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Novae are caused by runaway thermonuclear burning in the hydrogen-rich envelopes of accreting white dwarfs, which results in the envelope to expand rapidly and to eject most of its mass. For more than 30 years, nova theory has predicted the existence of a "fireball" phase following directly the runaway fusion, which should be observable as a short, bright, and soft X-ray flash before the nova becomes visible in the optical. Here we present the unequivocal detection of an extremely bright and very soft X-ray flash of the classical Galactic nova YZ Reticuli 11 hours prior to its 9 mag optical brightening. No X-ray source was detected 4 hours before and after the event, constraining the duration of the flash to shorter than 8 hours. In agreement with theoretical predictions, the source's spectral shape is consistent with a black body of $3.27^{+0.11}_{-0.33}\times 10^5$ K ($28.2^{+0.9}_{-2.8}$ eV), or a white dwarf atmosphere, radiating at the Eddington luminosity, with a photosphere that is only slightly larger than a typical white dwarf. This detection of the expanding white dwarf photosphere before the ejection of the envelope provides the last link of the predicted photospheric lightcurve evolution and opens a new window to measure the total nova energetics., Comment: Published in Nature on 11 May 2022

We examine four high resolution reflection grating spectrometers (RGS) spectra of the February 2009 outburst of the luminous recurrent nova LMC 2009a. They were very complex and rich in intricate absorption and emission features. The continuum was consistent with a dominant component originating in the atmosphere of a shell burning white dwarf (WD) with peak effective temperature between 810,000 K and a million K, and mass in the 1.2-1.4 M$_\odot$ range. A moderate blue shift of the absorption features of a few hundred km s$^{-1}$ can be explained with a residual nova wind depleting the WD surface at a rate of about 10$^{-8}$ M$_\odot$ yr$^{-1}$. The emission spectrum seems to be due to both photoionization and shock ionization in the ejecta. The supersoft X-ray flux was irregularly variable on time scales of hours, with decreasing amplitude of the variability. We find that both the period and the amplitude of another, already known 33.3 s modulation, varied within timescales of hours. We compared N LMC 2009a with other Magellanic Clouds novae, including 4 serendipitously discovered as supersoft X-ray sources (SSS) among 13 observed within 16 years after the eruption. The new detected targets were much less luminous than expected: we suggest that they were partially obscured by the accretion disk. Lack of SSS detections in the Magellanic Clouds novae more than 5.5 years after the eruption constrains the average duration of the nuclear burning phase., Comment: Accepted for publication in MNRAS

[Context] Photospheric radius expansion during X-ray bursts can be used to measure neutron star radii and help constrain the equation of state of neutron star matter. Understanding the stellar wind dynamics is important for interpreting observations, and therefore stellar wind models, though studied in past decades, have regained interest and need to be revisited with updated data and methods., [Aims] Here, we study the radiative wind model in the context of X-ray bursts with modern techniques and physics input. We focus on characterization of the solutions and the study of observable magnitudes as a function of free model parameters., [Methods] We implemented a spherically symmetric nonrelativistic wind model in a stationary regime, with updated opacity tables and modern numerical techniques. Total mass and energy outflows (Ṁ, Ė) were treated as free parameters., [Results] A high-resolution parameter-space exploration was performed to allow better characterization of observable magnitudes. High correlation was found between different photospheric magnitudes and free parameters. For instance, the photospheric ratio of gravitational energy outflow to radiative luminosity is directly proportional to the photospheric wind velocity., [Conclusions] The correlations found here could help determine the physical conditions of the inner layers, where nuclear reactions take place, by means of observable photospheric values. Further studies are needed to determine the range of physical conditions in which the correlations are valid., The authors would like to thank Arman Aryaeipour and Manuel Linares for fruitful discussions. This work has been partially supported by the Spanish MINECO grant AYA2017–86274–P, by the E.U. FEDER funds, and by the AGAUR/Generalitat de Catalunya grant SGR-661/2017. This article benefited from discussions within the “ChETEC” COST Action (CA16117).

Nova SMC 2016 has been the most luminous nova known in the direction of the Magellanic Clouds. It turned into a very luminous supersoft X-ray source between day 16 and 28 after the optical maximum. We observed it with Chandra, the HRC-S camera and the Low Energy Transmission Grating (LETG) on 2016 November and 2017 January (days 39 and 88 after optical maximum), and with XMM-Newton on 2016 December (day 75). We detected the compact white dwarf (WD) spectrum as a luminous supersoft X-ray continuum with deep absorption features of carbon, nitrogen, magnesium, calcium, probably argon and sulfur on day 39, and oxygen, nitrogen and carbon on days 75 and 88. The spectral features attributed to the WD atmosphere are all blue-shifted, by about 1800 km/s on day 39 and up to 2100 km/s in the following observations. Spectral lines attributed to low ionization potential transitions in the interstellar medium are also observed. Assuming the distance of the Small Magellanic Cloud, the bolometric luminosity exceeded Eddington level for at least three months. A preliminary analysis with atmospheric models indicates effective temperature around 700,000 K on day 39, peaking at the later dates in the 850,000-900,000 K range, as expected for a 1.25 m(sol) WD. We suggest a possible classification as an oxygen-neon WD, but more precise modeling is needed to accurately determine the abundances. The X-ray light curves show large, aperiodic ux variability, not associated with spectral variability. We detected red noise, but did not find periodic or quasi-periodic modulations., Accepted for publication in the Astrophysical Journal, pending final editorial review after "minor changes"

Classical novae occur on the surface of an accreting white dwarf in a binary system. After ejection of a fraction of the envelope and when the expanding shell becomes optically thin to X-rays, a bright source of supersoft X-rays arises, powered by residual H burning on the surface of the white dwarf. While the general picture of the nova event is well established, the details and balance of accretion and ejection processes in classical novae are still full of unknowns. The long-term balance of accreted matter is of special interest for massive accreting white dwarfs, which may be promising supernova Ia progenitor candidates. V5116 Sgr was observed as a bright and variable supersoft X-ray source by XMM-Newton 610~days after outburst. The light curve showed a periodicity consistent with the orbital period. During one third of the orbit the luminosity was a factor of seven brighter than during the other two thirds of the orbital period. In the present work we aim to disentangle the X-ray spectral components of V5116 Sgr and their variability. We present the high resolution spectra obtained with XMM-Newton RGS and Chandra LETGS/HRC-S in March and August 2007. The grating spectrum during the periods of high-flux shows a typical hot white dwarf atmosphere dominated by absorption lines of N VI and N VII. During the low-flux periods, the spectrum is dominated by an atmosphere with the same temperature as during the high-flux period, but with several emission features superimposed. Some of the emission lines are well modeled with an optically thin plasma in collisional equilibrium, rich in C and N, which also explains some excess in the spectra of the high-flux period. No velocity shifts are observed in the absorption lines, with an upper limit set by the spectral resolution of 500 km/s, consistent with the expectation of a non-expanding atmosphere so late in the evolution of the post-nova., Comment: 12 pages, 15 figures, 4 tables; accepted for publication in Astronomy and Astrophysics

A subset of ultraluminous X-ray sources (those with luminosities < 10^40 erg/s) are thought to be powered by the accretion of gas onto black holes with masses of ~5-20 M_solar, probably via an accretion disc. The X-ray and radio emission are coupled in such Galactic sources, with the radio emission originating in a relativistic jet thought to be launched from the innermost regions near the black hole, with the most powerful emission occurring when the rate of infalling matter approaches a theoretical maximum (the Eddington limit). Only four such maximal sources are known in the Milky Way, and the absorption of soft X-rays in the interstellar medium precludes determining the causal sequence of events that leads to the ejection of the jet. Here we report radio and X-ray observations of a bright new X-ray source whose peak luminosity can exceed 10^39 erg/s in the nearby galaxy, M31. The radio luminosity is extremely high and shows variability on a timescale of tens of minutes, arguing that the source is highly compact and powered by accretion close to the Eddington limit onto a stellar mass black hole. Continued radio and X-ray monitoring of such sources should reveal the causal relationship between the accretion flow and the powerful jet emission., 23 pages, 4 figures accepted by Nature (AOP 12/12/12)

The Andromeda Galaxy recurrent nova M31N 2008-12a had been observed in eruption ten times, including yearly eruptions from 2008-2014. With a measured recurrence period of $P_\mathrm{rec}=351\pm13$ days (we believe the true value to be half of this) and a white dwarf very close to the Chandrasekhar limit, M31N 2008-12a has become the leading pre-explosion supernova type Ia progenitor candidate. Following multi-wavelength follow-up observations of the 2013 and 2014 eruptions, we initiated a campaign to ensure early detection of the predicted 2015 eruption, which triggered ambitious ground and space-based follow-up programs. In this paper we present the 2015 detection; visible to near-infrared photometry and visible spectroscopy; and ultraviolet and X-ray observations from the Swift observatory. The LCOGT 2m (Hawaii) discovered the 2015 eruption, estimated to have commenced at Aug. $28.28\pm0.12$ UT. The 2013-2015 eruptions are remarkably similar at all wavelengths. New early spectroscopic observations reveal short-lived emission from material with velocities $\sim13000$ km s$^{-1}$, possibly collimated outflows. Photometric and spectroscopic observations of the eruption provide strong evidence supporting a red giant donor. An apparently stochastic variability during the early super-soft X-ray phase was comparable in amplitude and duration to past eruptions, but the 2013 and 2015 eruptions show evidence of a brief flux dip during this phase. The multi-eruption Swift/XRT spectra show tentative evidence of high-ionization emission lines above a high-temperature continuum. Following Henze et al. (2015a), the updated recurrence period based on all known eruptions is $P_\mathrm{rec}=174\pm10$ d, and we expect the next eruption of M31N 2008-12a to occur around mid-Sep. 2016., 46 pages, 19 figures, 14 tables, accepted for publication in ApJ (accepted version, minor changes made during the refereeing process)

We review the first results of a dedicated, on-going monitoring project for supersoft X-ray source (SSS) states of classical novae (CNe) in the central region of the Andromeda galaxy (M 31). So far, during three high-cadence observation campaigns with the X-ray telescopes XMM-Newton and Chandra from 2006 until 2009 we detected 21 CNe, only four of which were seen in X-rays before. This increased the total number of known CNe with X-ray counterparts in M 31 to 60, which is significantly larger than for any other galaxy, including the Milky Way. We compiled a catalog of X-ray and optical properties for these 60 sources which allowed us to perform source statistics and population studies of CNe with SSS state in M 31 for the first time. Here we review the results of this analysis: (i) several correlations between X-ray and optical parameters, (ii) an analysis of the intrinsic fraction of CNe with SSS state, and (iii) the first nova population study in X-rays. Our first results are promising, however a continuing monitoring of the unique target M 31 is strongly needed to extend this valuable data base and to refine the early statements and predictions.

Classical novae (CNe) have been found to represent the major class of supersoft X-ray sources (SSS) in our neighbour galaxy M 31. We determine properties and evolution of the two first SSSs ever discovered in the M 31 globular cluster (GC) system. We have used XMM-Newton, Chandra and Swift observations of the centre region of M 31 to discover both SSS and to determine their X-ray light curves and spectra. We performed detailed analysis of XMM-Newton EPIC PN spectra of the source in Bol 111 (SS1) using blackbody and NLTE white dwarf (WD) atmosphere models. For the SSS in Bol 194 (SS2) we used optical monitoring data to search for an optical counterpart. Both GC X-ray sources were classified as SSS. We identify SS1 with the CN M31N 2007-06b recently discovered in the M 31 GC Bol 111. For SS2 we did not find evidence for a recent nova outburst and can only provide useful constraints on the time of the outburst of a hypothetical nova. The only known CN in a M 31 GC can be identified with the first SSS found in a M31 GC. We discuss the impact of our observations on the nova rate for the M 31 GC system., Comment: 11 pages, 9 figures, 4 tables, submitted to A&A; v2: accepted version

We report on GROND observations of a 40 sec duration (rest-frame) optical flare from GRB 080129 at redshift 4.349. The rise- and decay time follow a power law with indices +12 and -8, respectively, inconsistent with a reverse shock and a factor 10$^5$ faster than variability caused by ISM interaction. While optical flares have been seen in the past (e.g. GRB 990123, 041219B, 060111B and 080319B), for the first time, our observations not only resolve the optical flare into sub-components, but also provide a spectral energy distribution from the optical to the near-infrared once every minute. The delay of the flare relative to the GRB, its spectral energy distribution as well as the ratio of pulse widths suggest it to arise from residual collisions in GRB outflows \cite{liw08}.If this interpretation is correct and can be supported by more detailed modelling or observation in further GRBs, the delay measurement provides an independent, determination of the Lorentz factor of the outflow., accepted for publ. in ApJ, 5 Figs

On 28 February 2007 a new outburst of the previously known transient source XTE J1856+053 was detected with RXTE/ASM. We present here the results of an XMM-Newton (0.5-10.0 keV) Target of Opportunity observation performed on 14 March 2007, aimed at constraining the mass of the compact object in this X-ray binary and determining its main properties. The EPIC-pn camera was used in Timing mode and its spectrum fit together with the RGS data. IR observations with GROND at the 2.2 m telescope in La Silla provide further information on the system. The X-ray light curve shows that both the 1996 and the 2007 outbursts had two peaks. The X-ray spectrum is well fit with a thermal accretion disk model, with kT=0.75+/-0.01 keV and foreground absorption N_H=4.5(+/-0.1)E22 cm**-2. The low disk temperature favours a black-hole as accreting object, with an estimated mass in the range 1.3-4.2 M_sun. From the IR upper limits we argue that XTE J1856+053 is a low mass X-ray binary. We estimate the orbital period of the system to be between 3 and 12 hours.

The purpose of this study is to find transient X-ray sources in M31, and to investigate and classify their nature. Three X-ray transients were observed with Swift. For each of the three X-ray transients we use the Swift X-ray and optical data together with observations from XMM-Newton and Chandra to investigate the lightcurves and the spectra of the outburst, and thereby to identify the source types. The outburst of XMMU J004215.8+411924 lasted for about one month. The source had a hard power-law spectrum with a photon index of 1.6. It was previously identified as a Be/X-ray binary based on the optical identification with a star. However, we show that with improved source coordinates it is clear that the optical source is not the counterpart to the X-ray source. The source SWIFT J004217.3+411532 had a bright outburst, after which it slowly decayed over half a year. The spectrum was soft, corresponding to a thermal accretion disk with innermost temperature of 250-600 eV. The source was not seen in the optical, and the soft spectrum indicates that the source is most likely a black hole low mass X-ray binary. M31N 2006-11a is a nova that was previously observed in the optical. We detected it both in X-rays and UV with Swift half a year after the optical maximum, after which it decayed below the Swift detection threshold within a month. The spectrum of the X-ray transient can be modelled by a black-body with a temperature of 50 eV. We use catalogues of X-ray transients in M31 to estimate their rate, and we find a lower limit of 9/yr., Comment: 5 pages, to appear in A&A

V5116 Sgr (Nova Sgr 2005 No. 2), discovered on 2005 July 4, was observed with XMM-Newton in March 2007, 20 months after the optical outburst. The X-ray spectrum shows that the nova had evolved to a pure supersoft X-ray source, with no significant emission at energies above 1 keV. The X-ray light-curve shows abrupt decreases and increases of the flux by a factor ~8. It is consistent with a periodicity of 2.97 h, the orbital period suggested by Dobrotka et al. (2007), although the observation lasted just a little more than a whole period. We estimate the distance to V5116 Sgr to be 11+/-3 kpc. A simple blackbody model does not fit correctly the EPIC spectra, with reduced chi^2>4. In contrast, ONe rich white dwarf atmosphere models provide a good fit, with nH=1.3(+/-0.1)e21 cm^-2, T=6.1(+/-0.1)e5 K, and L=3.9(+/-0.8)e37(D/10kpc)^2 erg/s (during the high-flux periods). This is consistent with residual hydrogen burning in the white dwarf envelope. The white dwarf atmosphere temperature is the same both in the low and the high flux periods, ruling out an intrinsic variation of the X-ray source as the origin of the flux changes. We speculate that the X-ray light-curve may result from a partial coverage by an asymmetric accretion disk in a high inclination system.

The new black hole candidate XTE J1817-330, discovered on 26 January 2006 with RXTE, was observed with XMM-Newton and INTEGRAL in February and March 2006, respectively. The X-ray spectrum is dominated by the thermal emission of the accretion disk in the soft band, with a low absorption column density (N H=1.77(±0.01)×1021 cm−2) and a maximum disk temperature kT max=0.68(±0.01) keV, plus a power law component, with the photon index decreasing from 2.66±0.02 to 1.98±0.07 between the two observations. Several interstellar absorption lines are detected in the X-ray spectrum, corresponding to O I, O II, O III, O VII and Fe XXIV. We constrain the distance to the system to be in the range 1–5 kpc.

The Andromeda Galaxy recurrent nova M31N 2008-12a had been caught in eruption eight times. The inter-eruption period of M31N 2008-12a is ~1 year, making it the most rapidly recurring system known, and a strong single-degenerate Type Ia Supernova progenitor candidate. Following the 2013 eruption, a campaign was initiated to detect the predicted 2014 eruption and to then perform high cadence optical photometric and spectroscopic monitoring using ground-based telescopes, along with rapid UV and X-ray follow-up with the Swift satellite. Here we report the results of a high cadence multicolour optical monitoring campaign, the spectroscopic evolution, and the UV photometry. We also discuss tantalising evidence of a potentially related, vastly-extended, nebulosity. The 2014 eruption was discovered, before optical maximum, on October 2, 2014. We find that the optical properties of M31N 2008-12a evolve faster than all Galactic recurrent novae known, and all its eruptions show remarkable similarity both photometrically and spectroscopically. Optical spectra were obtained as early as 0.26 days post maximum, and again confirm the nova nature of the eruption. A significant deceleration of the inferred ejecta expansion velocity is observed which may be caused by interaction of the ejecta with surrounding material, possibly a red giant wind. We find a low ejected mass and low ejection velocity, which are consistent with high mass-accretion rate, high mass white dwarf, and short recurrence time models of novae. We encourage additional observations, especially around the predicted time of the next eruption, towards the end of 2015., 23 pages, 11 figures, 5 tables, accepted for publication in Astronomy & Astrophysics

Highly luminous rapid flares are characteristic of processes around compact objects like white dwarfs, neutron stars or black holes. In the high energy regime of X- and gamma-rays, outbursts with variability time-scales of seconds and faster are routinely observed, e.g. in gamma-ray bursts or Soft Gamma Repeaters. In the optical, flaring activity on such time-scales has never been observed outside the prompt phase of GRBs. This is mostly due to the fact that outbursts with strong, fast flaring usually are discovered in the high-energy regime. Most optical follow-up observations of such transients employ instruments with integration times exceeding tens of seconds, which are therefore unable to resolve fast variability. Here we show the observation of extremely bright and rapid optical flaring in the galactic transient SWIFT J195509.6+261406. Flaring of this kind has never previously been reported. Our optical light-curves are phenomenologically similar to high energy light-curves of Soft Gamma Repeaters and Anomalous X-ray Pulsars, which are thought to be neutron stars with extremely high magnetic fields (magnetars). This suggests similar emission processes may be at work, but in contrast to the other known magnetars with strong emission in the optical., Comment: 8 pages, 3 figures. A substantially revised version of this manuscript was published in Nature. Due to license issues, the accepted manuscript will only be put on astro-ph as v2 6 months after this version

The Large Observatory For x-ray Timing (LOFT) was studied within ESA M3 Cosmic Vision framework and participated in the final down-selection for a launch slot in 2022-2024. Thanks to the unprecedented combination of effective area and spectral resolution of its main instrument, LOFT will study the behaviour of matter under extreme conditions, such as the strong gravitational field in the innermost regions of accretion flows close to black holes and neutron stars, and the supra-nuclear densities in the interior of neutron stars. The science payload is based on a Large Area Detector (LAD, 10 m 2 effective area, 2-30 keV, 240 eV spectral resolution, 1 deg collimated field of view) and a WideField Monitor (WFM, 2-50 keV, 4 steradian field of view, 1 arcmin source location accuracy, 300 eV spectral resolution). The WFM is equipped with an on-board system for bright events (e.g. GRB) localization. The trigger time and position of these events are broadcast to the ground within 30 s from discovery. In this paper we present the status of the mission at the end of its Phase A study., Proc. SPIE 9144, Space Telescopes and Instrumentation 2014: Ultraviolet to Gamma Ray, 91442T

The effect of the wind speed and direction on the arrays in a 50 kW plant was analyzed along several days. The wind is measured in different positions of the plant: the general plant meteorological station, the module's boundary layer and the close surroundings of the array. The averaged thermal resistance between the back of the module and the ambient is calculated and its dependence with the wind direction analyzed. Early results suggest that Northern wind has a better cooling effect than Western and Eastern winds.

1 Astrophysics Research Institute, Liverpool John Moores University, IC2 Liverpool Science Park, Liverpool, L3 5RF, UK e-mail: M.J.Darnley@ljmu.ac.uk 2 Institut de Ciencies de l’Espai (CSIC-IEEC), Campus UAB, C/Can Magrans s/n, 08193 Cerdanyola del Valles, Spain 3 Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands 4 Instituto de Astrofisica de Canarias, Via Lactea, s/n, La Laguna, 38205 Santa Cruz de Tenerife, Spain 5 Departamento de Astrofisica, Universidad de La Laguna, La Laguna, 38206 Santa Cruz de Tenerife, Spain 6 Department of Astronomy, San Diego State University, San Diego, CA 92182, USA 7 Physics Department, Lancaster University, Lancaster, LA1 4YB, UK 8 Department of Earth Science and Astronomy, College of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, 153-8902 Tokyo, Japan 9 Astronomical Institute, Academy of Sciences, 251 65 Ondřejov, Czech Republic 10 Astronomy Department, University of Illinois at Urbana-Champaign, 1002 W. Green Street, Urbana, IL 61801, USA 11 Department of Astronomy, Keio University, Hiyoshi, 223-8521 Yokohama, Japan 12 Variable Stars Observers League in Japan (VSOLJ), 7-1 Kitahatsutomi, 273-0126 Kamagaya, Japan 13 Astronomical Institute of the Charles University, Faculty of Mathemathics and Physics, V Holesovickach 2, 180 00 Praha 8, Czech Republic 14 Okayama Astrophysical Observatory, NAOJ, NINS, 3037-5 Honjo, Kamogata, Asakuchi, 719-0232 Okayama, Japan 15 European Space Astronomy Centre, Camino Bajo del Castillo s/n, Urb. Villafranca del Castillo, 28692 Villanueva de la Canada, Madrid, Spain 16 Departament de Fisica i Enginyeria Nuclear, EUETIB, Universitat Politecnica de Catalunya, c/ Compte d’Urgell 187, 08036 Barcelona, Spain 17 Institut d’Estudis Espacials de Catalunya, c/ Gran Capita 2-4, Ed. Nexus-201, 08034 Barcelona, Spain 18 Isaac Newton Group of Telescopes, Apartado de correos 321, 38700 Santa Cruz de La Palma, Spain

Type I X-ray bursts are thermonuclear explosions that occur in the envelopes of accreting neutron stars. Detailed observations of these phenomena have prompted numerous studies in theoretical astrophysics and experimental nuclear physics since their discovery over 35 years ago. In this review, we begin by discussing key observational features of these phenomena that may be sensitive to the particular patterns of nucleosynthesis from the associated thermonuclear burning. We then summarize efforts to model type I X-ray bursts, with emphasis on determining the nuclear physics processes involved throughout these bursts. We discuss and evaluate limitations in the models, particularly with regard to key uncertainties in the nuclear physics input. Finally, we examine recent, relevant experimental measurements and outline future prospects to improve our understanding of these unique environments from observational, theoretical and experimental perspectives., Accepted by Prog. Part. Nucl. Phys., 45 pages, 14 figures

The Rapid Burster (MXB 1730-335) is a unique object, showing both type I and type II X-ray bursts. A type I burst of the Rapid Burster was observed with Swift/XRT on 2009 March 5, showing photospheric radius expansion for the first time in this source. We report here on the mass and radius determination from this photospheric radius expansion burst using a Bayesian approach. After marginalization over the likely distance of the system (5.8-10 kpc) we obtain M=1.1+/-0.3 M_sun and R=9.6+/-1.5 km (1-sigma uncertainties) for the compact object, ruling out the stiffest equations of state for the neutron star. We study the sensitivity of the results to the distance, the color correction factor, and the hydrogen mass fraction in the envelope. We find that only the distance plays a crucial role., 7 pages, 8 figures, Accepted for publication in The Astrophysical Journal

Classical novae (CNe) represent a major class of supersoft X-ray sources (SSSs) in the central region of our neighbouring galaxy M 31. Significantly different SSS properties of CNe in the M 31 bulge and disk were indicated by recent X-ray population studies, which however considered only a small number of disk novae. We initiated a target of opportunity (ToO) program with XMM-Newton to observe the SSS phases of CNe in the disk of M 31 and improve the database for further population studies. We analysed two XMM-Newton ToO observations triggered in Aug 2011 and Jan 2012, respectively, and extracted X-ray spectra and light curves. We report the discovery of an X-ray counterpart to the M 31 disk nova M31N 2008-05d. The X-ray spectrum of the object allows us to classify it as a SSS parametrised by a blackbody temperature of 32+/-6 eV. More than three years after the nova outburst, the X-ray light curve of the SSS exhibits irregular, broad dip features. These dips affect primarily the very soft part of the X-ray spectrum, which might indicate absorption effects. Dipping SSS light curves are rarely observed in M 31 novae. As well as providing an unparalleled statistical sample, the M 31 population of novae with SSS counterparts produces frequent discoveries of unusual objects, thereby underlining the importance of regular monitoring., Comment: 6 pages, 4 figures, 1 table; accepted by Astronomy and Astrophysics

Results of several experimental tests performed to analyze the potential of CPV modules based on a single reflective stage and a fluid dielectric are summarized in this paper. Special emphasis is put on analyzing outdoor performance. The improved thermal design maintains the temperature difference between the solar cell and the ambient below 37°C during outdoor operation. In addition, the system performance under different spectra is analyzed experimentally determining the limiting subcell within the multijunction solar cell under variable spectral conditions.

Classical nova explosions are very energetic and frequent phenomena caused by explosive hydrogen burning on top of an accreting white dwarf. Observations of the recent nova V2487 Oph 1998 by the X-ray Multi-Mirror satellite (XMM-Newton) provide evidence that accretion (probably on a magnetic white dwarf) was reestablished as early as 2.7 years after the explosion. In addition, positional correlation with a source previously discovered by the Röntgen Satellite (ROSAT) in 1990 suggests that the site of a nova explosion had been seen in x-rays before the outburst.

The Rapid Burster (MXB1730-335) is a unique object showing both type I (thermonuclear) and type II (spasmodic accretion) X-ray bursts. It experiences regular outbursts every 100-200 days, lasting for 2-4 weeks. In January-March 2009 the Rapid Burster experienced a long (20 days) plateau. We obtained 20ks of Swift TOO observations during this period, between February 26 and March 5. A Type I X-ray burst with photospheric radius expansion was observed on 2009 March 5 among type II bursts.

We seek clarification of the nature of X-ray sources detected in M 31. Here we focus on CXOM31 J004253.1+411422, the brightness of which suggests that it belongs to the class of ultraluminous X-ray sources. We determine the X-ray properties of sources detected in the XMM-Newton Chandra monitoring program. We investigate spectral properties and search for periodic or quasi-periodic oscillations. A multi-component model is applied to the spectra obtained from XMM-Newton data to evaluate the relative contributions from thermal and non-thermal emission. The time dependence of this ratio is evaluated over a period of forty days. We simultaneously fit data from XMM-Newton EPIC-pn, MOS1 and MOS2 detectors with (non-thermal) powerlaw and (thermal) multicolored blackbody. The X-ray spectrum is best fit by the combination of a thermal component with kT ~ 1 keV and a powerlaw component with photon index approximately 2.6. From combined analysis of Chandra, Swift and XMM-Newton data, the unabsorbed total luminosity of this source decreases from ~ 3.8 x 10^{39} erg s^{-1} in the first observation to ~ 0.5 x 10^{39} ergs s^{-1} over a period of three months. The decay closely follows an exponential decline with a time constant of 32 days. The source spectrum evolves significantly, exhibiting a faster decline of the thermal component. We do not find evidence of any significant temporal features in the power density spectrum. The presence of a thermal component at kT ~ 1 keV in conjunction with a non-thermal high energy tail, is also consistent with spectral properties of other ULXs in the "high state". Our analysis indicates that the underlying source of this first ULX in M~31 is a black hole of mass, M > 13 M_{sun}, accreting near the Eddington limit, that underwent a transient outburst followed by an exponential decay reminiscent of transients associated with galactic X-ray novae., Accepted by Astronomy and Astrophysics. 6 pages, 8 figures

The 10th recorded outburst of the recurrent eclipsing nova USco was observed simultaneously in X-ray, UV, and optical by XMM-Newton on days 22.9 and 34.9 after outburst. Two full passages of the companion in front of the nova ejecta were observed, witnessing the reformation of the accretion disk. On day 22.9, we observed smooth eclipses in UV and optical but deep dips in the X-ray light curve which disappeared by day 34.9, then yielding clean eclipses in all bands. X-ray dips can be caused by clumpy absorbing material that intersects the line of sight while moving along highly elliptical trajectories. Cold material from the companion could explain the absence of dips in UV and optical light. The disappearance of X-ray dips before day 34.9 implies significant progress in the formation of the disk. The X-ray spectra contain photospheric continuum emission plus strong emission lines, but no clear absorption lines. Both continuum and emission lines in the X-ray spectra indicate a temperature increase from day 22.9 to day 34.9. We find clear evidence in the spectra and light curves for Thompson scattering of the photospheric emission from the white dwarf. Photospheric absorption lines can be smeared out during scattering in a plasma of fast electrons. We also find spectral signatures of resonant line scattering that lead to the observation of the strong emission lines. Their dominance could be a general phenomenon in high-inclination systems such as Cal87., Comment: Submitted to ApJ. 16 pages, 16 figures

[Abridged] Classical novae (CNe) represent the major class of supersoft X-ray sources (SSSs) in the central region of our neighbouring galaxy M 31. We performed a dedicated monitoring of the M 31 central region with XMM-Newton and Chandra between Nov 2007 and Feb 2008 and between Nov 2008 and Feb 2009 respectively, in order to find SSS counterparts of CNe, determine the duration of their SSS phase and derive physical outburst parameters. We systematically searched our data for X-ray counterparts of CNe and determined their X-ray light curves and spectral properties. We detected in total 17 X-ray counterparts of CNe in M 31, only four of which were known previously. These latter sources are still active 12.5, 11.0, 7.4 and 4.8 years after the optical outburst. From the 17 X-ray counterparts 13 were classified as SSSs. Four novae displayed short SSS phases (< 100 d). Based on these results and previous studies we compiled a catalogue of all novae with SSS counterparts in M 31 known so far. We used this catalogue to derive correlations between the following X-ray and optical nova parameters: turn-on time, turn-off time, effective temperature (X-ray), t2 decay time and expansion velocity of the ejected envelope (optical). Furthermore, we found a first hint for the existence of a difference between SSS parameters of novae associated with the stellar populations of the M 31 bulge and disk. Additionally, we conducted a Monte Carlo Markov Chain simulation on the intrinsic fraction of novae with SSS phase. This simulation showed that the relatively high fraction of novae without detected SSS emission might be explained by the inevitably incomplete coverage with X-ray observations in combination with a large fraction of novae with short SSS states, as expected from the WD mass distribution. In order to verify our results with an increased sample further monitoring observations are needed., 31 pages, 23 figures, 10 tables; submitted to A&A

Publicado en línea el 9 de febrero de 2010., Nova V5116 Sgr 2005 No. 2, discovered on 2005 July 4, was observed with XMM-Newton in March 2007, 20 months after the optical outburst. The X-ray spectrum showed that the nova had evolved to a pure supersoft X-ray source, indicative of residual H-burning on top of the white dwarf. The X-ray light-curve shows abrupt decreases and increases of the flux by a factor 8 with a periodicity of 2.97 h, consistent with the possible orbital period of the system. The EPIC spectra are well fit with an ONe white dwarf atmosphere model, with the same temperature both in the low and the high flux periods. This rules out an intrinsic variation of the X-ray source as the origin of the flux changes, and points to a possible partial eclipse as the origin of the variable light curve. The RGS high resolution spectra support this scenario showing a number of emission features in the low flux state, which either disappear or change into absorption features in the high flux state. A new XMM-Newton observation in March 2009 shows the SSS had turned off and V51 16 Sgr had evolved into a weaker and harder X-ray source.

We summarize our optical monitoring program of VY Scl stars with the SMARTS telescopes, and triggered X-ray as well as optical observations after/during state transitions of V504 Cen and VY Scl., Comment: 4 pages, Proc. of workshop "SuperSoft X-ray Sources - New Developments", ESTEC/Villafranca, May 2009, to appear in AN

Classical novae (CNe) have recently been reported to represent the major class of supersoft X-ray sources (SSSs) in the central area of our neighbouring galaxy M 31. This paper presents a review of results from recent X-ray observations of M 31 with XMM-Newton and Chandra. We carried out a dedicated optical and X-ray monitoring program of CNe and SSSs in the central area of M 31. We discovered the first SSSs in M 31 globular clusters (GCs) and their connection to the very first discovered CN in a M 31 GC. This result may have an impact on the CN rate in GCs. Furthermore, in our optical and X-ray monitoring data we discovered the CN M31N 2007-11a, which shows a very short SSS phase of 29 - 52 days. Short SSS states (durations < 100 days) of CNe indicate massive white dwarfs (WDs) that are candidate progenitors of supernovae type Ia. In the case of M31N 2007-11a, the optical and X-ray light curves suggest a binary containing a WD with M_WD > 1.0 M_sun. Finally, we present the discovery of the SSS counterpart of the CN M31N 2006-04a. The X-ray light curve of M31N 2006-04a shows short-time variability, which might indicate an orbital period of about 2 hours., 4 pages, 1 figure; Proc. of workshop "Supersoft X-ray Sources - New Developments", ESAC, May 2009; accepted for publication in Astronomische Nachrichten

6 pages, 1 figure, 1 table.-- Pre-print archive.-- Refereed workshop proceedings will be published as a regular issue of "Astronomische Nachrichten (Astronomical Notes)"., Detection of X-rays from classical novae, both in outburst and post-outburst, provides unique and crucial information about the explosion mechanism. Soft X-rays reveal the hot white dwarf photosphere, whenever hydrogen (H) nuclear burning is still on and expanding envelope is transparent enough, whereas harder X-rays give information about the ejecta and/or the accretion flow in the reborn cataclysmic variable. The duration of the supersoft X-ray emission phase is related to the turn-off of the classical nova, i.e., of the H-burning on top of the white dwarf core. A review of X-ray observations is presented, with a special emphasis on the implications for the duration of post-outburst steady H-burning and its theoretical explanation. The particular case of recurrent novae (both the ”standard” objects and the recently discovered ones) will also be reviewed, in terms of theoretical feasibility of short recurrence periods, as well as regarding implications for scenarios of type Ia supernovae., This research has been funded by theMICINN projects AYA 2008-01839/ESP, AYA 2008-04211-C02-01 and AYA 2007-66256, the AGAUR project 2009 SGR 315 and by FEDER funds.

Classical novae are phenomena caused by explosive hydrogen burning on an accreting white dwarf. So far, only one classical nova has been identified in X-rays before the actual optical outburst occurred (V2487 Oph). The recently discovered nova, V2491 Cyg, is one of the fastest (He/N) novae observed so far. Using archival ROSAT, XMM-Newton and Swift data, we show that V2491 Cyg was a persistent X-ray source during its quiescent time before the optical outburst. We present the X-ray spectral characteristics and derive X-ray fluxes. The pre-outburst X-ray emission is variable, and at least in one observation it shows a very soft X-ray source., Comment: 4 pages, 2 figures. Astronomy and Astrophysics, in press

Short supersoft X-ray source (SSS) states (durations < 100 days) of classical novae (CNe) indicate massive white dwarfs that are candidate progenitors of supernovae type Ia. We carry out a dedicated optical and X-ray monitoring program of CNe in the bulge of M 31. We discovered M31N 2007-11a and determined its optical and X-ray light curve. We used the robotic Super-LOTIS telescope to obtain the optical data and XMM-Newton and Chandra observations to discover an X-ray counterpart to that nova. Nova M31N 2007-11a is a very fast CN, exhibiting a very short SSS state with an appearance time of 6-16 days after outburst and a turn-off time of 45-58 days after outburst. The optical and X-ray light curves of M31N 2007-11a suggest a binary containing a white dwarf with a mass greater than one solar mass., 4 pages, 2 figures, 2 tables, submitted to A&A Letters; v2: accepted version

We present detailed analysis of three globular cluster X-ray sources in the XMM-Newton extended survey of M31. The X-ray counterpart to the M31 globular cluster Bo 45 (XBo 45) was observed with XMM-Newton on 2006 December 26. Its combined pn+MOS 0.3--10 keV lightcurve exhibited a r.m.s variability of ~10%, and its 0.3--7.0 keV emission spectrum was well described by an absorbed power law with photon index 1.44$\pm$0.12. Its variability and emission is characteristic of low mass X-ray binaries (LMXBs) in the low-hard state, whether the accretor is a neutron star or black hole. Such behaviour is typically observed at luminosities $\la$10% Eddington. However, XBo 45 exhibited this behaviour at an unabsorbed, 0.3--10 keV luminosity of 2.5$\pm0.2\times 10^{38}$ erg s$^{-1}$, or{~140%} Eddington for a 1.4 $M_{\odot}$ neutron star accreting hydrogen. Hence, we identify XBo 45 as a new candidate black hole LMXB. XBo 45 appears to have been consistently bright for ~30 years, consistent with theoretical prediction for a globular cluster black hole binary formed via tidal capture. Bo 375 was observed in the 2007, January 2 XMM-Newton observation, and has a two-component spectrum that is typical for a bright neutron star LMXB. Bo 135 was observed in the same field as Bo 45, and could contain either a black hole or neutron star., Accepted by ApJ, 16 pages, 5 figures. This version includes the final changes made at the request of the referee

The X-ray binary XTE J1817-330 was discovered in outburst on 26 January 2006 with RXTE/ASM. One year later, another X-ray transient discovered in 1996, XTE J1856+053, was detected by RXTE during a new outburst on 28 February 2007. We triggered XMM-Newton target of opportunity observations on these two objects to constrain their parameters and search for a stellar black holes. We summarize the properties of these two X-ray transients and show that the soft X-ray spectra indicate indeed the presence of an accreting stellar black hole in each of the two systems., to appear in the proceedings of the Second Kolkata Conference on Observational Evidence for Black Holes in the Universe, Feb. 2008, Editor Sandip Chakrabarti, AIP

The black hole candidate XTE J1817-330 was discovered in outburst on 26 January 2006 with RXTE/ASM. One year later, on 28 February 2007, another X-ray transient discovered in 1996, XTE J1856+053, was detected by RXTE during a new outburst. We report on the spectra obtained by XMM-Newton of these two black hole candidates., Comment: Replaced with corrected version

The galactic black hole candidate XTE J1817-330 was discovered in outburst by RXTE in January 2006. We present here the results of an XMM-Newton Target of opportunity observation (TOO), performed on 13 March 2006 (44 days after the maximum), and an INTEGRAL observation performed on 15-18 February 2006 (18 days after the maximum). The EPIC-pn camera on-board XMM-Newton was used in the fast read-out Burst mode to avoid photon pile-up, while the RGSs were used in Spectroscopy high count-rate mode. We fit both the XMM-Newton and the INTEGRAL spectra with a two-component model consisting of a thermal accretion disk and a comptonizing hot corona. The soft X-ray spectrum is dominated by an accretion disk component, with a maximum temperature decreasing from 0.96+/-0.04 keV at the time of the INTEGRAL observation to 0.70+/-m0.01 keV on 13 March. The Optical Monitors on board INTEGRAL and XMM-Newton showed the source with magnitudes V: 11.3-11.4, U:15.0-15.1 and UVW1:14.7-14.8. The soft X-ray spectrum, together with the optical and UV data, show a low hydrogen column density towards the source, and several absorption lines, most likely of interstellar origin, are detected in the RGS spectrum: OI K-alpha, OI K-beta, OII, OIII and OVII, which trace both cold and hot components of the ISM. The soft X-ray spectrum indicates the presence of a black hole, with an estimate for the upper limit of the mass of 6.0(+4.0/-2.5) Msun., Accepted for publication in A&A

Aims: The galactic superluminal microquasar GRO J1655-40 started a new outburst in February 2005, after seven years in quiescence, rising to a high/soft state in March 2005. In this paper we study the X-ray spectra during this rise. Methods: We observed GRO J1655-40 with XMM-Newton, on 27 February 2005, in the low/hard state, and on three consecutive days in March 2005, during the rise of the source to its high/soft state. The EPIC-pn camera was used in the fast-read Burst mode to avoid photon pile-up. Results: First, we contributed to the improvement of the calibration of the EPIC-pn, since the high flux received from the source required some refinements in the correction of the Charge Transfer Efficiency of the camera.Second, we find that the X-ray spectrum of GRO J1655-40 is dominated in the high/soft state by the thermal emission from the accretion disk, with an inner radius of 13-14(D/3.2kpc)km and a maximum temperature of 1.3 keV. Two absorption lines are detected in the EPIC-pn spectra, at 6.7-6.8 and 7.8-8.0 keV, which can be identified either as blended Fe XXV and Fe XXVI K-alpha and K-beta lines, or as blueshifted Fe XXV. We find no orbital dependence on the X-ray properties, which provides an upper limit for the inclination of the system of 73 degr. The RGS spectrometers reveal interstellar absorption features at 17.2AA, 17.5AA (Fe L edges) and 23.54AA (OI K-alpha). Finally, while checking the interstellar origin of the OI line, we find a general correlation of the OI K-alpha line equivalent width with the hydrogen column density using several sources available in the literature., Comment: 10 pages, 7 figures, 4 tables. Revised version with important changes

The M 31 nova M31N 2008-12a was recently found to be a recurrent nova (RN) with a recurrence time of about 1 year. This is by far the fastest recurrence time scale of any known RNe. Our optical monitoring programme detected the predicted 2014 outburst of M31N 2008-12a in early October. We immediately initiated an X-ray/UV monitoring campaign with Swift to study the multiwavelength evolution of the outburst. We monitored M31N 2008-12a with daily Swift observations for 20 days after discovery, covering the entire supersoft X-ray source (SSS) phase. We detected SSS emission around day six after outburst. The SSS state lasted for approximately two weeks until about day 19. M31N 2008-12a was a bright X-ray source with a high blackbody temperature. The X-ray properties of this outburst were very similar to the 2013 eruption. Combined X-ray spectra show a fast rise and decline of the effective blackbody temperature. The short-term X-ray light curve showed strong, aperiodic variability which decreased significantly after about day 14. Overall, the X-ray properties of M31N 2008-12a are consistent with the average population properties of M 31 novae. The optical and X-ray light curves can be scaled uniformly to show similar time scales as those of the Galactic RNe U Sco or RS Oph. The SSS evolution time scales and effective temperatures are consistent with a high-mass WD. We predict the next outburst of M31N 2008-12a to occur in autumn 2015., 13 pages, 7 figures, 3 tables; accepted for publication in A&A

Optical novae have recently been identified as the major class of supersoft X-ray sources in M31 based on ROSAT and early XMM-Newton and Chandra observations. This paper reports on a search for X-ray counterparts of optical novae in M31 based on archival Chandra HRC-I and ACIS-I as well as XMM-Newton observations of the galaxy center region obtained from July 2004 to February 2005. We systematically determine X-ray brightness or upper limit for counterparts of all known optical novae with outbursts between November 2003 to the end of the X-ray coverage. In addition, we determine the X-ray brightnesses for counterparts of four novae with earlier outbursts. For comparison with the X-ray data we created a catalogue of optical novae in M31 based on our own nova search programs and on all novae reported in the literature. We collected all known properties and named the novae consistently following the CBAT scheme. We detect eleven out of 34 novae within a year after the optical outburst in X-rays. While for eleven novae we detect the end of the supersoft source phase, seven novae are still bright more than 1200, 1600, 1950, 2650, 3100, 3370 and 3380 d after outburst. One nova is detected to turn on 50 d, another 200 d after outburst. Three novae unexpectedly showed short X-ray outbursts starting within 50 d after the optical outburst and lasting only two to three months. The X-ray emission of several of the novae can be characterized as supersoft from hardness ratios and/or X-ray spectra or by comparing HRC-I count rates with ACIS-I count rates or upper limits. The number of detected optical novae at supersoft X-rays is much higher than previously estimated (>30%). We use the X-ray light curves to estimate the burned masses of the White Dwarf and of the ejecta., 27 pages, 8 figures, Table A.1 and A.2 available at CDS or from http://www.mpe.mpg.de/~m31novae/opt/m31/index.php

A hydrostatic and stationary white dwarf envelope model has been developed for the study of the post-outburst phases of classical novae and their soft X-ray emission. Several white dwarf masses and chemical compositions typical for classical novae have been considered. The results show that the luminosity, maximum effective temperature and envelope masses depend on the white dwarf mass and on the chemical composition. Envelope masses for which equilibrium solutions exist are pretty small ~10^{-7}-10^{-6} Msun, thus leading to a short duration of the soft X-ray emitting phase of classical novae, in agreement with most of the observations. The models presented provide a useful tool for the determination of the white dwarf properties from observable parameters in the X-ray range., 15 pages, 11 figures, 2 tables, accepted for publication in Astronomy & Astrophysics