We also present a previously unpublished upper-limit on the late-time radio emission from supernova iPTF17cw. Finally, we conclude by discussing implications for follow-up in the X-rays.

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arXiv2019

Astro2020 Science White Paper: Radio Counterparts of Compact Object Mergers in the Era of Gravitational-Wave Astronomy

GHz radio astronomy has played a fundamental role in the recent dazzling discovery of GW170817, a neutron star (NS)-NS merger observed in both gravitational waves (GWs) and light at all wavelengths. Here we show how the expected progress in sensitivity of ground-based GW detectors over the next decade calls for U.S.-based GHz radio arrays to be improved beyond current levels. We discuss specifically how several new scientific opportunities would emerge in multi-messenger time-domain astrophysics if a next generation GHz radio facility with sensitivity and resolution $10\times$ better than the current Jansky Very Large Array (VLA) were to work in tandem with ground-based GW detectors. These opportunities include probing the properties, structure, and size of relativistic jets and wide-angle ejecta from NS-NS mergers, as well as unraveling the physics of their progenitors via host galaxy studies.

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arXiv2019

AGN (and other) astrophysics with Gravitational Wave Events

The stellar mass binary black hole (sBBH) mergers presently detected by LIGO may originate wholly or in part from binary black hole mergers embedded in disks of gas around supermassive black holes. Determining the contribution of these active galactic nucleus (AGN) disks to the sBBH merger rate enables us to uniquely measure important parameters of AGN disks, including their typical density, aspect ratio, and lifetime, thereby putting unique limits on an important element of galaxy formation. For the first time, gravitational waves will allow us to reveal the properties of the hidden interior of AGN disks, while electromagnetic radiation (EM) probes the disk photosphere. The localization of sBBH merger events from LIGO is generally insufficient for association with a single EM counterpart. However, the contribution to the LIGO event rate from rare source types (such as AGNs) can be determined on a statistical basis. To determine the contribution to the sBBH rate from AGNs in the next decade requires: {\em 1) a complete galaxy catalog for the LIGO search volume, 2) strategic multi-wavelength EM follow-up of LIGO events and 3) significant advances in theoretical understanding of AGN disks and the behavior of objects embedded within them.}

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arXiv2019

Multi-Messenger Astronomy with Extremely Large Telescopes

The field of time-domain astrophysics has entered the era of Multi-messenger Astronomy (MMA). One key science goal for the next decade (and beyond) will be to characterize gravitational wave (GW) and neutrino sources using the next generation of Extremely Large Telescopes (ELTs). These studies will have a broad impact across astrophysics, informing our knowledge of the production and enrichment history of the heaviest chemical elements, constrain the dense matter equation of state, provide independent constraints on cosmology, increase our understanding of particle acceleration in shocks and jets, and study the lives of black holes in the universe. Future GW detectors will greatly improve their sensitivity during the coming decade, as will near-infrared telescopes capable of independently finding kilonovae from neutron star mergers. However, the electromagnetic counterparts to high-frequency (LIGO/Virgo band) GW sources will be distant and faint and thus demand ELT capabilities for characterization. ELTs will be important and necessary contributors to an advanced and complete multi-messenger network.

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arXiv2019

Radio Forensics Could Unmask Nearby Off-axis Gamma-ray Bursts

The multi-messenger observation of gamma-ray burst (GRB)\,170817A from the nearby binary neutron-star merger GW170817 demonstrated that low-energy $γ$-ray emission can be observed at relatively large angles from GRB jet axes. If such structured emission is typical, then the currently known sample of short GRBs with no distance measurements may contain multiple nearby off-axis events whose delayed afterglows could have gone undetected. These nearby neutron star mergers may produce telltale radio flares peaking years after the prompt GRB emission that could still be observable. Here, we show that several short GRBs observed by the Burst Alert Telescope (BAT) on the Neil Gehrels \textit{Swift} satellite, with no identified afterglow and no distance measurement, could potentially be associated with radio flares detectable by sensitive cm-wavelength radio facilities such as the Karl G. Jansky Very Large Array. We also examine optical follow-up observations that have been carried out for these events, and find that a nearby GW170817-like kilonova is ruled out for only a third of them.

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arXiv2019

STROBE-X: X-ray Timing and Spectroscopy on Dynamical Timescales from Microseconds to Years

We present the Spectroscopic Time-Resolving Observatory for Broadband Energy X-rays (STROBE-X), a probe-class mission concept selected for study by NASA. It combines huge collecting area, high throughput, broad energy coverage, and excellent spectral and temporal resolution in a single facility. STROBE-X offers an enormous increase in sensitivity for X-ray spectral timing, extending these techniques to extragalactic targets for the first time. It is also an agile mission capable of rapid response to transient events, making it an essential X-ray partner facility in the era of time-domain, multi-wavelength, and multi-messenger astronomy. Optimized for study of the most extreme conditions found in the Universe, its key science objectives include: (1) Robustly measuring mass and spin and mapping inner accretion flows across the black hole mass spectrum, from compact stars to intermediate-mass objects to active galactic nuclei. (2) Mapping out the full mass-radius relation of neutron stars using an ensemble of nearly two dozen rotation-powered pulsars and accreting neutron stars, and hence measuring the equation of state for ultradense matter over a much wider range of densities than explored by NICER. (3) Identifying and studying X-ray counterparts (in the post-Swift era) for multiwavelength and multi-messenger transients in the dynamic sky through cross-correlation with gravitational wave interferometers, neutrino observatories, and high-cadence time-domain surveys in other electromagnetic bands. (4) Continuously surveying the dynamic X-ray sky with a large duty cycle and high time resolution to characterize the behavior of X-ray sources over an unprecedentedly vast range of time scales. STROBE-X's formidable capabilities will also enable a broad portfolio of additional science.

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arXiv2019

Optical follow-up of gravitational wave events during the second advanced LIGO/VIRGO observing run with the DLT40 Survey

We describe the GW follow-up strategy and subsequent results of the DLT40 during the second science run (O2) of the LVC. Depending on the information provided in the GW alert together with the localization map sent by the LVC, DLT40 would respond promptly to image the corresponding galaxies selected by our ranking algorithm in order to search for possible EM counterparts in real time. During the LVC O2 run, DLT40 followed ten GW triggers, observing between $\sim$20-100 galaxies within the GW localization area of each event. From this campaign, we identified two real transient sources within the GW localizations with an appropriate on-source time -- one was an unrelated type Ia supernova (SN~2017cbv), and the other was the optical kilonova, AT 2017fgo/SSS17a/DLT17ck, associated with the binary neutron star coalescence GW170817 (a.k.a gamma-ray burst GRB170817A). We conclude with a discussion of the DLT40 survey's plans for the upcoming LVC O3 run, which include expanding our galaxy search fields out to $D\approx$65 Mpc to match the LVC's planned three-detector sensitivity for binary neutron star mergers.

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arXiv2018

Science with an ngVLA: Compact binary mergers as traced by gravitational waves

In light of the recent dazzling discovery of GW170817, we discuss several new scientific opportunities that would emerge in multi-messenger time-domain astrophysics if a facility like the next generation Very Large Array (ngVLA) were to work in tandem with ground-based gravitational wave (GW) detectors. These opportunities include probing wide-angle ejecta and off-axis afterglows of neutron star (NS)-NS mergers; enabling direct size measurements of radio ejecta from NS-NS mergers; and unraveling the physics behind the progenitors of compact binary mergers via host galaxy studies at radio wavelengths. Our results show that, thanks to its unprecedented sensitivity and resolution, the ngVLA will enable transformational results in the multi-messenger exploration of the transient radio sky.

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arXiv2018

Optimized Radio Follow Up of Binary Neutron-Star Mergers

Motivated by the recent discovery of the binary neutron-star (BNS) merger GW170817, we determine the optimal observational setup for detecting and characterizing radio counterparts of nearby ($d_L\sim40$\,Mpc) BNS mergers. We simulate GW170817-like radio transients, and radio afterglows generated by fast jets with isotropic energy $E_{\rm iso}\sim 10^{50}$\,erg, expanding in a low-density interstellar medium (ISM; $n_{\rm ISM}=10^{-4}-10^{-2}$\,cm$^{-3}$), observed from different viewing angles (from slightly off-axis to largely off-axis). We then determine the optimal timing of GHz radio observations following the precise localization of the BNS radio counterpart candidate, assuming a sensitivity comparable to that of the Karl G. Jansky Very Large Array. The optimization is done so as to ensure that properties such as viewing angle and circumstellar density can be correctly reconstructed with the minimum number of observations. We show that radio is the optimal band to explore the fastest ejecta from BNSs in low-density ISM, since the optical emission is likelyto be dominated by the so-called `kilonova' component, while X-rays from the jet are detectable only for a small subset of the BNS models considered here. Finally, we discuss how future radio arrays like the next generation VLA (ngVLA) would improve the detectability of BNS mergers with physical parameters similar to the ones here explored.

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arXiv2018

An upper-limit on the linear polarization fraction of the GW170817 radio continuum

We present late-time radio observations of GW170817, the first binary neutron star merger discovered through gravitational waves by the advanced LIGO and Virgo detectors. Our observations, carried out with the Karl G. Jansky Very Large Array, were optimized to detect polarized radio emission, and thus to constrain the linear polarization fraction of GW170817. At an epoch of ~244 days after the merger, we rule out linearly polarized emission above a fraction of ~12% at a frequency of 2.8 GHz (99% confidence). Within the structured jet scenario (a.k.a. successful jet plus cocoon system) for GW170817, the derived upper-limit on the radio continuum linear polarization fraction strongly constrains the magnetic field configuration in the shocked ejecta. We show that our results for GW170817 are compatible with the low level of linear polarization found in afterglows of cosmological long gamma-ray bursts. Finally, we discuss our findings in the context of future expectations for the study of radio counterparts of binary neutron star mergers identified by ground-based gravitational-wave detectors.

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arXiv2018

A turnover in the radio light curve of GW170817

We present 2-9 GHz radio observations of GW170817 covering the period 125-200 days post-merger, taken with the Australia Telescope Compact Array and the Karl G. Jansky Very Large Array. Our observations demonstrate that the radio afterglow peaked at $149\pm2$ days post-merger and is now declining in flux density. We see no evidence for evolution in the radio-only spectral index, which remains consistent with optically-thin synchrotron emission connecting the radio, optical, and X-ray regimes. The peak implies a total energy in the synchrotron-emitting component of a ${\rm few}\times 10^{50}\,$erg. The temporal decay rate is most consistent with mildly- or non-relativistic material and we do not see evidence for a very energetic off-axis jet, but we cannot distinguish between a lower-energy jet and more isotropic emission.

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