The recent discovery of a gamma-ray burst (GRB) coincident with the gravitational-wave (GW) event GW170817 revealed the existence of a population of low-luminosity short duration gamma-ray transients ...produced by neutron star mergers in the nearby Universe. These events could be routinely detected by existing gamma-ray monitors, yet previous observations failed to identify them without the aid of GW triggers. Here we show that GRB150101B is an analogue of GRB170817A located at a cosmological distance. GRB150101B is a faint short burst characterized by a bright optical counterpart and a long-lived X-ray afterglow. These properties are unusual for standard short GRBs and are instead consistent with an explosion viewed off-axis: the optical light is produced by a luminous kilonova, while the observed X-rays trace the GRB afterglow viewed at an angle of ~13°. Our findings suggest that these properties could be common among future electromagnetic counterparts of GW sources.
ABSTRACT
A significant fraction (30 per cent) of well-localized short gamma-ray bursts (sGRBs) lack a coincident host galaxy. This leads to two main scenarios: (i) that the progenitor system merged ...outside of the visible light of its host, or (ii) that the sGRB resided within a faint and distant galaxy that was not detected by follow-up observations. Discriminating between these scenarios has important implications for constraining the formation channels of neutron star mergers, the rate and environments of gravitational wave sources, and the production of heavy elements in the Universe. In this work, we present the results of our observing campaign targeted at 31 sGRBs that lack a putative host galaxy. Our study effectively doubles the sample of well-studied sGRB host galaxies, now totaling 72 events of which $28{{\ \rm per\ cent}}$ lack a coincident host to deep limits (r ≳ 26 or F110W ≳ 27 AB mag), and represents the largest homogeneously selected catalogue of sGRB offsets to date. We find that 70 per cent of sub-arcsecond localized sGRBs occur within 10 kpc of their host’s nucleus, with a median projected physical offset of 5.6 kpc. Using this larger population, we discover an apparent redshift evolution in their locations: bursts at low-z occur at 2 × larger offsets compared to those at z > 0.5. This evolution could be due to a physical evolution of the host galaxies themselves or a bias against faint high-z galaxies. Furthermore, we discover a sample of hostless sGRBs at z ≳ 1 that are indicative of a larger high-z population, constraining the redshift distribution and disfavoring lognormal delay time models.
ABSTRACT
We present a detailed multiwavelength analysis of two short gamma-ray bursts (sGRBs) detected by the Neil Gehrels Swift Observatory: GRB 160624A at $z$ = 0.483 and GRB 200522A at $z$ = ...0.554. These sGRBs demonstrate very different properties in their observed emission and environment. GRB 160624A is associated with a late-type galaxy with an old stellar population (≈3 Gyr) and moderate ongoing star formation (≈1 M⊙ yr−1). Hubble and Gemini limits on optical/near-infrared emission from GRB 160624A are among the most stringent for sGRBs, leading to tight constraints on the allowed kilonova properties. In particular, we rule out any kilonova brighter than AT2017gfo, disfavouring large masses of wind ejecta (≲0.03 M⊙). In contrast, observations of GRB 200522A uncovered a luminous (LF125W ≈ 1042 erg s−1 at 2.3 d) and red (r − H ≈ 1.3 mag) counterpart. The red colour can be explained either by bright kilonova emission powered by the radioactive decay of a large amount of wind ejecta (0.03 M⊙ ≲ M ≲ 0.1 M⊙) or moderate extinction, E(B − V) ≈ 0.1−0.2 mag, along the line of sight. The location of this sGRB in the inner regions of a young (≈0.1 Gyr) star-forming (≈2−6 M⊙ yr−1) galaxy and the limited sampling of its counterpart do not allow us to rule out dust effects as contributing, at least in part, to the red colour.
ABSTRACT
We report on our observing campaign of the compact binary merger GW190814, detected by the Advanced LIGO and Advanced Virgo detectors on 2019 August 14. This signal has the best localization ...of any observed gravitational wave (GW) source, with a 90 per cent probability area of 18.5 deg2, and an estimated distance of ≈240 Mpc. We obtained wide-field observations with the Deca-Degree Optical Transient Imager (DDOTI) covering 88 per cent of the probability area down to a limiting magnitude of w = 19.9 AB. Nearby galaxies within the high probability region were targeted with the Lowell Discovery Telescope (LDT), whereas promising candidate counterparts were characterized through multicolour photometry with the Reionization and Transients InfraRed (RATIR) and spectroscopy with the Gran Telescopio de Canarias (GTC). We use our optical and near-infrared limits in conjunction with the upper limits obtained by the community to constrain the possible electromagnetic counterparts associated with the merger. A gamma-ray burst seen along its jet’s axis is disfavoured by the multiwavelength data set, whereas the presence of a burst seen at larger viewing angles is not well constrained. Although our observations are not sensitive to a kilonova similar to AT2017gfo, we can rule out high-mass (>0.1 M⊙) fast-moving (mean velocity ≥0.3c) wind ejecta for a possible kilonova associated with this merger.
A significant fraction (30\%) of well-localized short gamma-ray bursts
(sGRBs) lack a coincident host galaxy. This leads to two main scenarios:
\textit{i}) that the progenitor system merged outside ...of the visible light of
its host, or \textit{ii}) that the sGRB resided within a faint and distant
galaxy that was not detected by follow-up observations. Discriminating between
these scenarios has important implications for constraining the formation
channels of neutron star mergers, the rate and environments of gravitational
wave sources, and the production of heavy elements in the Universe. In this
work, we present the results of our observing campaign targeted at 31 sGRBs
that lack a putative host galaxy. Our study effectively doubles the sample of
well-studied sGRB host galaxies, now totaling 72 events of which $28\%$ lack a
coincident host to deep limits ($r$\,$\gtrsim$\,$26$ or
$F110W$\,$\gtrsim$\,$27$ AB mag), and represents the largest homogeneously
selected catalog of sGRB offsets to date. We find that 70\% of sub-arcsecond
localized sGRBs occur within 10 kpc of their host's nucleus, with a median
projected physical offset of $5.6$ kpc. Using this larger population, we
discover an apparent redshift evolution in their locations: bursts at low-$z$
occur at $2\times$ larger offsets compared to those at $z$\,$>$\,$0.5$. This
evolution could be due to a physical evolution of the host galaxies themselves
or a bias against faint high-$z$ galaxies. Furthermore, we discover a sample of
hostless sGRBs at $z$\,$\gtrsim$\,$1$ that are indicative of a larger high-$z$
population, constraining the redshift distribution and disfavoring log-normal
delay time models.
We present a detailed multi-wavelength analysis of two short Gamma-Ray Bursts
(sGRBs) detected by the Neil Gehrels Swift Observatory: GRB 160624A at
$z=0.483$ and GRB 200522A at $z=0.554$. These ...sGRBs demonstrate very different
properties in their observed emission and environment. GRB 160624A is
associated to a late-type galaxy with an old stellar population ($\approx$3
Gyr) and moderate on-going star formation ($\approx$1 $M_{\odot}$ yr$^{-1}$).
Hubble and Gemini limits on optical/nIR emission from GRB 160624A are among the
most stringent for sGRBs, leading to tight constraints on the allowed kilonova
properties. In particular, we rule out any kilonova brighter than AT2017gfo,
disfavoring large masses of wind ejecta ($\lesssim$0.03 $M_\odot$). In
contrast, observations of GRB 200522A uncovered a luminous
($L_\textrm{F125W}\approx 10^{42}$ erg s$^{-1}$ at 2.3~d) and red ($r-H\approx
1.3$ mag) counterpart. The red color can be explained either by bright kilonova
emission powered by the radioactive decay of a large amount of wind ejecta
(0.03 $M_\odot$ $\lesssim$ $M$ $\lesssim$ 0.1 $M_\odot$) or moderate
extinction, $E(B-V)\approx0.1-0.2$ mag, along the line of sight. The location
of this sGRB in the inner regions of a young ($\approx$0.1 Gyr) star-forming
($\approx$2-6 $M_{\odot}$ yr$^{-1}$) galaxy and the limited sampling of its
counterpart do not allow us to rule out dust effects as contributing, at least
in part, to the red color.
The recent discovery of a faint gamma-ray burst (GRB) coincident with the gravitational wave (GW) event GW 170817 revealed the existence of a population of low-luminosity short duration gamma-ray ...transients produced by neutron star mergers in the nearby Universe. These events could be routinely detected by existing gamma-ray monitors, yet previous observations failed to identify them without the aid of GW triggers. Here we show that GRB150101B was an analogue of GRB170817A located at a cosmological distance. GRB 150101B was a faint short duration GRB characterized by a bright optical counterpart and a long-lived X-ray afterglow. These properties are unusual for standard short GRBs and are instead consistent with an explosion viewed off-axis: the optical light is produced by a luminous kilonova component, while the observed X-rays trace the GRB afterglow viewed at an angle of ~13 degrees. Our findings suggest that these properties could be common among future electromagnetic counterparts of GW sources.
A significant fraction (30\%) of well-localized short gamma-ray bursts (sGRBs) lack a coincident host galaxy. This leads to two main scenarios: \textit{i}) that the progenitor system merged outside ...of the visible light of its host, or \textit{ii}) that the sGRB resided within a faint and distant galaxy that was not detected by follow-up observations. Discriminating between these scenarios has important implications for constraining the formation channels of neutron star mergers, the rate and environments of gravitational wave sources, and the production of heavy elements in the Universe. In this work, we present the results of our observing campaign targeted at 31 sGRBs that lack a putative host galaxy. Our study effectively doubles the sample of well-studied sGRB host galaxies, now totaling 72 events of which \(28\%\) lack a coincident host to deep limits (\(r\)\,\(\gtrsim\)\,\(26\) or \(F110W\)\,\(\gtrsim\)\,\(27\) AB mag), and represents the largest homogeneously selected catalog of sGRB offsets to date. We find that 70\% of sub-arcsecond localized sGRBs occur within 10 kpc of their host's nucleus, with a median projected physical offset of \(5.6\) kpc. Using this larger population, we discover an apparent redshift evolution in their locations: bursts at low-\(z\) occur at \(2\times\) larger offsets compared to those at \(z\)\,\(>\)\,\(0.5\). This evolution could be due to a physical evolution of the host galaxies themselves or a bias against faint high-\(z\) galaxies. Furthermore, we discover a sample of hostless sGRBs at \(z\)\,\(\gtrsim\)\,\(1\) that are indicative of a larger high-\(z\) population, constraining the redshift distribution and disfavoring log-normal delay time models.
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