With rapid response capabilities, and a daily planning of its observing schedule, the Neil Gehrels Swift Observatory is ideal for monitoring transient and variable sources. Here we present a sample ...of the 12 novae with the most detailed ultraviolet (UV) follow-up by Swift—the first uniform analysis of such UV light-curves. The fading of these specific light-curves can be modelled as power-law decays (plotting magnitude against log time), showing that the same physical processes dominate the UV emission for extended time intervals in individual objects. After the end of the nuclear burning interval, the X-ray emission drops significantly, fading by a factor of around 10–100. The UV changes, however, are of a lower amplitude, declining by 1–2 mag over the same time period. The UV light-curves typically show a break from flatter to steeper around the time at which the X-ray light-curve starts a steady decline from maximum, ∼0.7–1.3 Tsub.SSSend. Considering populations of both classical and recurrent novae, and those with main sequence or giant companions, we do not find any strong differences in the UV light-curves or their evolution, although the long-period recurrent novae are more luminous than the majority of the classical novae.
Swift discovered XRF 050416A with the Burst Alert Telescope and began observing it with its narrow-field instruments only 64.5 s after the burst onset. Its very soft spectrum classifies this event as ...an X-ray flash. The afterglow X-ray emission was monitored up to 74 days after the burst. The X-ray light curve initially decays very fast (decay slope a 6 2.4), subsequently flattens (a 6 0.44), and eventually steepens again (a 6 0.88), similar to many X-ray afterglows. The first and second phases end 6172 and 61450 s after the burst onset, respectively. We find evidence of spectral evolution from a softer emission with photon index 6 3.0 during the initial steep decay, to a harder emission with 6 2.0 during the following evolutionary phases. The spectra show intrinsic absorption in the host galaxy with column density of 66.8 x 10 super(21) cm super(-2). The consistency of the initial photon index with the high-energy BAT photon index suggests that the initial fast decaying phase of the X-ray light curve may be the low-energy tail of the prompt emission. The lack of jet break signatures in the X-ray afterglow light curve is not consistent with empirical relations between the source rest-frame peak energy and the collimation-corrected energy of the burst. The standard uniform jet model can give a possible description of the XRF 050416A X-ray afterglow for an opening angle larger than a few tens of degrees, although numerical simulations show that the late-time decay is slightly flatter than expected from on-axis viewing of a uniform jet. A structured Gaussian-type jet model with uniform Lorentz factor distribution and viewing angle outside the Gaussian core is another possibility, although a full agreement with data is not achieved with the numerical models explored.
The Phases of X-Ray Emission of RS Oph Ness, Jan-Uwe; Starrfield, Sumner; Page, Kim L ...
Progress of theoretical physics. Supplement,
01/2007, Letnik:
169
Journal Article
Recenzirano
Odprti dostop
The recurrent symbiotic nova RS Oph reoccurred after 21 years on 12 February 2006. In contrast to the 1985 outburst, much denser coverage with X-ray observations was achieved. Swift observed RS Oph ...up to several times a day while Chandra and XMM-Newton observed two to four times during each phase of evolution. While the Swift observations provide high resolution in time, the Chandra and XMM-Newton observations provide high spectral resolution. Refined models can be constrained by the grating spectra, and interpolation of the model parameters can be constrained by the wealth of Swift observations. We compared the Swift light curve with six X-ray observations taken with EXOSAT during the 1985 outburst. We found that the decay from the supersoft X-ray binary (SSS) phase had been observed.
ABSTRACT We present the results of an intensive multiwavelength campaign on nova LMC 2012. This nova evolved very rapidly in all observed wavelengths. The time to fall two magnitudes in the V band ...was only 2 days. In X-rays the super soft phase began 13 5 days after discovery and ended around day 50 after discovery. During the super soft phase, the Swift/XRT and Chandra spectra were consistent with the underlying white dwarf (WD) being very hot, ∼1 MK, and luminous, ∼1038 erg s−1. The UV, optical, and near-IR photometry showed a periodic variation after the initial and rapid fading had ended. Timing analysis revealed a consistent 19.24 0.03 hr period in all UV, optical, and near-IR bands with amplitudes of ∼0.3 mag which we associate with the orbital period of the central binary. No periods were detected in the corresponding X-ray data sets. A moderately high inclination system, i = 60 10 , was inferred from the early optical emission lines. The HST/STIS UV spectra were highly unusual with only the N v (1240 ) line present and superposed on a blue continuum. The lack of emission lines and the observed UV and optical continua from four epochs can be fit with a low mass ejection event, ∼10−6 , from a hot and massive WD near the Chandrasekhar limit. The WD, in turn, significantly illuminated its subgiant companion which provided the bulk of the observed UV/optical continuum emission at the later dates. The inferred extreme WD characteristics and low mass ejection event favor nova LMC 2012 being a recurrent nova of the U Sco subclass.
We report Swift observations of a sample of 92 bright soft X-ray selected active galactic nuclei (AGNs). This sample represents the largest number of AGNs observed to study the spectral energy ...distribution (SED) of AGNs with simultaneous optical/UV and X-ray data. The principal motivation of this study is to understand the SEDs of AGNs in the optical/UV to X-ray regime and to provide bolometric corrections which are important in determining the Eddington ratio L/L {sub Edd}. In particular, we rigorously explore the dependence of the UV-EUV contribution to the bolometric correction on the assumed EUV spectral shape. We find strong correlations of the spectral slopes {alpha}{sub X} and {alpha}{sub UV} with L/L {sub Edd}. Although narrow-line Seyfert 1 galaxies (NLS1s) have steeper {alpha}{sub X} and higher L/L {sub Edd} than broad-line Seyfert 1 galaxies (BLS1s), their optical/UV to X-ray spectral slopes {alpha}{sub ox} and optical/UV slopes {alpha}{sub UV} are very similar. The mean SED of NLS1s shows that in general this type of AGNs appears to be fainter in the UV and at hard X-ray energies than BLS1s. We find a strong correlation between {alpha}{sub X} and {alpha}{sub UV} for AGNs with X-ray spectral slopes {alpha}{sub X}<1.6. For AGNs with steeper X-ray spectra, both this relation and the relation between {alpha}{sub X} and L/L {sub Edd} break down. At {alpha}{sub X}{approx}1.6, L/L {sub Edd} reaches unity. We note an offset in the {alpha}{sub UV}-L/L {sub Edd} relation between NLS1s and BLS1s. We argue that {alpha}{sub UV} is a good estimator of L/L {sub Edd} and suggest that {alpha}{sub UV} can be used to estimate L/L {sub Edd} in high-redshift QSOs. Although NLS1s appear to be highly variable in X-rays, they only vary marginally in the UV.
Fast radio bursts (FRBs) are bright, millisecond-duration radio bursts of cosmic origin. There have been several dozen FRBs found to repeat. Among them, those precisely localized provide the best ...opportunity to probe their multi-wavelength counterparts, local environment, and host galaxy that would reveal their origins. Here we report our X-ray, ultraviolet (UV) and optical observations with the \(Swift\) satellite that were performed simultaneously in the radio band with the Five-hundred-meter Aperture Spherical radio Telescope (FAST) observations of the repeating FRB 20190520B, aiming at detection of possible multi-wavelength bursts in association with radio bursts and multi-wavelength counterpart of the persistent radio source (PRS). While a total of 10 radio bursts were detected by FAST at the same time of \(Swift\) observations, we detected neither X-ray, UV or optical bursts in accompany of the radio bursts, nor persistent multi-wavelength counterpart of the PRS. We obtained the energy upper limits (\(3\sigma\)) on any multi-wavelength bursts as \(5.03 \times 10^{47}\) erg in the hard X-ray band (15-150 keV), \(7.98 \times 10^{45}\) erg in the soft X-ray band (0.3-10 keV), and \(4.51 \times 10^{44}\) erg in the U band, respectively. The energy ratio between soft X-ray (0.3-10 keV) and radio emission of the bursts is constrained as \(<6\times10^{7}\), and the ratio between optical (U band) and radio as \(<1.19\times10^{6}\). The 3\(\sigma\) luminosity upper limits at the position of PRS are 1.04\(\times10^{47}\) (15-150 keV), 8.81\(\times10^{42}\) (0.3-10 keV), 9.26\(\times10^{42}\) (UVW1), and 2.54\(\times10^{42}\) erg s\(^{-1}\) (U), respectively. We show that the PRS is much more radio loud than representative pulsar wind nebulae, supernova remnants, extended jet of Galactic X-ray binaries and ultraluminous X-ray sources, suggestive of boosted radio emission of the PRS.
The shell of the classical nova V5668 Sgr was resolved by ALMA at the frequency of 230 GHz 927 days after eruption, showing that most of the continuum bremsstrahlung emission originates in clumps ...with diameter smaller than \(10^{15}\) cm. Using VLA radio observations, obtained between days 2 and 1744 after eruption, at frequencies between 1 and 35 GHz, we modeled the nova spectra, assuming first that the shell is formed by a fixed number of identical clumps, and afterwards with the clumps having a power law distribution of sizes, and were able to obtain the clump's physical parameters (radius, density and temperature). We found that the density of the clumps decreases linearly with the increase of the shell's volume, which is compatible with the existence of a second media, hotter and thinner, in pressure equilibrium with the clumps. We show that this thinner media could be responsible for the emission of the hard X-rays observed at the early times of the nova eruption, and that the clump's temperature evolution follows that of the super-soft X-ray luminosity. We propose that the clumps were formed in the radiative shock produced by the collision of the fast wind of the white dwarf after eruption, with the slower velocity of the thermonuclear ejecta. From the total mass of the clumps, the observed expansion velocity and thermonuclear explosion models, we obtained an approximate value of 1.25 M\(_{\odot}\) for the mass of the white dwarf, a central temperature of \(10^7\) K and an accretion rate from the secondary star of \(10^{-9}-10^{-8}\) M\(_{\odot}\) y\(^{-1}\).
\(\gamma\)-ray observations of the Cygnus Cocoon, an extended source surrounding the Cygnus X star-forming region, suggest the presence of a cosmic ray accelerator reaching energies up to a few PeV. ...The very-high-energy (VHE; 0.1-100~TeV) \(\gamma\)-ray emission may be explained by the interaction of cosmic-ray hadrons with matter inside the Cocoon, but an origin of inverse Compton radiation by relativistic electrons cannot be ruled out. Inverse Compton \(\gamma\)-rays at VHE are accompanied by synchrotron radiation peaked in X-rays. Hence, X-ray observations may probe the electron population and magnetic field of the source. We observed eleven fields in or near the Cygnus Cocoon with the Neil Gehrels Swift Observatory's X-Ray Telescope (Swift-XRT) totaling 110 ksec. We fit the fields to a Galactic and extra-galactic background model and performed a log-likelihood ratio test for an additional diffuse component. We found no significant additional emission and established upper limits in each field. By assuming that the X-ray intensity traces the TeV intensity and follows an \(dN/dE\propto E^{-2.5}\) spectrum, we obtained a 90\% upper limit of \(F_X < 8.7\times 10^{-11}\rm~erg\,cm^{-2}\,s^{-1}\) or \(< 5.2\times 10^{-11}\rm~erg\,cm^{-2}\,s^{-1}\) on the X-ray flux of the entire Cygnus Cocoon between 2 and 10 keV depending on the choice of hydrogen column density model. This suggests that no more than one quarter of the \(\gamma\)-ray flux at 1 TeV is produced by inverse Compton scattering, when assuming an equipartition magnetic field of \(\sim 20\,\mu\)G.
We present a detailed compilation and analysis of the X-ray phase space of low- to intermediate-redshift (\( 0\le z \le 1\)) transients that consolidates observed light curves (and theory where ...necessary) for a large variety of classes of transient/variable phenomena in the 0.3--10 keV energy band. We include gamma-ray burst afterglows, supernovae, supernova shock breakouts and shocks interacting with the environment, tidal disruption events and active galactic nuclei, fast blue optical transients, cataclysmic variables, magnetar flares/outbursts and fast radio bursts, cool stellar flares, X-ray binary outbursts, and ultraluminous X-ray sources. Our overarching goal is to offer a comprehensive resource for the examination of these ephemeral events, extending the X-ray duration-luminosity phase space (DLPS) to show luminosity evolution. We use existing observations (both targeted and serendipitous) to characterize the behavior of various transient/variable populations. Contextualizing transient signals in the larger DLPS serves two primary purposes: to identify areas of interest (i.e., regions in the parameter space where one would expect detections, but in which observations have historically been lacking) and to provide initial qualitative guidance in classifying newly discovered transient signals. We find that while the most luminous (largely extragalactic) and least luminous (largely Galactic) part of the phase space is well-populated at \(t > 0.1\) days, intermediate luminosity phenomena (L\(_x = 10^{34} - 10^{42}\) erg s\(^{-1}\)) represent a gap in the phase space. We thus identify L\(_x = 10^{34} - 10^{42}\) erg s\(^{-1}\) and \(t = 10^{-4} - 0.1\) days as a key discovery phase space in transient X-ray astronomy.
PDF
