Discovery of the peculiar supernova 1998bw in the error box of GRB980425

The discovery of X-ray, optical and radio afterglows of gamma-ray bursts (GRBs) and the measurements of the distances to some of them have established that these events come from Gpc distances and are the most powerful photon emitters known in the Universe, with peak luminosities up to 10^52 erg/s. We here report the discovery of an optical transient, in the BeppoSAX Wide Field Camera error box of GRB980425, which occurred within about a day of the gamma-ray burst. Its optical light curve, spectrum and location in a spiral arm of the galaxy ESO 184-G82, at a redshift z = 0.0085, show that the transient is a very luminous type Ic supernova, SN1998bw. The peculiar nature of SN1998bw is emphasized by its extraordinary radio properties which require that the radio emitter expand at relativistical speed. Since SN1998bw is very different from all previously observed afterglows of GRBs, our discovery raises the possibility that very different mechanisms may give rise to GRBs, which differ little in their gamma-ray properties.Comment: Under press embargo at Nature (submitted June 10, 1998

Spectrophotometric analysis of gamma-ray burst afterglow extinction curves with X-shooter

In this work we use gamma-ray burst (GRB) afterglow spectra observed with the VLT/X-shooter spectrograph to measure rest-frame extinction in GRB lines-of-sight by modeling the broadband near-infrared (NIR) to X-ray afterglow spectral energy distributions (SEDs). Our sample consists of nine Swift GRBs, eight of them belonging to the long-duration and one to the short-duration class. Dust is modeled using the average extinction curves of the Milky Way and the two Magellanic Clouds. We derive the rest-frame extinction of the entire sample, which fall in the range $0 \lesssim {\it A}_{\rm V} \lesssim 1.2$. Moreover, the SMC extinction curve is the preferred extinction curve template for the majority of our sample, a result which is in agreement with those commonly observed in GRB lines-of-sights. In one analysed case (GRB 120119A), the common extinction curve templates fail to reproduce the observed extinction. To illustrate the advantage of using the high-quality X-shooter afterglow SEDs over the photometric SEDs, we repeat the modeling using the broadband SEDs with the NIR-to-UV photometric measurements instead of the spectra. The main result is that the spectroscopic data, thanks to a combination of excellent resolution and coverage of the blue part of the SED, are more successful in constraining the extinction curves and therefore the dust properties in GRB hosts with respect to photometric measurements. In all cases but one the extinction curve of one template is preferred over the others. We show that the modeled values of the extinction and the spectral slope, obtained through spectroscopic and photometric SED analysis, can differ significantly for individual events. Finally we stress that, regardless of the resolution of the optical-to-NIR data, the SED modeling gives reliable results only when the fit is performed on a SED covering a broader spectral region

What do γ\gamma-ray bursts look like?

There have been great and rapid progresses in the field of $\gamma$-ray bursts (denoted as GRBs) since BeppoSAX and other telescopes discovered their afterglows in 1997. Here, we will first give a brief review on the observational facts of GRBs and direct understanding from these facts, which lead to the standard fireball model. The dynamical evolution of the fireball is discussed, especially a generic model is proposed to describe the whole dynamical evolution of GRB remnant from highly radiative to adiabatic, and from ultra-relativistic to non-relativistic phase. Then, Various deviations from the standard model are discussed to give new information about GRBs and their environment. In order to relax the energy crisis, the beaming effects and their possible observational evidences are also discussed in GRB's radiations.Comment: 10 pages, Latex. Invited talk at the Pacific Rim Conference on Stellar Astrophysics, Hong Kong, China, Aug. 199

The unpolarized macronova associated with the gravitational wave event GW 170817

The merger of two dense stellar remnants including at least one neutron star (NS) is predicted to produce gravitational waves (GWs) and short duration gamma ray bursts (GRBs). In the process, neutron-rich material is ejected from the system and heavy elements are synthesized by r-process nucleosynthesis. The radioactive decay of these heavy elements produces additional transient radiation termed "kilonova" or "macronova". We report the detection of linear optical polarization P = (0.50 +/- 0.07)% at 1.46 days after detection of the GWs from GW170817, a double neutron star merger associated with an optical macronova counterpart and a short GRB. The optical emission from a macronova is expected to be characterized by a blue, rapidly decaying, component and a red, more slowly evolving, component due to material rich of heavy elements, the lanthanides. The polarization measurement was made when the macronova was still in its blue phase, during which there is an important contribution from a lanthanide-free outflow. The low degree of polarization is consistent with intrinsically unpolarized emission scattered by Galactic dust, suggesting a symmetric geometry of the emitting region and low inclination of the merger system. Stringent upper limits to the polarization degree from 2.45 - 9.48 days post-burst are consistent with the lanthanides-rich macronova interpretation.Comment: 18 pages, 1 figure, 2 tables, Nature Astronomy, in pres

The Environment of the Binary Neutron Star Merger GW170817

We present Hubble Space Telescope (HST) and Chandra imaging, combined with Very Large Telescope MUSE integral field spectroscopy of the counterpart and host galaxy of the first binary neutron star merger detected via gravitational-wave emission by LIGO and Virgo, GW170817. The host galaxy, NGC 4993, is an S0 galaxy at z = 0.009783. There is evidence for large, face-on spiral shells in continuum imaging, and edge-on spiral features visible in nebular emission lines. This suggests that NGC 4993 has undergone a relatively recent ($\lesssim 1$ Gyr) "dry" merger. This merger may provide the fuel for a weak active nucleus seen in Chandra imaging. At the location of the counterpart, HST imaging implies there is no globular or young stellar cluster, with a limit of a few thousand solar masses for any young system. The population in the vicinity is predominantly old with lesssim1% of any light arising from a population with ages $\lt 500\,\mathrm{Myr}$. Both the host galaxy properties and those of the transient location are consistent with the distributions seen for short-duration gamma-ray bursts, although the source position lies well within the effective radius (${r}_{e}\sim 3$ kpc), providing an r e -normalized offset that is closer than $\sim 90 \%$ of short GRBs. For the long delay time implied by the stellar population, this suggests that the kick velocity was significantly less than the galaxy escape velocity. We do not see any narrow host galaxy interstellar medium features within the counterpart spectrum, implying low extinction, and that the binary may lie in front of the bulk of the host galaxy

The Emergence of a Lanthanide-rich Kilonova Following the Merger of Two Neutron Stars

We report the discovery and monitoring of the near-infrared counterpart (AT2017gfo) of a binary neutron-star merger event detected as a gravitational wave source by Advanced LIGO/Virgo (GW170817) and as a short gamma-ray burst by Fermi/GBM and Integral/SPI-ACS (GRB170817A). The evolution of the transient light is consistent with predictions for the behaviour of a "kilonova/macronova", powered by the radioactive decay of massive neutron-rich nuclides created via r-process nucleosynthesis in the neutron-star ejecta. In particular, evidence for this scenario is found from broad features seen in Hubble Space Telescope infrared spectroscopy, similar to those predicted for lanthanide dominated ejecta, and the much slower evolution in the near-infrared Ks-band compared to the optical. This indicates that the late-time light is dominated by high-opacity lanthanide-rich ejecta, suggesting nucleosynthesis to the 3rd r-process peak (atomic masses A~195). This discovery confirms that neutron-star mergers produce kilo-/macronovae and that they are at least a major - if not the dominant - site of rapid neutron capture nucleosynthesis in the universe

Accreting Millisecond X-Ray Pulsars

Accreting Millisecond X-Ray Pulsars (AMXPs) are astrophysical laboratories without parallel in the study of extreme physics. In this chapter we review the past fifteen years of discoveries in the field. We summarize the observations of the fifteen known AMXPs, with a particular emphasis on the multi-wavelength observations that have been carried out since the discovery of the first AMXP in 1998. We review accretion torque theory, the pulse formation process, and how AMXP observations have changed our view on the interaction of plasma and magnetic fields in strong gravity. We also explain how the AMXPs have deepened our understanding of the thermonuclear burst process, in particular the phenomenon of burst oscillations. We conclude with a discussion of the open problems that remain to be addressed in the future.Comment: Review to appear in "Timing neutron stars: pulsations, oscillations and explosions", T. Belloni, M. Mendez, C.M. Zhang Eds., ASSL, Springer; [revision with literature updated, several typos removed, 1 new AMXP added

A LOFAR DETECTION of the LOW-MASS YOUNG STAR T TAU at 149 MHz

© 2017 Published by Elsevier B.V. Radio observations of young stellar objects (YSOs) enable the study of ionized plasma outflows from young protostars via their free-free radiation. Previous studies of the low-mass young system T Tau have used radio observations to model the spectrum and estimate important physical properties of the associated ionized plasma (local electron density, ionized gas content, and emission measure). However, without an indication of the low-frequency turnover in the free-free spectrum, these properties remain difficult to constrain. This paper presents the detection of T Tau at 149 MHz with the Low Frequency Array (LOFAR)-the first time a YSO has been observed at such low frequencies. The recovered total flux indicates that the free-free spectrum may be turning over near 149 MHz. The spectral energy distribution is fitted and yields improved constraints on local electron density ((7.2 ± 2.1) × 103 cm-3), ionized gas mass ( ± × -1.0 1.8 10-6Ṁ), and emission measure ((1.67 ± 0.14) × 105 pc cm-6)

Multi-messenger observations of a binary neutron star merger

On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ~1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40+8-8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 Mo. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ~40 Mpc) less than 11 hours after the merger by the One- Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ~10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ~9 and ~16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta

Localization and Broadband Follow-Up of the Gravitational-Wave Transient GW150914

A gravitational-wave (GW) transient was identified in data recorded by the Advanced Laser InterferometerGravitational-wave Observatory (LIGO) detectors on 2015 September 14. The event, initially designated G184098and later given the name GW150914, is described in detail elsewhere. By prior arrangement, preliminary estimatesof the time, significance, and sky location of the event were shared with 63 teams of observers covering radio,optical, near-infrared, X-ray, and gamma-ray wavelengths with ground- and space-based facilities. In this Letter wedescribe the low-latency analysis of the GW data and present the sky localization of the first observed compactbinary merger. We summarize the follow-up observations reported by 25 teams via private Gamma-rayCoordinates Network circulars, giving an overview of the participating facilities, the GW sky localizationcoverage, the timeline, and depth of the observations. As this event turned out to be a binary black hole merger,there is little expectation of a detectable electromagnetic (EM) signature. Nevertheless, this first broadbandcampaign to search for a counterpart of an Advanced LIGO source represents a milestone and highlights the broadcapabilities of the transient astronomy community and the observing strategies that have been developed to pursueneutron star binary merger events. Detailed investigations of the EM data and results of the EM follow-upcampaign are being disseminated in papers by the individual teams