We present very early UV to optical photometric and spectroscopic observations of the peculiar Type IIn supernova (SN) 2011ht in UGC 5460. The UV observations of the rise to peak are only the second ...ever recorded for a Type IIn SN and are by far the most complete. The optical spectra are dominated by strong, Balmer emission with narrow peaks (FWHM ~ 600 km s super(-1)), very broad asymmetric wings (FWHM ~ 4200 km s super(-1)), and blueshifted absorption (~300 km s super(-1)) superposed on a strong blue continuum. The UV spectra are dominated by Fe II, MgII, Si II, and Si III absorption lines broadened by ~1500 km s super(-1). Merged X-ray observations reveal a L sub(0.2-10) = (1.0 + or - 0.2) x 10 super(39) erg s super(-1). Some properties of SN 2011ht are similar to SN impostors, while others are comparable to Type IIn SNe. Early spectra showed features typical of luminous blue variables at maximum and during giant eruptions.
We present the observations of GRB090510 performed by the Fermi Gamma-Ray Space Telescope and the Swift observatory. This is a bright, short burst that shows an extended emission detected in the GeV ...range. Furthermore, its optical emission initially rises, a feature so far observed only in long bursts, while the X-ray flux shows an initial shallow decrease, followed by a steeper decay. This exceptional behavior enables us to investigate the physical properties of the gamma-ray burst outflow, poorly known in short bursts. We discuss internal and external shock models for the broadband energy emission of this object.
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 \(\sim 1.7\,{\rm{s}}\) 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 \(\,{M}_{\odot }\). 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 \(\sim 40\,{\rm{Mpc}}\)) 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 \(\sim 9\) and \(\sim 16\) 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 NGC 4993 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.
We present the systematic analysis of the Ultraviolet/Optical Telescope (UVOT) and X-ray Telescope (XRT) light curves for a sample of 26 Swift gamma-ray bursts (GRBs). By comparing the optical/UV and ...X-ray light curves, we found that they are remarkably different during the first 500 s after the Burst Alert Telescope trigger, while they become more similar during the middle phase of the afterglow, i.e. between 2000 and 20 000 s.
If we take literally the average properties of the sample, we find that the mean temporal indices observed in the optical/UV and X-rays after 500 s are consistent with a forward-shock scenario, under the assumptions that electrons are in the slow cooling regime, the external medium is of constant density and the synchrotron cooling frequency is situated between the optical/UV and X-ray observing bands. While this scenario describes well the averaged observed properties, some individual GRB afterglows require different or additional assumptions, such as the presence of late energy injection.
We show that a chromatic break (a break in the X-ray light curve that is not seen in the optical) is present in the afterglows of three GRBs and demonstrate evidence for chromatic breaks in a further four GRBs. The average properties of these breaks cannot be explained in terms of the passage of the synchrotron cooling frequency through the observed bands, nor a simple change in the external density. It is difficult to reconcile chromatic breaks in terms of a single component outflow and instead, more complex jet structure or additional emission components are required.
The origins of gamma-ray burst (GRB) prompt emission are currently not well understood and in this context long, well-observed events are particularly important to study. We present the case of GRB ...070616, analysing the exceptionally long-duration multipeaked prompt emission, and later afterglow, captured by all the instruments on-board Swift and by Suzaku Wide-Band All-Sky Monitor (WAM). The high-energy light curve remained generally flat for several hundred seconds before going into a steep decline. Spectral evolution from hard to soft is clearly taking place throughout the prompt emission, beginning at 285 s after the trigger and extending to 1200 s. We track the movement of the spectral peak energy, whilst observing a softening of the low-energy spectral slope. The steep decline in flux may be caused by a combination of this strong spectral evolution and the curvature effect. We investigate origins for the spectral evolution, ruling out a superposition of two power laws and considering instead an additional component dominant during the late prompt emission. We also discuss origins for the early optical emission and the physics of the afterglow. The case of GRB 070616 clearly demonstrates that both broad-band coverage and good time resolution are crucial to pin down the origins of the complex prompt emission in GRBs.
The probability of photon measurement in some photon-counting instrumentation, such as the Optical Monitor (OM) on the XMM–Newton satellite, and the Ultraviolet/Optical Telescope (UVOT) on the Swift ...satellite, does not follow a Poisson distribution due to the detector characteristics, but a binomial distribution. For a single-pixel approximation, an expression was derived for the incident count rate as a function of the measured count rate by Fordham, Moorhead & Galbraith. We show that the measured count rate error is binomial, and extend their formalism to derive the error in the incident count rate. The error on the incident count rate at large count rates is larger than the Poisson error of the incident count rate.
We present the earliest ultraviolet (UV) spectrum of a gamma-ray burst (GRB) as observed with the Swift Ultra-Violet/Optical Telescope (UVOT). The GRB 081203A spectrum was observed for 50 s with the ...UV-grism starting 251 s after the Swift-Burst-Alert-Telescope (BAT) trigger. During this time, the GRB was ≈13.4 mag (u filter) and was still rising to its peak optical brightness. In the UV-grism spectrum, we find a damped Lyα line, Lyβ and the Lyman continuum break at a redshift z= 2.05 ± 0.01. A model fit to the Lyman absorption implies a gas column density of log NH i= 22.0 ± 0.1 cm−2, which is typical of GRB host galaxies with damped Lyα absorbers. This observation of GRB 081203A demonstrates that for brighter GRBs (v≈ 14 mag) with moderate redshift (0.5 < z < 3.5) the UVOT is able to provide redshifts, and probe for damped Lyα absorbers within 4–6 min from the time of the Swift-BAT trigger.
We present Swift UV/Optical Telescope (UVOT) imaging of the galaxies M81 and Holmberg IX. We combine UVOT imaging in three near-ultraviolet (NUV) filters (uvw2: 1928 A; uvm2: 2246 A; uvw1: 2600 A) ...with ground-based optical imaging from the Sloan Digital Sky Survey to constrain the stellar populations of both galaxies. Our analysis consists of three different methods. First, we use the NUV imaging to identify UV star-forming knots and then perform spectral energy distribution (SED) modeling on the UV/optical photometry of these sources. Second, we measure surface brightness profiles of the disk of M81 in the NUV and optical. Lastly, we use SED fitting of individual pixels to map the properties of the two galaxies. In agreement with earlier studies, we find evidence for a burst in star formation in both galaxies starting ~200 Myr ago coincident with the suggested time of an M81-M82 interaction. In line with theories of its origin as a tidal dwarf, we find that the luminosity-weighted age of Holmberg IX is a few hundred million years. Both galaxies are best fit by a Milky Way dust extinction law with a prominent 2175 A bump. In addition, we describe a stacked median filter technique for modeling the diffuse background light within a galaxy and a Markov chain method for cleaning segment maps generated by SExtractor.
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