The flaring X-ray corona in the quasar PDS 456

International audienceNew Swift monitoring observations of the variable, radio-quiet quasar, PDS 456, are presented. A bright X-ray flare was captured in 2018 September, the flux increasing by a factor of 4 and with a doubling time-scale of 2 d. From the light crossing argument, the coronal size is inferred to be ≲30 gravitational radii for a black hole mass of 10^9 M_⊙ and the total flare energy exceeds 10^51 erg. A hardening of the X-ray emission accompanied the flare, with the photon index decreasing from Γ = 2.2 to Γ = 1.7 and back again. The flare is produced in the X-ray corona, the lack of any optical or UV variability being consistent with a constant accretion rate. Simultaneous XMM–Newton and NuSTAR observations were performed, 1–3 d after the flare peak and during the decline phase. These caught PDS 456 in a bright, bare state, where no disc wind absorption features are apparent. The hard X-ray spectrum shows a high energy roll-over, with an e-folding energy of |$E_{\rm fold}=51^{+11}_{-8}$| keV. The deduced coronal temperature, of kT = 13 keV, is one of the coolest measured in any AGN and PDS 456 lies well below the predicted pair annihilation line in X-ray corona. The spectral variability, becoming softer when fainter following the flare, is consistent with models of cooling X-ray coronae. Alternatively, an increase in a non-thermal component could contribute towards the hard X-ray flare spectrum

The story of Seyfert galaxy RE J2248-511: from intriguingly ultrasoft to unremarkably average

RE J2248−511 is one of only 14 non-blazar active galactic nuclei (AGN) detected in the far-ultraviolet (FUV) by the ROSAT Wide Field Camera implying a large ultrasoft X-ray flux. This soft X-ray excess is strongly variable on year time-scales, a common property of narrow-line Seyfert 1s, yet its optical line widths classify this source as a broad-lined Seyfert 1 (BLS1). We use four nearly simultaneous optical–X-ray spectral energy distributions (SEDs) spanning 7 yr to study the spectral shape and long-term variability of RE J2248−511. Here we show that the continuum SED for the brightest epoch data set is consistent with the mean SED of a standard quasar, and matches well to that from an XMM–Sloan Digital Sky Survey sample of AGN with 〈M/M⊙〉 ∼ 108 and 〈L/LEdd〉 ∼ 0.2. All the correlated optical and soft X-ray variability can be due entirely to a major absorption event. The only remarkable aspect of this AGN is that there is no measurable intrinsic X-ray absorption column in the brightest epoch data set. The observed FUV flux is determined by the combination of this and the fact that the source lies within a local absorption ‘hole’. RE J2248−511, whose variable, ultrasoft X-ray flux once challenged its BLS1 classification, demonstrates that characterization of such objects requires multi-epoch, multiwavelength campaigns

X-ray, UV, and optical time delays in the bright Seyfert galaxy Ark 120 with co-ordinated Swift and ground-based observations

International audienceWe report on the results of a multiwavelength monitoring campaign of the bright, nearby Seyfert galaxy Ark 120, using a ∼50-d observing programme with Swift and a ∼4-month co-ordinated ground-based observing campaign, pre-dominantly using the Skynet Robotic Telescope Network. We find Ark 120 to be variable at all optical, UV, and X-ray wavelengths, with the variability observed to be well correlated between wavelength bands on short time-scales. We perform cross-correlation analysis across all available wavelength bands, detecting time delays between emission in the X-ray band and the Swift V, B, and UVW1 bands. In each case, we find that the longer wavelength emission is delayed with respect to the shorter wavelength emission. Within our measurement uncertainties, the time delays are consistent with the τ ∼ λ^4/3 relation, as predicted by a disc reprocessing scenario. The measured lag centroids are τ_cent = 11.90 ± 7.33, 10.80 ± 4.08, and 10.60 ± 2.87 d between the X-ray and V, B, and UVW1 bands, respectively. These time delays are longer than those expected from standard accretion theory and, as such, Ark 120 may be another example of an active galaxy whose accretion disc appears to exist on a larger scale than predicted by the standard thin-disc model. Additionally, we detect further inter-band time delays: most notably between the ground-based I and B bands (τ_cent = 3.46 ± 0.86 d), and between both the Swift XRT and UVW1 bands and the I band (τ_cent = 12.34 ± 4.83 and 2.69 ± 2.05 d, respectively), highlighting the importance of co-ordinated ground-based optical observations

Measurement of the B⁺ total cross section and B⁺ differential cross section d sigma/dp_T in p(p)over-bar collisions at root s=1.8 TeV

We present measurements of the B+ meson total cross section and differential cross section dsigma/dp(T). The measurements use a 98+/-4 pb(-1) sample of p (p) over bar collisions at roots = 1.8 TeV collected by the CDF detector. Charged B meson candidates are reconstructed through the decay B+/- → J/psiK(+/-) with J/psi → mu(+)mu(-). The total cross section, measured in the central rapidity region \y\ lt 1.0 for p(T)(B) gt 6.0 GeV/c, is 3.6+/-0.6(stat + syst) mub. The measured differential cross section is substantially larger than typical QCD predictions calculated to next-to-leading order

Central pseudorapidity gaps in events with a leading antiproton at the Fermilab tevatron pp collider

The results from a similar measurement performed in a subsample of pp events containing a leading (high longitudinal momentum) antiproton. As such, large pseudorapidity gaps are presumed to be due to Pomeron exchanges and are the signature for diffraction. The process with a leading beam particle in the final state, which is kinematically associated with an adjacent pseudorapidity gap, is known as single diffraction dissociation (SD), while that with a central gap as double diffraction dissociation (DD)