arXiv subjects

K. L. Page

Publications and source records attributed to K. L. Page.

At least 73 records · Page 4Linked to original sources

arXiv2016

The panchromatic spectroscopic evolution of the classical CO nova V339 Del (Nova Del 2013) until X-ray turnoff

Classical novae are the product of thermonuclear runaway-initiated explosions occurring on accreting white dwarfs. V339 Del (Nova Delphinus 2013) was one of the brightest classical novae of the last hundred years. Spectroscopy and photometry are available from $γ$-rays through infrared at stages that have frequently not been well observed. The complete data set is intended to provide a benchmark for comparison with modeling and for understanding more sparsely monitored historical classical and recurrent novae. This paper is the first in the series of reports on the development of the nova. We report here on the early stages of the outburst, through the X-ray active stage. A time sequence of optical, flux calibrated high resolution spectra was obtained with the Nordic Optical Telescope (NOT) using FIES simultaneously, or contemporaneously, with the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope during the early stages of the outburst. These were supplemented with MERCATOR/HERMES optical spectra. High resolution IUE ultraviolet spectra of OS And 1986, taken during the Fe curtain phase, served as a template for the distance determination. We used standard plasma diagnostics (e.g., [O III] and [N II] line ratios, and the H$β$ line flux) to constrain electron densities and temperatures of the ejecta. Using Monte Carlo modeling of the ejecta, we derived the structure, filling factor, and mass from comparisons of the optical and ultraviolet line profiles.

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arXiv2016

The reproducible radio outbursts of SS Cygni

We present the results of our intensive radio observing campaign of the dwarf nova SS Cyg during its 2010 April outburst. We argue that the observed radio emission was produced by synchrotron emission from a transient radio jet. Comparing the radio light curves from previous and subsequent outbursts of this system (including high-resolution observations from outbursts in 2011 and 2012) shows that the typical long and short outbursts of this system exhibit reproducible radio outbursts that do not vary significantly between outbursts, which is consistent with the similarity of the observed optical, ultraviolet and X-ray light curves. Contemporaneous optical and X-ray observations show that the radio emission appears to have been triggered at the same time as the initial X-ray flare, which occurs as disk material first reaches the boundary layer. This raises the possibility that the boundary region may be involved in jet production in accreting white dwarf systems. Our high spatial resolution monitoring shows that the compact jet remained active throughout the outburst with no radio quenching.

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arXiv2016

Swift follow-up of the Gravitational Wave source GW150914

The Advanced LIGO observatory recently reported the first direct detection of gravitational waves (GW) which triggered ALIGO on 2015 September 14. We report on observations taken with the Swift satellite two days after the trigger. No new X-ray, optical, UV or hard X-ray sources were detected in our observations, which were focussed on nearby galaxies in the GW error region and covered 4.7 square degrees (~2% of the probability in the rapidly-available GW error region; 0.3% of the probability from the final GW error region, which was produced several months after the trigger). We describe the rapid Swift response and automated analysis of the X-ray telescope and UV/Optical Telescope data, and note the importance to electromagnetic follow up of early notification of the progenitor details inferred from GW analysis.

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arXiv2016

Pan-Chromatic observations of the Recurrent Nova LMC 2009a (LMC 1971b)

Nova LMC 2009a is confirmed as a Recurrent Nova (RN) from positional coincidence with nova LMC 1971b. The observational data set is one of the most comprehensive for any Galactic or extragalactic RN: optical and near-IR photometry from outburst until over 6 years later; optical spectra for the first 6 months, and Swift satellite Ultraviolet and X-ray observations from 9 days to almost 1 year post-outburst. We find $M_V = -8.4\pm0.8_{\mathrm{r}}\pm0.7_{\mathrm{s}}$ and expansion velocities between 1000 and 4000 km s$^{-1}$. Coronal line emission before day 9 indicates shocks in the ejecta. Strengthening of He II $λ$4686 preceded the emergence of the Super-Soft Source (SSS) in X-rays at $\sim63-70$ days, which was initially very variable. Periodic modulations, $P=1.2$ days, most probably orbital in nature, were evident in the UV and optical from day 43. Subsequently, the SSS shows an oscillation with the same period but with a delay of 0.28P. The progenitor system has been identified; the secondary is most likely a sub-giant feeding a luminous accretion disk. Properties of the SSS infer a white dwarf (WD) mass $1.1 \mathrm{M}_\odot \lesssim M_{\rm WD} \lesssim 1.3 \mathrm{M}_\odot$. If the accretion occurs at constant rate, $\dot{\it{M}}_{\rm acc} \simeq 3.6^{+4.7}_{-2.5} \times 10^{-7} \mathrm{M}_\odot$ yr$^{-1}$ is needed, consistent with nova models for an inter-eruption interval of 38 years, low outburst amplitude, progenitor position in the color-magnitude diagram, and spectral energy distribution at quiescence. We note striking similarities between LMC 2009a and the Galactic nova KT Eri, suggesting that KT Eri is a candidate RN.

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arXiv2015

Late time multi wavelength observations of Swift J1644+5734: A luminous optical/IR bump and quiescent X-ray emission

We present late-time multi-wavelength observations of Swift J1644+57, suggested to be a relativistic tidal disruption flare (TDF). Our observations extend to >4 years from discovery, and show that 1.4 years after outburst the relativistic jet switched-off on a timescale less than tens of days, corresponding to a power-law decay faster than $t^{-70}$. Beyond this point weak X-rays continue to be detected at an approximately constant luminosity of $L_X \sim 5 \times 10^{42}$ erg s$^{-1}$, and are marginally inconsistent with a continuing decay of $t^{-5/3}$, similar to that seen prior to the switch-off. Host photometry enables us to infer a black hole mass of $M_{BH}=3 \times 10^6$ M$_{\odot}$, consistent with the late time X-ray luminosity arising from sub-Eddington accretion onto the black hole in the form of either an unusually optically faint AGN or a slowly varying phase of the transient. Optical/IR observations show a clear bump in the light curve at timescales of 30-50 days, with a peak magnitude (corrected for host galaxy extinction) of $M_R \sim -22-23$. The luminosity of the bump is significantly higher than seen in other, non-relativistic TDFs and does not match any re-brightening seen at X-ray or radio wavelengths. Its luminosity, light curve shape and spectrum are broadly similar to those seen in superluminous SNe, although subject to large uncertainties in the correction of the significant host extinction. We discuss these observations in the context of both TDF and massive star origins for Swift J1644+5734 and other candidate relativistic tidal flares.

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arXiv2015

Swift detection of the super-swift switch-on of the super-soft phase in nova V745 Sco (2014)

V745 Sco is a recurrent nova, with the most recent eruption occurring in February 2014. V745 Sco was first observed by Swift a mere 3.7 hr after the announcement of the optical discovery, with the super-soft X-ray emission being detected around four days later and lasting for only ~two days, making it both the fastest follow-up of a nova by Swift and the earliest switch-on of super-soft emission yet detected. Such an early switch-on time suggests a combination of a very high velocity outflow and low ejected mass and, together with the high effective temperature reached by the super-soft emission, a high mass white dwarf (>1.3 M_sun). The X-ray spectral evolution was followed from an early epoch where shocked emission was evident, through the entirety of the super-soft phase, showing evolving column density, emission lines, absorption edges and thermal continuum temperature. UV grism data were also obtained throughout the super-soft interval, with the spectra showing mainly emission lines from lower ionization transitions and the Balmer continuum in emission. V745 Sco is compared with both V2491 Cyg (another nova with a very short super-soft phase) and M31N 2008-12a (the most rapidly recurring nova yet discovered). The longer recurrence time compared to M31N 2008-12a could be due to a lower mass accretion rate, although inclination of the system may also play a part. Nova V745 Sco (2014) revealed the fastest evolving super-soft source phase yet discovered, providing a detailed and informative dataset for study.

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arXiv2014

A NuSTAR observation of the fast symbiotic nova V745 Sco in outburst

The fast recurrent nova V745 Sco was observed in the 3-79 keV X-rays band with NuSTAR 10 days after the optical discovery. The measured X-ray emission is consistent with a collisionally ionized optically thin plasma at temperature of about 2.7 keV. A prominent iron line observed at 6.7 keV does not require enhanced iron in the ejecta. We attribute the X-ray flux to shocked circumstellar material. No X-ray emission was observed at energies above 20 keV, and the flux in the 3-20 keV range was about 1.6 $\times$ 10$^{-11}$ erg cm$^{-2}$ s$^{-1}$. The emission measure indicates an average electron density of order of 10$^7$ cm$^{-3}$. The X-ray flux in the 0.3-10 keV band almost simultaneously measured with Swift was about 40 times larger, mainly due to the luminous central supersoft source emitting at energy below 1 keV. The fact that the NuSTAR spectrum cannot be fitted with a power law, and the lack of hard X-ray emission, allow us to rule out Comptonized gamma rays, and to place an upper limit of the order of 10$^{-11}$ erg cm$^{-2}$ s$^{-1}$ on the gamma-ray flux of the nova on the tenth day of the outburst.

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arXiv2014

XMMFITCAT: The XMM-Newton spectral-fit database

The XMM-Newton spectral-fit database (XMMFITCAT) is a catalogue of spectral fitting results for the source detections within the XMM-Newton Serendipitous source catalogue with more than 50 net (background-subtracted) counts per detector in the 0.5-10 keV energy band. Its most recent version, constructed from the latest version of the XMM-Newton catalogue, the 3XMM Data Release 4 (3XMM-DR4), contains spectral-fitting results for $\gtrsim$ 114,000 detections, corresponding to $\simeq$ 78,000 unique sources. Three energy bands are defined and used in the construction of XMMFITCAT: Soft (0.5-2 keV), Hard (2-10 keV), and Full (0.5-10 keV) bands. Six spectral models, three simple and three more complex models, were implemented and applied to the spectral data. Simple models are applied to all sources, whereas complex models are applied to observations with more than 500 counts (30%). XMMFITCAT includes best-fit parameters and errors, fluxes, and goodness of fit estimates for all fitted models. XMMFITCAT has been conceived to provide the astronomical community with a tool to construct large and representative samples of X-ray sources by allowing source selection according to spectral properties, as well as characterise the X-ray properties of samples selected in different wavelengths. We present in this paper the main details of the construction of this database, and summarise its main characteristics.

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arXiv2014

X-ray and UV observations of V751 Cyg in an optical high state

Aims: The VY Scl system (anti-dwarf nova) V751 Cyg is examined following a claim of a super-soft spectrum in the optical low state. Methods: A serendipitous XMM-Newton X-ray observation and, 21 months later, Swift X-ray and UV observations, have provided the best such data on this source so far. These optical high-state datasets are used to study the flux and spectral variability of V751 Cyg. Results: Both the XMM-Newton and Swift data show evidence for modulation of the X-rays for the first time at the known 3.467 hr orbital period of V751 Cyg. In two Swift observations, taken ten days apart, the mean X-ray flux remained unchanged, while the UV source brightened by half a magnitude. The X-ray spectrum was not super-soft during the optical high state, but rather due to multi-temperature optically thin emission, with significant (10^{21-22} cm^-2) absorption, which was higher in the observation by Swift than that of XMM-Newton. The X-ray flux is harder at orbital minimum, suggesting that the modulation is related to absorption, perhaps linked to the azimuthally asymmetric wind absorption seen previously in H-alpha.

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arXiv2014

Searching for highly obscured AGN in the XMM-Newton serendipitous source catalog

The majority of active galactic nuclei (AGN) are obscured by large amounts of absorbing material that makes them invisible at many wavelengths. X-rays, given their penetrating power, provide the most secure way for finding these AGN. The XMM-Newton serendipitous source catalog is the largest catalog of X-ray sources ever produced; it contains about half a million detections. These sources are mostly AGN. We have derived X-ray spectral fits for very many 3XMM-DR4 sources ($\gtrsim$ 114 000 observations, corresponding to $\sim$ 77 000 unique sources), which contain more than 50 source photons per detector. Here, we use a subsample of $\simeq$ 1000 AGN in the footprint of the SDSS area (covering 120 deg$^2$) with available spectroscopic redshifts. We searched for highly obscured AGN by applying an automated selection technique based on X-ray spectral analysis that is capable of efficiently selecting AGN. The selection is based on the presence of either a) flat rest-frame spectra; b) flat observed spectra; c) an absorption turnover, indicative of a high rest-frame column density; or d) an Fe K$α$ line with an equivalent width > 500 eV. We found 81 highly obscured candidate sources. Subsequent detailed manual spectral fits revealed that 28 of them are heavily absorbed by column densities higher than 10$^{23}$ cm$^{-2}$. Of these 28 AGN, 15 are candidate Compton-thick AGN on the basis of either a high column density, consistent within the 90% confidence level with N$_{\rm H}$ $>$10$^{24}$ cm$^{-2}$, or a large equivalent width (>500 eV) of the Fe K$α$ line. Another six are associated with near-Compton-thick AGN with column densities of $\sim$ 5$\times$10$^{23}$ cm$^{-2}$. A combination of selection criteria a) and c) for low-quality spectra, and a) and d) for medium- to high-quality spectra, pinpoint highly absorbed AGN with an efficiency of 80%.

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arXiv2014

GRB 130427A: a Nearby Ordinary Monster

Long-duration Gamma-Ray Bursts (GRBs) are an extremely rare outcome of the collapse of massive stars, and are typically found in the distant Universe. Because of its intrinsic luminosity ($L\sim 3 \times 10^{53}$ erg s$^{-1}$) and its relative proximity ($z=0.34$), GRB 130427A was a unique event that reached the highest fluence observed in the gamma-ray band. Here we present a comprehensive multiwavelength view of GRB 130427A with Swift, the 2-m Liverpool and Faulkes telescopes and by other ground-based facilities, highlighting the evolution of the burst emission from the prompt to the afterglow phase. The properties of GRB 130427A are similar to those of the most luminous, high-redshift GRBs, suggesting that a common central engine is responsible for producing GRBs in both the contemporary and the early Universe and over the full range of GRB isotropic energies.

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arXiv2014

Hubble Space Telescope Far Ultraviolet Spectroscopy of the Recurrent Nova T Pyxidis

With six recorded nova outbursts, the prototypical recurrent nova T Pyxidis is the ideal cataclysmic variable system to assess the net change of the white dwarf mass within a nova cycle. Recent estimates of the mass ejected in the 2011 outburst ranged from a few 1.E-5 sollar mass to 3.3E-4 sollar mass, and assuming a mass accretion rate of 1.E-8 to 1.E-7 Sollar mass/yr for 44yrs, it has been concluded that the white dwaf in T Pyx is actually losing mass. Using NLTE disk modeling spectra to fit our recently obtained Hubble Space Telescope (HST) COS and STIS spectra, we find a mass accretion rate of up to two orders of magnitude larger than previously estimated. Our larger mass accretion rate is due mainly to the newly derived distance of T Pyx (4.8kpc; Sokoloski et al. 2013, larger than the previous 3.5kpc estimate), our derived reddening of E(B-V)=0.35 (based on combined IUE and GALEX spectra) and NLTE disk modeling (compared to black body and raw flux estimates in earlier works). We find that for most values of the reddening (0.25 < E(B-V) < 0.50) and white dwaf mass (0.70 to 1.35 Sollar mass) the accreted mass is larger than the ejected mass. Only for a low reddening (0.25 and smaller) combined with a large white dwaf mass (0.9 sollar mass and larger) is the ejected mass larger than the accreted one. However, the best spectral fitting results are obtained for a larger value of the reddening.

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arXiv2014

GRB 120422A/SN 2012bz: Bridging the Gap between Low- And High-Luminosity GRBs

At low redshift, a handful of gamma-ray bursts (GRBs) have been discovered with peak luminosities ($L_{\rm iso} < 10^{48.5}~\rm{erg\,s}^{-1}$) substantially lower than the average of the more distant ones ($L_{\rm iso} > 10^{49.5}~\rm{erg\,s}^{-1}$). The properties of several low-luminosity (low-$L$) GRBs indicate that they can be due to shock break-out, as opposed to the emission from ultrarelativistic jets. Owing to this, it is highly debated how both populations are connected, and whether there is a continuum between them. The burst at redshift $z=0.283$ from 2012 April 22 is one of the very few examples of intermediate-$L$ GRBs with a $γ$-ray luminosity of $L\sim10^{48.9}~\rm{erg\,s}^{-1}$ that have been detected up to now. Together with the robust detection of its accompanying supernova SN 2012bz, it has the potential to answer important questions on the origin of low- and high-$L$ GRBs and the GRB-SN connection. We carried out a spectroscopy campaign using medium- and low-resolution spectrographs at 6--10-m class telescopes, covering the time span of 37.3 days, and a multi-wavelength imaging campaign from radio to X-ray energies over a duration of $\sim270$ days. Furthermore, we used a tuneable filter centred at H$α$ to map star formation in the host galaxy and the surrounding galaxies. We used these data to extract and model the properties of different radiation components and incorporate spectral-energy-distribution fitting techniques to extract the properties of the host galaxy. Modelling the light curve and spectral energy distribution from the radio to the X-rays revealed the blast-wave to expand with an initial Lorentz factor of $Γ_0\sim60$, low for a high-$L$ GRB, and that the afterglow had an exceptional low peak luminosity-density of $\lesssim2\times10^{30}~\rm{erg\,s}^{-1}\,\rm{Hz}^{-1}$ in the sub-mm. [Abridged]

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arXiv2013

1SXPS: A deep Swift X-ray Telescope point source catalog with light curves and spectra

We present the 1SXPS (Swift-XRT Point Source) catalog of 151,524 X-ray point-sources detected by the Swift-XRT in 8 years of operation. The catalog covers 1905 square degrees distributed approximately uniformly on the sky. We analyze the data in two ways. First we consider all observations individually, for which we have a typical sensitivity of ~3e-13 erg/cm2/s (0.3--10 keV). Then we co-add all data covering the same location on the sky: these images have a typical sensitivity of ~9e-14 erg/cm2/s (0.3--10 keV). Our sky coverage is nearly 2.5 times that of 3XMM-DR4, although the catalog is a factor of ~1.5 less sensitive. The median position error is 5.5" (90% confidence), including systematics. Our source detection method improves on that used in previous XRT catalogs and we report >68,000 new X-ray sources. The goals and observing strategy of the Swift satellite allow us to probe source variability on multiple timescales, and we find ~30,000 variable objects in our catalog. For every source we give positions, fluxes, time series (in four energy bands and two hardness ratios), estimates of the spectral properties, spectra and spectral fits for the brightest sources, and variability probabilities in multiple energy bands and timescales.

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arXiv2013

Obscuration effects in Super-Soft-Source X-ray spectra

Super-Soft-Source (SSS) X-ray spectra are blackbody-like spectra with effective temperatures ~3-7x10^5 K and luminosities of 10^{35-38} erg/s. SSS grating spectra display atmospheric absorption lines. Radiation transport atmosphere models can be used to derive physical parameters, but more sophisticated models are required. We bypass the complications of spectral models and concentrate on the data in a comparative, qualitative study. We inspect all available X-ray grating SSS spectra to determine systematic, model-independent trends. We use comparative plots of spectra of different systems to find common and different features. The results are interpreted in the context of system parameters obtained from the literature. We find two distinct types of SSS spectra which we name SSa and SSe. Their main observational characteristics are either clearly visible absorption lines or emission lines, respectively, while both types contain atmospheric continuum emission. SSe may be obscured SSa systems, which is supported by similarities between SSe and SSa with obscured and unobscured AGN, respectively. Further, we find all known or suspected high-inclination systems to emit permanently in an SSe state. Some sources are found to transition between SSa and SSe states, becoming SSe when fainter. SSS spectra are subject to various occultation processes. In Cal 87, the accretion disc blocks the central hot source when viewed edge on. In novae, the accretion disc may have been destroyed during the initial explosion but could have reformed by the time of the SSS phase. In addition, clumpy ejecta may lead to temporary obscuration events. The emission lines originate from reprocessed emission in the accretion disc, its wind or further out in clumpy ejecta while Thomson scattering allows continuum emission to be visible also during total obscuration of the central hot source.

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arXiv2013

Extraordinary luminous soft X-ray transient MAXI J0158-744 as an ignition of a nova on a very massive O-Ne white dwarf

We present the observation of an extraordinary luminous soft X-ray transient, MAXI J0158-744, by the Monitor of All-sky X-ray Image (MAXI) on 2011 November 11. This transient is characterized by a soft X-ray spectrum, a short duration (1.3 x 10^3 s < ΔT_d < 1.10 x 10^4 s), a very rapid rise (< 5.5 x 10^3 s), and a huge peak luminosity of 2 x 10^40 erg s^-1 in 0.7-7.0 keV band. With Swift observations and optical spectroscopy from the Small and Moderate Aperture Research Telescope System (SMARTS), we confirmed that the transient is a nova explosion, on a white dwarf in a binary with a Be star, located near the Small Magellanic Cloud. An extremely early turn-on of the super-soft X-ray source (SSS) phase (< 0.44 d), the short SSS phase duration of about one month, and a 0.92 keV neon emission line found in the third MAXI scan, 1296 s after the first detection, suggest that the explosion involves a small amount of ejecta and is produced on an unusually massive O-Ne white dwarf close to, or possibly over, the Chandrasekhar limit. We propose that the huge luminosity detected with MAXI was due to the fireball phase, a direct manifestation of the ignition of the thermonuclear runaway process in a nova explosion.

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arXiv2013

The 7.1 hour X-ray-UV-NIR period of the gamma-ray classical Nova Monocerotis 2012

Nova Mon 2012 is the third gamma-ray transient identified with a thermonuclear runaway on a white dwarf, that is, a nova event. Swift monitoring has revealed the distinct evolution of the harder and super-soft X-ray spectral components, while Swift-UV and V and I-band photometry show a gradual decline with subtle changes of slope. During the super-soft emission phase, a coherent 7.1 hr modulation was found in the soft X-ray, UV, optical and near-IR data, varying in phase across all wavebands. Assuming this period to be orbital, the system has a near-main sequence secondary, with little appreciable stellar wind. This distinguishes it from the first GeV nova, V407 Cyg, where the gamma-rays were proposed to form through shock-accelerated particles as the ejecta interacted with the red giant wind. We favor a model in which the gamma-rays arise from the shock of the ejecta with material close to the white dwarf in the orbital plane. This suggests that classical novae may commonly be GeV sources. We ascribe the orbital modulation to a raised section of an accretion disk passing through the line of sight, periodically blocking and reflecting much of the emission. The disk must, therefore, have reformed by day 150 after outburst.

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arXiv2013

A new population of ultra-long duration gamma-ray bursts

We present comprehensive multiwavelength observations of three gamma-ray bursts (GRBs) with durations of several thousand seconds. We demonstrate that these events are extragalactic transients; in particular we resolve the long-standing conundrum of the distance of GRB 101225A (the "Christmas-day burst"), finding it to have a redshift z=0.847, and showing that two apparently similar events (GRB 111209A and GRB 121027A) lie at z=0.677 and z=1.773 respectively. The systems show extremely unusual X-ray and optical lightcurves, very different from classical GRBs, with long lasting highly variable X-ray emission and optical light curves that exhibit little correlation with the behaviour seen in the X-ray. Their host galaxies are faint, compact, and highly star forming dwarf galaxies, typical of "blue compact galaxies". We propose that these bursts are the prototypes of a hitherto largely unrecognized population of ultra-long GRBs, that while observationally difficult to detect may be astrophysically relatively common. The long durations may naturally be explained by the engine driven explosions of stars of much larger radii than normally considered for GRB progenitors which are thought to have compact Wolf-Rayet progenitor stars. However, we cannot unambiguously identify supernova signatures within their light curves or spectra. We also consider the alternative possibility that they arise from the tidal disruption of stars by supermassive black holes.

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