Space Telescope and Optical Reverberation Mapping Project. VII. Understanding the Ultraviolet Anomaly in NGC 5548 with X-Ray Spectroscopy

During the Space Telescope and Optical Reverberation Mapping Project observations of NGC 5548, the continuum and emission-line variability became decorrelated during the second half of the six-month-long observing campaign. Here we present Swift and Chandra X-ray spectra of NGC 5548 obtained as part of the campaign. The Swift spectra show that excess flux (relative to a power-law continuum) in the soft X-ray band appears before the start of the anomalous emission-line behavior, peaks during the period of the anomaly, and then declines. This is a model-independent result suggesting that the soft excess is related to the anomaly. We divide the Swift data into on- and off-anomaly spectra to characterize the soft excess via spectral fitting. The cause of the spectral differences is likely due to a change in the intrinsic spectrum rather than to variable obscuration or partial covering. The Chandra spectra have lower signal-to-noise ratios, but are consistent with the Swift data. Our preferred model of the soft excess is emission from an optically thick, warm Comptonizing corona, the effective optical depth of which increases during the anomaly. This model simultaneously explains all three observations: the UV emission-line flux decrease, the soft-excess increase, and the emission-line anomaly

X-ray flares in the early Swift observations of the possible naked gamma-ray burst 050421

We present the Swift observations of the faint burst GRB 050421. The X-ray light-curve shows at least two flares: the first flare peaking at ~110 s after the BAT trigger (T0) and the second one peaking at ~154 s. The first flare presents a flux variation of δF/Fpeak ~ 3.7 and a short timescale ratio δt/tpeak ~ 0.07. The second flare is smaller and presents a flux variation of δF/Fpeak ~ 1.7 and a short timescale ratio δt/tpeak ~ 0.03. We argue that the mechanism producing these flares is probably late internal shocks. The X-ray light-curve is consistent with a rapid decline with a temporal index α ~ 3.1, which decays from ~10−9 erg cm−2 s−1 at T0 + 100 s to <7 × 10−13 erg cm−2 s−1 at T0 + 900 s. A possible spectral softening is also observed with time, from β ~ 0.1 to ~ 1.2. A good joint fit to the BAT and XRT spectra before T0 + 171 s with βXRT−BAT ~ 0.2 indicates that the early X-ray and Gamma-ray emissions are likely produced by the same mechanism. We argue that the X-ray spectral softening, if any, is due to a shift of the peak of the prompt emission spectrum down to lower energies, and that the rapid decline of the X-ray emission is probably the tail of the prompt emission. This suggests that the X-ray emission is completely dominated by high latitude radiation and the external shock, if any, is extremely faint and below the detection threshold. GRB 050421 is likely the first “naked burst” detected by Swift