Short term X-ray spectral variability of the quasar PDS 456 observed in a low flux state

We present an analysis of the 2013 Suzaku campaign on the nearby luminous quasar PDS 456, covering a total duration of ~1 Ms and a net exposure of 455 ks. During these observations, the X-ray flux was suppressed by a factor of >10 in the soft X-ray band when compared to other epochs. We investigated the broadband continuum by constructing a spectral energy distribution, making use of the optical/UV photometry and hard X-ray spectra from the later XMM-Newton/NuSTAR campaign in 2014. The high energy part of this low flux state cannot be accounted for by self-consistent accretion disc and corona models without attenuation by absorbing gas, which partially covers a substantial fraction of the line of sight towards the X-ray source. Two absorption layers are required, of column density $\log (N_{\rm{H,low}}/{\rm cm^{-2}})=22.3\pm0.1$ and $\log (N_{\rm{H,high}}/{\rm cm^{-2}})=23.2\pm0.1$, with average covering factors of ~80% (with typical 5% variations) and 60% ($\pm$10-15%), respectively. In these observations PDS 456 displays significant short term X-ray spectral variability, on timescales of ~100 ks, which can be accounted for by variable covering of the absorbing gas. The partial covering absorber prefers an outflow velocity of $v_{\rm pc} = 0.25^{+0.01}_{-0.05}c$ at the >99.9% confidence level over the case where $v_{\rm pc}=0$. This is consistent with the velocity of the highly ionised outflow responsible for the blueshifted iron K absorption profile. We therefore suggest that the partial covering clouds could be the denser, or clumpy part of an inhomogeneous accretion disc wind. Finally we estimate the size-scale of the X-ray source from its variability. The radial extent of the X-ray emitter is found to be of the order ~15-20 $R_{\rm g}$, although the hard X-ray (>2 keV) emission may originate from a more compact or patchy corona of hot electrons, which is ~6-8 $R_{\rm g}$ in size.Comment: 38 pages, 13 figures, accepted for publication in MNRA

On why the Iron K-shell absorption in AGN is not a signature of the local Warm/Hot Intergalactic Medium

We present a comparison between the 2001 XMM-Newton and 2005 Suzaku observations of the quasar, PG1211+143 at z=0.0809. Variability is observed in the 7 keV iron K-shell absorption line (at 7.6 keV in the quasar frame), which is significantly weaker in 2005 than during the 2001 XMM-Newton observation. From a recombination timescale of <4 years, this implies an absorber density n>0.004 particles/cm3, while the absorber column is 5e22<N_H <1 1e24 particles/cm2. Thus the sizescale of the absorber is too compact (pc scale) and the surface brightness of the dense gas too high (by 9-10 orders of magnitude) to arise from local hot gas, such as the local bubble, group or Warm/Hot Intergalactic Medium (WHIM), as suggested by McKernan et al. (2004, 2005). Instead the iron K-shell absorption must be associated with an AGN outflow with mildly relativistic velocities. Finally we show that the the association of the absorption in PG1211+143 with local hot gas is simply a coincidence, the comparison between the recession and iron K absorber outflow velocities in other AGN does not reveal a one to one kinematic correlation.Comment: accepted for publication in MNRAS LETTERS. 5 pages, 4 figure

The Connection between Ultraviolet and X-ray Outflows in AGN: the case of PDS 456

High-velocity outflows from AGN are a well-known phenomena in terms of the Broad Absorption Lines seen in the UV/optical. More recently, similar, possibly related, outflows have been reported in the X-ray. The most extreme example is seen in the nearby, luminous QSO PDS 456, which displays a massive, high velocity (50000 km s-1), high-ionization X-ray outflow of 10 solar masses yr-1. Here we present the UV spectrum of PDS 456 as observed by the Hubble Space Telescope. We find the UV spectrum is also extreme, displaying very broad emission-lines, with CIV 1549 blueshifted by 5000 km s-1 and a broad Ly-alpha absorption trough blueshifted by 14000-24000 km s-1. No strong, broad high-ionization absorption features are seen. We interpret the combined UV and X-ray spectrum of PDS 456 as the signature of a decelerating, cooling outflow, which may be driven by radiation and/or magnetic field. This outflow may be the source of some of the broad emission and absorption-line gas.Comment: Accepted for publication in MNRAS. 6 pages. 6 figure

Probing the face-on disc-corona system of the bare AGN Mrk 110 from UV to hard X-rays: a moderate changing-state AGN?

[Abridged] The X-ray broadband spectra of the bare AGN Mrk 110, obtained by simultaneous XMM-Newton and NuSTAR observations (Nov 2019 and April 2020), are characterised by the presence of a prominent and absorption-free smooth soft X-ray excess, moderately broad OVII and Fe Kalpha emission lines, and a lack of a strong Compton hump. While relativistic reflection as the sole emission is ruled out, a simplified combination of soft and hard Comptonisation from a warm and a hot coronae, plus mild relativistic disc reflection reproduces the data very well. We aim to confirm the physical origin of the soft X-ray excess of Mrk 110 and to determine its disc-corona system properties from its energetics using two new sophisticated models: reXcor and relagn, respectively. At both epochs, the inferred high-values of the warm-corona heating from the X-ray broadband spectral analysis using reXcor confirm that the soft X-ray excess originates mainly from a warm corona rather than relativistic reflection. The intrinsic best-fit SED determined at both epochs using relagn show a high X-ray contribution relative to the UV and are very well reproduced by a warm and hot coronae plus mild relativistic reflection. The outer radii of the hot and warm coronae are located at a few 10s and ~100 Rg, respectively. Moreover, combining the inferred low Eddington ratio (~ a few %) from this work, and previous multi-wavelength spectral and timing studies suggests that Mrk 110 could be classified as a moderate changing-state AGN. Our analysis confirms the existence of a warm corona as a significant contribution to the soft X-ray excess and UV emission in Mrk 110, adding to growing evidence that AGN accretion deviates from standard disc theory. This strengthens the importance of long-term multi-wavelength monitoring on both single targets and large AGN surveys to reveal the real nature of disc-corona system in AGN.Comment: 12 pages, 7 figures, accepted for publication in A&

Prompt Problems: A New Programming Exercise for the Generative AI Era

Large Language Models (LLMs) are revolutionizing the field of computing education with their powerful code-generating capabilities. Traditional pedagogical practices have focused on code writing tasks, but there is now a shift in importance towards code reading, comprehension and evaluation of LLM-generated code. Alongside this shift, an important new skill is emerging -- the ability to solve programming tasks by constructing good prompts for code-generating models. In this work we introduce a new type of programming exercise to hone this nascent skill: 'Prompt Problems'. Prompt Problems are designed to help students learn how to write effective prompts for AI code generators. A student solves a Prompt Problem by crafting a natural language prompt which, when provided as input to an LLM, outputs code that successfully solves a specified programming task. We also present a new web-based tool called Promptly which hosts a repository of Prompt Problems and supports the automated evaluation of prompt-generated code. We deploy Promptly for the first time in one CS1 and one CS2 course and describe our experiences, which include student perceptions of this new type of activity and their interactions with the tool. We find that students are enthusiastic about Prompt Problems, and appreciate how the problems engage their computational thinking skills and expose them to new programming constructs. We discuss ideas for the future development of new variations of Prompt Problems, and the need to carefully study their integration into classroom practice.Comment: Accepted to SIGCSE'24. arXiv admin note: substantial text overlap with arXiv:2307.1636

The Suzaku View of Highly Ionized Outflows in AGN

We present the results of a new spectroscopic study of Fe K-band absorption in active galactic nuclei (AGN). Using data obtained from the Suzaku public archive we have performed a statistically driven blind search for Fe XXV Healpha and/or Fe XXVI Lyalpha absorption lines in a large sample of 51 Type 1.01.9 AGN. Through extensive Monte Carlo simulations we find that statistically significant absorption is detected at E greater than or approximately equal to 6.7 keV in 20/51 sources at the P(sub MC) greater than or equal tov 95 per cent level, which corresponds to approximately 40 per cent of the total sample. In all cases, individual absorption lines are detected independently and simultaneously amongst the two (or three) available X-ray imaging spectrometer detectors, which confirms the robustness of the line detections. The most frequently observed outflow phenomenology consists of two discrete absorption troughs corresponding to Fe XXV Healpha and Fe XXVI Lyalpha at a common velocity shift. From xstar fitting the mean column density and ionization parameter for the Fe K absorption components are log (N(sub H) per square centimeter)) is approximately equal to 23 and log (Xi/erg centimeter per second) is approximately equal to 4.5, respectively. Measured outflow velocities span a continuous range from less than1500 kilometers per second up to approximately100 000 kilometers per second, with mean and median values of approximately 0.1 c and approximately 0.056 c, respectively. The results of this work are consistent with those recently obtained using XMM-Newton and independently provides strong evidence for the existence of very highly ionized circumnuclear material in a significant fraction of both radio-quiet and radio-loud AGN in the local universe

"It's Weird That it Knows What I Want": Usability and Interactions with Copilot for Novice Programmers

Recent developments in deep learning have resulted in code-generation models that produce source code from natural language and code-based prompts with high accuracy. This is likely to have profound effects in the classroom, where novices learning to code can now use free tools to automatically suggest solutions to programming exercises and assignments. However, little is currently known about how novices interact with these tools in practice. We present the first study that observes students at the introductory level using one such code auto-generating tool, Github Copilot, on a typical introductory programming (CS1) assignment. Through observations and interviews we explore student perceptions of the benefits and pitfalls of this technology for learning, present new observed interaction patterns, and discuss cognitive and metacognitive difficulties faced by students. We consider design implications of these findings, specifically in terms of how tools like Copilot can better support and scaffold the novice programming experience.Comment: 26 pages, 2 figures, TOCH

Wide-band spectroscopy of the Compton thick Seyfert 2 galaxy Mrk 3 with Suzaku

We obtained a wide-band spectrum of the Compton-thick Seyfert 2 galaxy Mrk 3 with Suzaku. The observed spectrum was clearly resolved into weak, soft power-law emission, a heavily absorbed power-law component, cold reflection, and many emission lines. The heavily absorbed component, absorbed by gas with a column density of 1.1x10^24 cm^-2, has an intrinsic 2--10 keV luminosity of ~1.6x10^43 erg s^-1, and is considered to be direct emission from the Mrk 3 nucleus. The reflection component was interpreted as reflection of the direct light off cold, thick material; the reflection fraction $R$ was 1.36+/-0.20. The cold material is inferred to be located > 1 pc from the central black hole of Mrk 3 due to the low ionization parameter of iron (xi < 1 erg cm s^-1) and the narrow iron line width (s < 22 eV). A Compton shoulder to the iron line was detected, but the intensity of the shoulder component was less than that expected from spherically distributed Compton-thick material. The weak, soft power-law emission is considered to be scattered light by ionized gas. The existence of many highly-ionized lines from O, Ne, Mg, Si, S, and Fe in the observed spectrum indicates that the ionized gas has a broad ionized structure, with xi=10--1000. The scattering fraction with respect to the direct light was estimated to be 0.9+/-0.2%, which indicates that the column density of the scattering region is about 3.6x10^22 cm^-2. This high-quality spectrum obtained by Suzaku can be considered a template for studies of Seyfert 2 galaxies.Comment: 24 pages, 8 figures, accepted in PASJ for publicatio

Loading rate and contraction duration effects on in vivo human Achilles tendon mechanical properties.

Tendons are viscoelastic, which implies loading rate dependency, but loading rates of contractions are often not controlled during assessment of human tendon mechanical properties in vivo. We investigated the effects of sustained submaximal isometric plantarflexion contractions, which potentially negate loading rate dependency, on the stiffness of the human Achilles tendon in vivo using dynamometry and ultrasonography. Maximum voluntary contractions (high loading rate), ramp maximum force contractions with 3 s loading (lower loading rate) and sustained contractions (held for 3 s) at 25%, 50% and 80% of maximal tendon force were conducted. No loading rate effect on stiffness (25-80% max. tendon force) was found. However, loading rate effects were seen up to 25% of maximum tendon force, which were reduced by the sustained method. Sustained plantarflexion contractions may negate loading rate effects on tendon mechanical properties and appear suitable for assessing human Achilles tendon stiffness in vivo