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 high-cadence multifrequency radio observations of the long gamma-ray burst (GRB) 190829A, which was detected at photon energies above 100 GeV by the High Energy Stereoscopic ...System (H.E.S.S.). Observations with the Meer Karoo Array Telescope (MeerKAT, 1.3 GHz) and Arcminute Microkelvin Imager – Large Array (AMI-LA, 15.5 GHz) began one day post-burst and lasted nearly 200 d. We used complementary data from Swift X-Ray Telescope (XRT), which ran to 100 d post-burst. We detected a likely forward shock component with both MeerKAT and XRT up to over 100 d post-burst. Conversely, the AMI-LA light curve appears to be dominated by reverse shock emission until around 70 d post-burst when the afterglow flux drops below the level of the host galaxy. We also present previously unpublished observations of the other H.E.S.S.-detected GRB, GRB 180720B from AMI-LA, which shows likely forward shock emission that fades in less than 10 d. We present a comparison between the radio emission from the three GRBs with detected very high energy (VHE) gamma-ray emission and a sensitivity-limited radio afterglow sample. GRB 190829A has the lowest isotropic radio luminosity of any GRB in our sample, but the distribution of luminosities is otherwise consistent, as expected, with the VHE GRBs being drawn from the same parent distribution as the other radio-detected long GRBs.
Magnetars are neutron stars with extremely strong magnetic fields (10
to 10
gauss)
, which episodically emit X-ray bursts approximately 100 milliseconds long and with energies of 10
to 10
erg. ...Occasionally, they also produce extremely bright and energetic giant flares, which begin with a short (roughly 0.2 seconds), intense flash, followed by fainter, longer-lasting emission that is modulated by the spin period of the magnetar
(typically 2 to 12 seconds). Over the past 40 years, only three such flares have been observed in our local group of galaxies
, and in all cases the extreme intensity of the flares caused the detectors to saturate. It has been proposed that extragalactic giant flares are probably a subset
of short γ-ray bursts, given that the sensitivity of current instrumentation prevents us from detecting the pulsating tail, whereas the initial bright flash is readily observable out to distances of around 10 to 20 million parsecs. Here we report X-ray and γ-ray observations of the γ-ray burst GRB 200415A, which has a rapid onset, very fast time variability, flat spectra and substantial sub-millisecond spectral evolution. These attributes match well with those expected for a giant flare from an extragalactic magnetar
, given that GRB 200415A is directionally associated
with the galaxy NGC 253 (roughly 3.5 million parsecs away). The detection of three-megaelectronvolt photons provides evidence for the relativistic motion of the emitting plasma. Radiation from such rapidly moving gas around a rotating magnetar may have generated the rapid spectral evolution that we observe.
Low-Magnetic-Field Soft Gamma Repeater Rea, N; Esposito, P; Turolla, R ...
Science (American Association for the Advancement of Science),
11/2010, Letnik:
330, Številka:
6006
Journal Article
Recenzirano
Odprti dostop
Soft gamma repeaters (SGRs) and anomalous x-ray pulsars form a rapidly increasing group of x-ray sources exhibiting sporadic emission of short bursts. They are believed to be magnetars, that is, ...neutron stars powered by extreme magnetic fields, B approximately 10¹⁴ to 10¹⁵ gauss. We report on a soft gamma repeater with low magnetic field, SGR 0418+5729, recently detected after it emitted bursts similar to those of magnetars. X-ray observations show that its dipolar magnetic field cannot be greater than 7.5 x 10¹² gauss, well in the range of ordinary radio pulsars, implying that a high surface dipolar magnetic field is not necessarily required for magnetar-like activity. The magnetar population may thus include objects with a wider range of B-field strengths, ages, and evolutionary stages than observed so far.
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 the study of 14 XMM-Newton observations of the magnetar SGR 1806-20 spread over a period of 8 years, starting in 2003 and extending to 2011. We find that in mid 2005, a year and ...a half after a giant flare (GF), the torques on the star increased to the largest value yet seen, with a long term average rate between 2005 and 2011 of Hz s−1, an order of magnitude larger than its historical level measured in 1995. The pulse morphology of the source is complex in the observations following the GF, while its pulsed-fraction remained constant at about 7% in all observations. Spectrally, the combination of a blackbody (BB) and power-law (PL) components is an excellent fit to all observations. The BB and PL fluxes increased by a factor of 2.5 and 4, respectively, while the spectra hardened, in concordance with the 2004 major outburst that preceded the GF. The fluxes decayed exponentially back to quiescence with a characteristic timescale of years, although they did not reach a constant value until at least 3.5 years later (2009). The long-term timing and spectral behavior of the source point to a decoupling between the mechanisms responsible for their respective behavior. We argue that low level seismic activity causing small twists in the open field lines can explain the long lasting large torques on the star, while the spectral behavior is due to a twist imparted onto closed field lines after the 2004 large outburst.
GRB 130427A was extremely bright as a result of occurring at low redshift whilst the energetics were more typical of high-redshift gamma-ray bursts (GRBs). We collected well-sampled light curves at ...1.4 and 4.8 GHz of GRB 130427A with the Westerbork Synthesis Radio Telescope (WSRT); and we obtained its most accurate position with the European Very Long Baseline Interferometry Network (EVN). Our flux density measurements are combined with all the data available at radio, optical and X-ray frequencies to perform broad-band modelling in the framework of a reverse–forward shock model and a two-component jet model, and we discuss the implications and limitations of both models. The low density inferred from the modelling implies that the GRB 130427A progenitor is either a very low metallicity Wolf–Rayet star, or a rapidly rotating, low-metallicity O star. We also find that the fraction of the energy in electrons is evolving over time, and that the fraction of electrons participating in a relativistic power-law energy distribution is less than 15 per cent. We observed intraday variability during the earliest WSRT observations, and the source sizes inferred from our modelling are consistent with this variability being due to interstellar scintillation effects. Finally, we present and discuss our limits on the linear and circular polarization, which are among the deepest limits of GRB radio polarization to date.
ABSTRACT We report on the analysis of two deep XMM-Newton observations of the magnetar Swift J1834.9−0846 and its surrounding extended emission taken in 2014 March and October, 2.5 and 3.1 yr after ...the source went into outburst. The magnetar is only weakly detected in the first observation, with an absorption-corrected flux erg s−1 cm−2 and a upper limit during the second observation of about 3 × 10−14 erg s−1 cm−2. This flux level is more than 3 orders of magnitude lower than the flux measured at the outburst onset in 2011 September. The extended emission, centered at the magnetar position and elongated toward the southwest, is clearly seen in both observations; it is best fit by a highly absorbed power law (PL), with a hydrogen column density of cm−2 and PL photon index . Its flux is constant between the two observations at erg s−1 cm−2. We find no statistically significant changes in the spectral shape or the flux of this extended emission over a period of 9 yr from 2005 to 2014. These new results strongly support the extended emission nature as a wind nebula and firmly establish Swift J1834.9−0846 as the first magnetar to show a surrounding wind nebula. Further, our results imply that such nebulae are no longer exclusive to rotation-powered pulsars and narrow the gap between these two subpopulations of isolated neutron stars. The size and spectrum of the nebula are compatible with those of pulsar-wind nebulae, but its radiative efficiency is markedly high, possibly pointing to an additional wind component in Swift J1834.9−0846.
Long γ-ray bursts are associated with energetic, broad-lined, stripped-envelope supernovae
and as such mark the death of massive stars. The scarcity of such events nearby and the brightness of the ...γ-ray burst afterglow, which dominates the emission in the first few days after the burst, have so far prevented the study of the very early evolution of supernovae associated with γ-ray bursts
. In hydrogen-stripped supernovae that are not associated with γ-ray bursts, an excess of high-velocity (roughly 30,000 kilometres per second) material has been interpreted as a signature of a choked jet, which did not emerge from the progenitor star and instead deposited all of its energy in a thermal cocoon
. Here we report multi-epoch spectroscopic observations of the supernova SN 2017iuk, which is associated with the γ-ray burst GRB 171205A. Our spectra display features at extremely high expansion velocities (around 115,000 kilometres per second) within the first day after the burst
. Using spectral synthesis models developed for SN 2017iuk, we show that these features are characterized by chemical abundances that differ from those observed in the ejecta of SN 2017iuk at later times. We further show that the high-velocity features originate from the mildly relativistic hot cocoon that is generated by an ultra-relativistic jet within the γ-ray burst expanding and decelerating into the medium that surrounds the progenitor star
. This cocoon rapidly becomes transparent
and is outshone by the supernova emission, which starts to dominate the emission three days after the burst.
GRB 120323A is a very intense short gamma -ray burst (GRB) detected simultaneously during its prompt γ-ray emission phase with the Gamma-ray Burst Monitor (GBM) on board the Fermi Gamma-ray Space ...Telescope and the Konus experiment on board the Wind satellite. GBM and Konus operate in the keV-MeV regime; however, the GBM range is broader toward both the low and the high parts of the γ-ray spectrum. Analyses of such bright events provide a unique opportunity to check the consistency of the data analysis as well as cross-calibrate the two instruments. We performed time-integrated and coarse time-resolved spectral analysis of GRB 120323A prompt emission. We conclude that the analyses of GBM and Konus data are only consistent when using a double-hump spectral shape for both data sets; in contrast, the single hump of the empirical Band function, traditionally used to fit GRB prompt emission spectra, leads to significant discrepancies between GBM and Konus analysis results. Our two-hump model is a combination of a thermal-like and a non-thermal component. We interpret the first component as a natural manifestation of the jet photospheric emission.
PDF
