arXiv subjects

M. de Pasquale

Publications and source records attributed to M. de Pasquale.

16 recordsLinked to original sources

arXiv2025

The most distant optically polarised GRB afterglow: GRB 240419A at z = 5.178

Gamma-ray bursts (GRBs) are extremely bright phenomena powered by relativistic jets arising from explosive events at cosmological distances. The nature of the jet and the configuration of the local magnetic fields are still unclear, with the distinction between different models possibly provided by the detection of early-time polarisation. Past observations do not agree on a universal scenario describing early-time polarisation in GRB afterglows, and new studies are necessary to investigate this open question. We present here the discovery of GRB\,240419A, its redshift determination of $z=5.178$, its early-time optical polarimetry observations, and the multi-wavelength monitoring of its afterglow. We analysed three epochs of polarimetric data to derive the early-time evolution of the polarisation. The multi-wavelength light curve from the X-rays to the near-infrared band was also investigated to give a broader perspective on the whole event. We find a high level of polarisation, $P=6.97^{+1.84}_{-1.52}$\,\%, at 1740~s after the GRB trigger, followed by a slight decrease up to $P=4.81^{+1.87}_{-1.53}$\,\% at 3059~s. On the same timescale, the polarisation position angle is nearly constant. The multi-band afterglow at the time of the polarisation measurements is consistent with a forward shock (FS), while the earlier evolution at $t-t_0\lesssim700$ s can be associated with the interplay between the forward and the reverse shocks or with energy injection. The detected polarised radiation when the afterglow is FS-dominated and the stable position angle are consistent with an ordered magnetic field plus a turbulent component driven by large-scale magnetohydrodynamic instabilities. The lack of a jet break in the light curve prevents a comparison of the polarisation temporal evolution with theoretical expectations from magnetic fields amplified by microscopic-scale turbulence, limiting ...

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arXiv2020

Swift Multiwavelength Follow-up of LVC S200224ca and the Implications for Binary Black Hole Mergers

On 2020 February 24, during their third observing run ("O3"), the Laser Interferometer Gravitational-wave Observatory and Virgo Collaboration (LVC) detected S200224ca: a candidate gravitational wave (GW) event produced by a binary black hole (BBH) merger. This event was one of the best-localized compact binary coalescences detected in O3 (with 50%/90% error regions of 13/72 deg$^2$), and so the Neil Gehrels Swift Observatory performed rapid near-UV/X-ray follow-up observations. Swift-XRT and UVOT covered approximately 79.2% and 62.4% (respectively) of the GW error region, making S200224ca the BBH event most thoroughly followed-up in near-UV (u-band) and X-ray to date. No likely EM counterparts to the GW event were found by the Swift BAT, XRT, or UVOT, nor by other observatories. Here we report on the results of our searches for an EM counterpart, both in the BAT data near the time of the merger, and in follow-up UVOT/XRT observations. We also discuss the upper limits we can place on EM radiation from S200224ca, and the implications these limits have on the physics of BBH mergers. Namely, we place a shallow upper limit on the dimensionless BH charge, $\hat{q} < 1.4 \times10^{-4}$, and an upper limit on the isotropic-equivalent energy of a blast wave $E < 4.1\times10^{51}$ erg (assuming typical GRB parameters).

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arXiv2019

Swift-XRT Follow-up of Gravitational Wave Triggers in the Second Advanced LIGO/Virgo Observing Run

The Neil Gehrels Swift Observatory carried out prompt searches for gravitational wave (GW) events detected by the LIGO/Virgo Collaboration (LVC) during the second observing run ("O2"). Swift performed extensive tiling of eight LVC triggers, two of which had very low false-alarm rates (GW 170814 and the epochal GW 170817), indicating a high confidence of being astrophysical in origin; the latter was the first GW event to have an electromagnetic counterpart detected. In this paper we describe the follow-up performed during O2 and the results of our searches. No GW electromagnetic counterparts were detected; this result is expected, as GW 170817 remained the only astrophysical event containing at least one neutron star after LVC's later retraction of some events. A number of X-ray sources were detected, with the majority of identified sources being active galactic nuclei. We discuss the detection rate of transient X-ray sources and their implications in the O2 tiling searches. Finally, we describe the lessons learned during O2, and how these are being used to improve the \swift\ follow-up of GW events. In particular, we simulate a population of GRB afterglows to evaluate our source ranking system's ability to differentiate them from unrelated and uncatalogued X-ray sources. We find that $\approx$60-70% of afterglows whose jets are oriented towards Earth will be given high rank (i.e., "interesting" designation) by the completion of our second follow-up phase (assuming their location in the sky was observed), but that this fraction can be increased to nearly 100% by performing a third follow-up observation of sources exhibiting fading behavior.

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arXiv2017

Cosmic evolution and metal aversion in super-luminous supernova host galaxies

The SUperluminous Supernova Host galaxIES (SUSHIES) survey aims to provide strong new constraints on the progenitors of superluminous supernovae (SLSNe) by understanding the relationship to their host galaxies. We present the photometric properties of 53 H-poor and 16 H-rich SLSN host galaxies out to $z\sim4$. We model their spectral energy distributions to derive physical properties, which we compare with other galaxy populations. At low redshift, H-poor SLSNe are preferentially found in very blue, low-mass galaxies with high average specific star-formation rates. As redshift increases, the host population follows the general evolution of star-forming galaxies towards more luminous galaxies. After accounting for secular evolution, we find evidence for differential evolution in galaxy mass, but not in the $B$-band and the far UV luminosity ($3σ$ confidence). Most remarkable is the scarcity of hosts with stellar masses above $10^{10}~M_\odot$ for both classes of SLSNe. In the case of H-poor SLSNe, we attribute this to a stifled production efficiency above $\sim0.4$ solar metallicity. However, we argue that, in addition to low metallicity, a short-lived stellar population is also required to regulate the SLSN production. H-rich SLSNe are found in a very diverse population of star-forming galaxies. Still, the scarcity of massive hosts suggests a stifled production efficiency above $\sim0.8$ solar metallicity. The large dispersion of the H-rich SLSNe host properties is in stark contrast to those of gamma-ray burst, regular core-collapse SN, and H-poor SLSNe host galaxies. We propose that multiple progenitor channels give rise to this sub-class.

astro-ph.GA↗

arXiv2017

Swift and NuSTAR observations of GW170817: detection of a blue kilonova

With the first direct detection of merging black holes in 2015, the era of gravitational wave (GW) astrophysics began. A complete picture of compact object mergers, however, requires the detection of an electromagnetic (EM) counterpart. We report ultraviolet (UV) and X-ray observations by Swift and the Nuclear Spectroscopic Telescope ARray (NuSTAR) of the EM counterpart of the binary neutron star merger GW170817. The bright, rapidly fading ultraviolet emission indicates a high mass ($\approx0.03$ solar masses) wind-driven outflow with moderate electron fraction ($Y_{e}\approx0.27$). Combined with the X-ray limits, we favor an observer viewing angle of $\approx 30^{\circ}$ away from the orbital rotation axis, which avoids both obscuration from the heaviest elements in the orbital plane and a direct view of any ultra-relativistic, highly collimated ejecta (a gamma-ray burst afterglow).

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arXiv2016

A Correlation Between the Intrinsic Brightness and Average Decay Rate of Gamma-ray Burst X-ray Afterglow Light Curves

We present a correlation between the average temporal decay (αX,avg,>200s) and early-time luminosity (LX,200s) of X-ray afterglows of gamma-ray bursts as observed by Swift-XRT. Both quantities are measured relative to a rest frame time of 200 s after the γ-ray trigger. The luminosity average decay correlation does not depend on specific temporal behavior and contains one scale independent quantity minimizing the role of selection effects. This is a complementary correlation to that discovered by Oates et al. (2012) in the optical light curves observed by Swift-UVOT. The correlation indicates that on average, more luminous X-ray afterglows decay faster than less luminous ones, indicating some relative mechanism for energy dissipation. The X-ray and optical correlations are entirely consistent once corrections are applied and contamination is removed. We explore the possible biases introduced by different light curve morphologies and observational selection effects, and how either geometrical effects or intrinsic properties of the central engine and jet could explain the observed correlation.

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arXiv2014

Herschel/PACS Observations of the Host Galaxy of GRB 031203

We present Herschel/PACS observations of the nearby (z=0.1055) dwarf galaxy that has hosted the long gamma ray burst (LGRB) 031203. Using the PACS data we have been able to place constraints on the dust temperature, dust mass, total infrared luminosity and infrared-derived star-formation rate (SFR) for this object. We find that the GRB host galaxy (GRBH) 031203 has a total infrared luminosity of 3x10^10 L_sun placing it in the regime of the IR-luminous galaxy population. Its dust temperature and specific SFR are comparable to that of many high-redshift (z=0.3-2.5) infrared (IR)-detected GRB hosts (T_dust>40K ; sSFR>10 Gyr^-1), however its dust-to-stellar mass ratio is lower than what is commonly seen in IR-luminous galaxies. Our results suggest that GRBH 031203 is undergoing a strong starburst episode and its dust properties are different to those of local dwarf galaxies within the same metallicity and stellar mass range. Furthermore, our measurements place it in a distinct class to the well studied nearby host of GRB 980425 (z=0.0085), confirming the notion that GRB host galaxies can span a large range in properties even at similar cosmological epochs, making LGRBs an ideal tool in selecting samples of star-forming galaxies up to high redshift.

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arXiv2011

Fermi and Swift Gamma-Ray Burst Afterglow Population Studies

The new and extreme population of GRBs detected by Fermi-LAT shows several new features in high energy gamma-rays that are providing interesting and unexpected clues into GRB prompt and afterglow emission mechanisms. Over the last 6 years, it has been Swift that has provided the robust dataset of UV/optical and X-ray afterglow observations that opened many windows into components of GRB emission structure. The relationship between the LAT detected GRBs and the well studied, fainter, less energetic GRBs detected by Swift-BAT is only beginning to be explored by multi-wavelength studies. We explore the large sample of GRBs detected by BAT only, BAT and Fermi-GBM, and GBM and LAT, focusing on these samples separately in order to search for statistically significant differences between the populations, using only those GRBs with measured redshifts in order to physically characterize these objects. We disentangle which differences are instrumental selection effects versus intrinsic properties, in order to better understand the nature of the special characteristics of the LAT bursts.

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arXiv2010

On the electron energy distribution index of Swift Gamma-Ray Burst Afterglows

The electron energy distribution index, p, is a fundamental parameter of the synchrotron emission from a range of astronomical sources. Here we examine one such source of synchrotron emission, Gamma-Ray Burst afterglows observed by the Swift satellite. Within the framework of the blast wave model, we examine the constraints placed on the distribution of p by the observed X-ray spectral indices and parametrise the distribution. We find that the observed distribution of spectral indices are inconsistent with an underlying distribution of p composed of a single discrete value but consistent with a Gaussian distribution centred at p = 2.36 and having a width of 0.59. Furthermore, accepting that the underlying distribution is a Gaussian, we find the majority (>94%) of GRB afterglows in our sample have cooling break frequencies less than the X-ray frequency.

astro-ph.HE↗

arXiv2010

Paper II: Calibration of the Swift ultraviolet/optical telescope

The Ultraviolet/Optical Telescope (UVOT) is one of three instruments onboard the Swift observatory. The photometric calibration has been published, and this paper follows up with details on other aspects of the calibration including a measurement of the point spread function with an assessment of the orbital variation and the effect on photometry. A correction for large scale variations in sensitivity over the field of view is described, as well as a model of the coincidence loss which is used to assess the coincidence correction in extended regions. We have provided a correction for the detector distortion and measured the resulting internal astrometric accuracy of the UVOT, also giving the absolute accuracy with respect to the International Celestial Reference System. We have compiled statistics on the background count rates, and discuss the sources of the background, including instrumental scattered light. In each case we describe any impact on UVOT measurements, whether any correction is applied in the standard pipeline data processing or whether further steps are recommended.

astro-ph.IM↗

arXiv2009

A statistical study of gamma-ray burst afterglows measured by the Swift Ultra-violet Optical Telescope

We present the first statistical analysis of 27 UVOT optical/ultra-violet lightcurves of GRB afterglows. We have found, through analysis of the lightcurves in the observer's frame, that a significant fraction rise in the first 500s after the GRB trigger, that all lightcurves decay after 500s, typically as a power-law with a relatively narrow distribution of decay indices, and that the brightest optical afterglows tend to decay the quickest. We find that the rise could either be produced physically by the start of the forward shock, when the jet begins to plough into the external medium, or geometrically where an off-axis observer sees a rising lightcurve as an increasing amount of emission enters the observers line of sight, which occurs as the jet slows. We find that at 99.8% confidence, there is a correlation, in the observed frame, between the apparent magnitude of the lightcurves at 400s and the rate of decay after 500s. However, in the rest frame a Spearman Rank test shows only a weak correlation of low statistical significance between luminosity and decay rate. A correlation should be expected if the afterglows were produced by off-axis jets, suggesting that the jet is viewed from within the half-opening angle theta or within a core of uniform energy density theta_c. We also produced logarithmic luminosity distributions for three rest frame epochs. We find no evidence for bimodality in any of the distributions. Finally, we compare our sample of UVOT lightcurves with the XRT lightcurve canonical model. The range in decay indices seen in UVOT lightcurves at any epoch is most similar to the range in decay of the shallow decay segment of the XRT canonical model. However, in the XRT canonical model there is no indication of the rising behaviour observed in the UVOT lightcurves.

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arXiv2008

Optimal Coaddition of Imaging Data for Rapidly Fading Gamma-Ray Burst Afterglows

We present a technique for optimal coaddition of image data for rapidly varying sources, with specific application to gamma-ray burst (GRB) afterglows. Unweighted coaddition of rapidly fading afterglow lightcurve data becomes counterproductive relatively quickly. It is better to stop coaddition of the data once noise dominates late exposures. A better alternative is to optimally weight each exposure to maximize the signal-to-noise ratio (S/N) of the final coadded image data. By using information about GRB lightcurves and image noise characteristics, optimal image coaddition increases the probability of afterglow detection and places the most stringent upper limits on non-detections. For a temporal power law flux decay typical of GRB afterglows, optimal coaddition has the greatest potential to improve the S/N of afterglow imaging data (relative to unweighted coaddition), when the decay rate is high, the source count rate is low, and the background rate is high. The optimal coaddition technique is demonstrated with applications to Swift Ultraviolet/Optical Telescope (UVOT) data of several GRBs, with and without detected afterglows.

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arXiv2008

An extremely luminous X-ray outburst at the birth of a supernova

Massive stars end their short lives in spectacular explosions, supernovae, that synthesize new elements and drive galaxy evolution. Throughout history supernovae were discovered chiefly through their delayed optical light, preventing observations in the first moments (hours to days) following the explosion. As a result, the progenitors of some supernovae and the events leading up to their violent demise remain intensely debated. Here we report the serendipitous discovery of a supernova at the time of explosion, marked by an extremely luminous X-ray outburst. We attribute the outburst to the break-out of the supernova shock-wave from the progenitor, and show that the inferred rate of such events agrees with that of all core-collapse supernovae. We forecast that future wide-field X-ray surveys will catch hundreds of supernovae each year in the act of explosion, and thereby enable crucial neutrino and gravitational wave detections that may ultimately unravel the explosion mechanism.

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arXiv2007

Photometric Calibration of the Swift Ultraviolet/Optical Telescope

We present the photometric calibration of the Swift UltraViolet/Optical Telescope (UVOT) which includes: optimum photometric and background apertures, effective area curves, colour transformations, conversion factors for count rates to flux, and the photometric zero points (which are accurate to better than 4 per cent) for each of the seven UVOT broadband filters. The calibration was performed with observations of standard stars and standard star fields that represent a wide range of spectral star types. The calibration results include the position dependent uniformity, and instrument response over the 1600-8000A operational range. Because the UVOT is a photon counting instrument, we also discuss the effect of coincidence loss on the calibration results. We provide practical guidelines for using the calibration in UVOT data analysis. The results presented here supersede previous calibration results.

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arXiv2005

Multi-wavelength analysis of the field of the dark burst GRB 031220

We have collected and analyzed data taken in different spectral bands (from X-ray to optical and infrared) of the field of GRB031220 and we present results of such multiband observations. Comparison between images taken at different epochs in the same filters did not reveal any strong variable source in the field of this burst. X-ray analysis shows that only two of the seven Chandra sources have a significant flux decrease and seem to be the most likely afterglow candidates. Both sources do not show the typical values of the R-K colour but they appear to be redder. However, only one source has an X-ray decay index (1.3 +/- 0.1) that is typical for observed afterglows. We assume that this source is the best afterglow candidate and we estimate a redshift of 1.90 +/- 0.30. Photometric analysis and redshift estimation for this object suggest that this GRB can be classified as a Dark Burst and that the obscuration is the result of dust extinction in the circum burst medium or inside the host galaxy.

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arXiv2005

Ultraviolet, Optical, and X-Ray Observations of the Type Ia Supernova 2005am with Swift

We present ultraviolet and optical light curves in six broadband filters and grism spectra obtained by Swift's Ultraviolet/Optical Telescope for the Type Ia supernova SN2005am. The data were collected beginning about four days before the B-band maximum, with excellent coverage of the rapid decline phase and later observations extending out to 69 days after the peak. The optical and near UV light curve match well those of SN1992A. The other UV observations constitute the first set of light curves shorter than 2500 Angstroms and allow us to compare the light curve evolution in three UV bands. The UV behavior of this and other low redshift supernovae can be used to constrain theories of progenitor evolution or to interpret optical light curves of high redshift supernovae. Using Swift's X-Ray Telescope, we also report the upper limit to SN2005am's X-ray luminosity to be 1.77 x 10^40 erg s^-1 in the 0.3--10 keV range from 58,117 s of exposure time.

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