Very early observations with the Swift satellite of gamma-ray burst (GRB) afterglows reveal that the optical component is not detected in a large number of cases. This is in contrast to the bright optical flashes previously discovered in some GRBs (e.g. GRB 990123 and GRB 021211). Comparisons of the X-ray afterglow flux to the optical afterglow flux and prompt gamma-ray fluence is used to quantify the seemingly deficient optical, and in some cases X-ray, light at these early epochs. This comparison reveals that some of these bursts appear to have higher than normal gamma-ray efficiencies. We discuss possible mechanisms and their feasibility for explaining the apparent lack of early optical emission. The mechanisms considered include: foreground extinction, circumburst absorption, Ly-alpha blanketing and absorption due to high redshift, low density environments, rapid temporal decay, and intrinsic weakness of the reverse shock. Of these, foreground extinction, circumburst absorption, and high redshift provide the best explanations for most of the non-detections in our sample. There is tentative evidence of suppression of the strong reverse shock emission. This could be because of a Poynting-flux-dominated flow or a pure non-relativistic hydrodynamical reverse shock.
We present a photometric survey of the optical counterparts of ultraluminous X-ray sources (ULXs) observed with the Hubble Space Telescope in nearby (< 5 Mpc) galaxies. Of the 33 ULXs with Hubble & Chandra data, 9 have no visible counterpart, placing limits on their M_V of ~ -4 to -9, enabling us to rule out O-type companions in 4 cases. The refined positions of two ULXs place them in the nucleus of their host galaxy. They are removed from our sample. Of the 22 remaining ULXs, 13 have one possible optical counterpart, while multiple are visible within the error regions of other ULXs. By calculating the number of chance coincidences, we estimate that 13 +/- 5 are the true counterparts. We attempt to constrain the nature of the companions by fitting the SED and M_V to obtain candidate spectral types. We can rule out O-type companions in 20 cases, while we find that one ULX (NGC 253 ULX2) excludes all OB-type companions. Fitting with X-ray irradiated models provides constraints on the donor star mass and radius. For 7 ULXs, we are able to impose inclination-dependent upper and/or lower limits on the black holes mass, if the extinction to the assumed companion star is not larger than the Galactic column. These are NGC 55 ULX1, NGC 253 ULX1, NGC 253 ULX2, NGC 253 XMM6, Ho IX X-1, IC342 X-1 & NGC 5204 X-1. This suggests that 10 ULXs do not have O companions, while none of the 18 fitted rule out B-type companions.
Recent intensive campaigns to monitor the variable broad emission lines of AGN have provided a wealth of observational data. To reliably derive the BLR structure from such data requires a detailed comparison with physically realistic models. To provide such models we have combined photoionization calculations with a BLR modelling code. We find that the line emission is significantly anisotropic for the majority of lines at some radii in our models, particularly where the ionization parameter is large. As both anisotropic line emission and negative responsivity can have a substantial impact on the form of response functions, they must be taken into account when comparing BLR models to monitoring data. Differentiating between the effects of anisotropy and negative responsivity requires comparison of the variability behaviour of lines covering a wide range in ionization state with detailed photoionization models similar to those presented here. Failure to take such physical effects into account will lead to incorrect conclusions regarding the BLR structure. Based on the form of the anisotropic response functions for our models, we argue that in NGC5548 the high ionization lines (including CIII] λ1909) can be explained by a spherical BLR component which has a significant radial ionization parameter gradient. The clouds in this component may be optically thin at the Lyman limit in the inner part of the BLR due to the high ionization parameter. The bulk of the low ionization lines, and in particular almost all of the Balmer line emission, arises from a non-spherical BLR component, possibly an accretion disc. We have also investigated the accuracy of the linear response approximation by implicitly calculating the emissivity as a function of continuum level. For several lines the response is significantly non-linear over the typical continuum variability range of AGN. The form of the response functions is therefore luminosity dependent. Work is under way to incorporate this effect into the maximum-entropy-method used to recover response functions from monitoring data.
Abstract Previous examination of fully-convective M-dwarf stars highlighted unexplained enhanced rates of nanoflare activity. A potential explanation was linked to the helical turbulence dynamo which operates in fully convective stars. However, recent studies have found this helical dynamo does not appear significantly different to the Solar dynamo. The specific role the convective boundary plays on observed nanoflare rates, until now, was not known. Here we find evidence that fully convective M2.5V (and later) stars display greatly enhanced nanoflare rates compared with their pre-convective boundary counterparts. Importantly, the rate of nanoflare activity increases with increasing spectral sub-type, with nanoflares exhibiting greatly enhanced flaring rates via Sweet-Parker reconnection. This occurs more favourably at increased plasma resistivities experienced in these later MV stars, suggesting a direct interplay between the rate of nanoflare occurrence and the intrinsic plasma parameters. As such, nanoflare behaviour is likely to be unrelated to the behaviour of the local dynamo.
The unique capability of the Swift satellite to perform a prompt and autonomous slew to a newly detected Gamma-Ray Burst (GRB) has yielded the discovery of interesting new properties of GRB X-ray afterglows, such as the steep early lightcurve decay and the frequent presence of flares detected up to a few hours after the GRB trigger. We present observations of GRB 050607, the fourth case of a GRB discovered by Swift with flares superimposed on the overall fading X-ray afterglow. The flares of GRB 050607 were not symmetric as in previously reported cases, showing a very steep rise and a shallower decay, similar to the Fast Rise, Exponential Decay that are frequently observed in the gamma-ray prompt emission. The brighter flare had a flux increase by a factor of approximately 25,peaking for 30 seconds at a count rate of approximately 30 counts s-1, and it presented hints of addition short time scale activity during the decay phase. There is evidence of spectral evolution during the flares. In particular, at the onset of the flares the observed emission was harder, with a gradual softening as each flare decayed. The very short time scale and the spectral variability during the flaring activity are indicators of possible extended periods of energy emission by the GRB central engine. The flares were followed by a phase of shallow decay, during which the forward shock was being refreshed by a long-lived central engine or by shells of lower Lorentz factors, and by a steepening after approximately 12 ks to a decay slope considered typical of X-ray afterglows.
We report the discovery of a 1.32$^{+0.10}_{-0.10}$ $\mathrm{M_{\rm Jup}}$ planet orbiting on a 75.12 day period around the G3V $10.8^{+2.1}_{-3.6}$ Gyr old star TOI-5542 (TIC 466206508; TYC 9086-1210-1). The planet was first detected by the Transiting Exoplanet Survey Satellite (TESS) as a single transit event in TESS Sector 13. A second transit was observed 376 days later in TESS Sector 27. The planetary nature of the object has been confirmed by ground-based spectroscopic and radial velocity observations from the CORALIE and HARPS spectrographs. A third transit event was detected by the ground-based facilities NGTS, EulerCam, and SAAO. We find the planet has a radius of 1.009$^{+0.036}_{-0.035}$ $\mathrm{R_{\rm Jup}}$ and an insolation of 9.6$^{+0.9}_{-0.8}$ $S_{\oplus}$, along with a circular orbit that most likely formed via disk migration or in situ formation, rather than high-eccentricity migration mechanisms. Our analysis of the HARPS spectra yields a host star metallicity of [Fe/H] = $-$0.21$\pm$0.08, which does not follow the traditional trend of high host star metallicity for giant planets and does not bolster studies suggesting a difference among low- and high-mass giant planet host star metallicities. Additionally, when analyzing a sample of 216 well-characterized giant planets, we find that both high masses (4 $\mathrm{M_{\rm Jup}}$ $$ 10 days) and hot (P $<$ 10 days) giant planets are preferentially located around metal-rich stars (mean [Fe/H] $>$ 0.1). TOI-5542b is one of the oldest known warm Jupiters and it is cool enough to be unaffected by inflation due to stellar incident flux, making it a valuable contribution in the context of planetary composition and formation studies.
The Swift X‐ray Telescope is designed to make astrometric, spectroscopic and photometric observations of the X‐ray emission from Gamma‐Ray Bursts and their afterglows in the 0.2–10 keV energy band. Here we report some results on the in‐flight calibration of the Swift XRT effective area obtained analyzing observations of cosmic sources with different the analysis of cosmic sources intrinsic spectra and using the on‐ground calibration and ray‐tracing simulations as a starting point Our analysis includes the study of the effective area for different XRT operating modes.
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