arXiv subjects

R. D. Saxton

Publications and source records attributed to R. D. Saxton.

At least 37 records · Page 2Linked to original sources

arXiv2012

A tidal disruption-like X-ray flare from the quiescent galaxy SDSS J120136.02+300305.5

SDSS J120136.02+300305.5 was detected in an XMM-Newton slew from June 2010 with a flux 56 times higher than an upper limit from ROSAT, corresponding to Lx~3x10^44 ergs/s. It has the optical spectrum of a quiescent galaxy (z=0.146). Overall the X-ray flux has evolved consistently with the canonical t^-5/3 model, expected for returning stellar debris from a tidal disruption event, fading by a factor ~300 over 300 days. In detail the source is very variable and became invisible to Swift between 27 and 48 days after discovery, perhaps due to self-absorption. The X-ray spectrum is soft but is not the expected tail of optically thick thermal emission. It may be fit with a Bremsstrahlung or double-power-law model and is seen to soften with time and declining flux. Optical spectra taken 12 days and 11 months after discovery indicate a deficit of material in the broad line and coronal line regions of this galaxy, while a deep radio non-detection implies that a jet was not launched during this event.

astro-ph.CO↗

arXiv2011

A New Comprehensive 2-D Model of the Point Spread Functions of the XMM-Newton EPIC Telescopes : Spurious Source Suppression and Improved Positional Accuracy

We describe here a new full 2-D parameterization of the PSFs of the three XMM-Newton EPIC telescopes as a function of instrument, energy, off-axis angle and azimuthal angle, covering the whole field-of-view of the three EPIC detectors. It models the general PSF envelopes, the primary and secondary spokes, their radial dependencies, and the large-scale azimuthal variations. This PSF model has been constructed via the stacking and centering of a large number of bright, but not significantly piled-up point sources from the full field-of-view of each EPIC detector, and azimuthally filtering the resultant PSF envelopes to form the spoke structures and the gross azimuthal shapes observed. This PSF model is available for use within the XMM-Newton Science Analysis System via the usage of Current Calibration Files XRTi_XPSF_0011.CCF and later versions. Initial source-searching tests showed substantial reductions in the numbers of spurious sources being detected in the wings of bright point sources. Furthermore, we have uncovered a systematic error in the previous PSF system, affecting the entire mission to date, whereby returned source RA and Dec values are seen to vary sinusoidally about the true position (amplitude ~0.8") with source azimuthal position. The new PSF system is now available and is seen as a major improvement with regard to the detection of spurious sources. The new PSF also largely removes the discovered astrometry error and is seen to improve the positional accuracy of EPIC. The modular nature of the PSF system allows for further refinements in the future.

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arXiv2010

Swift follow-up of unidentified X-ray sources in the XMM-Newton Slew Survey

We present deep Swift follow-up observations of a sample of 94 unidentified X-ray sources from the XMM-Newton Slew Survey. The X-ray Telescope on-board Swift detected 29% of the sample sources; the flux limits for undetected sources suggests the bulk of the Slew Survey sources are drawn from one or more transient populations. We report revised X-ray positions for the XRT-detected sources, with typical uncertainties of 2.9", reducing the number of catalogued optical matches to just a single source in most cases. We characterise the sources detected by Swift through their X-ray spectra and variability and via UVOT photometry and catalogued nIR, optical and radio observations. Six sources can be associated with known objects and 8 may be associated with unidentified ROSAT sources within the 3-sigma error radii of our revised X-ray positions. We find 10 of the 30 XRT-detected sources are clearly stellar in nature, including one periodic variable star and 2 high proper motion stars. For 11 sources we propose an AGN classification, among which 4 are detected with BAT and 3 have redshifts spanning z = 0.2 - 0.9 obtained from the literature or from optical spectroscopy presented here. The 67 Slew Survey sources we do not detect with Swift are studied via their characteristics in the Slew Survey and by comparison with the XRT and BAT detected population. We suggest that these are mostly if not all extragalactic, though unlikely to be highly absorbed sources in the X-rays such as Compton thick AGN. A large number of these are highly variable soft X-ray sources. A small fraction of mainly hard-band detections may be spurious. This follow-up programme brings us a step further to completing the identifications of a substantial sample of XMM-Newton Slew Survey sources, important for understanding the nature of the transient sky and allowing flux-limited samples to be constructed.

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arXiv2009

The XMM-Newton Slew Survey: Towards The Whole X-ray Sky and the Rarest X-ray Events

The data collected by XMM-Newton as it slews between pointings currently cover almost half the entire sky, and many familiar features and new sources are visible. The soft-band sensitivity limit of the Slew is close to that of the RASS, and a large-area Slew-RASS comparison now provides the best opportunity for discovering extremely rare high-variability objects.

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arXiv2009

XMMSL1 J060636.2-694933: An XMM-Newton Slew discovery and Swift/Magellan follow up of a new Classical Nova in the LMC

In order to discover new X-ray transients, the data taken by XMM-Newton as it slews between targets are being processed and cross-correlated with other X-ray observations. A bright source, XMMSL1 J060636.2-694933, was detected on 18 July 2006 at a position where no previous X-ray source had been seen. The XMM-Newton slew data, plus follow-up dedicated XMM-Newton and Swift observations, plus optical data acquired with the Magellan Clay telescope, and archival All-Sky Automated Survey (ASAS) data were used to classify the new object, and to investigate its properties. No XMM-Newton slew X-ray counts are detected above 1 keV and the source is seen to be over five hundred times brighter than the ROSAT All-Sky Survey upper limit at that position. The line-rich optical spectrum acquired with the Magellan telescope allows the object to be classified as an Ao auroral phase nova, and the soft X-ray spectrum indicates that the nova was in a super-soft source state in the X-ray decline seen in the follow-up X-ray observations. The archival ASAS data suggests that the nova at onset (Oct 2005) was a 'very fast' nova, and an estimate of its distance is consistent with the nova being situated within the LMC. With the discovery presented here of a new classical nova in the LMC, it is clear that XMM-Newton slew data are continuing to offer a powerful opportunity to find new X-ray transient objects.

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arXiv2009

Pre-nova X-ray observations of V2491 Cyg (Nova Cyg 2008b)

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.

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arXiv2009

Discovery of a large and bright bow shock nebula associated with low mass X-ray binary SAX J1712.6-3739

In a multiwavelength program dedicated to identifying optical counterparts of faint persistent X-ray sources in the Galactic Bulge, we find an accurate X-ray position of SAX J1712.6-3739 through Chandra observations, and discover its faint optical counterpart using our data from EFOSC2 on the ESO 3.6m telescope. We find this source to be a highly extincted neutron star LMXB with blue optical colours. We serendipitously discover a relatively bright and large bow shock shaped nebula in our deep narrowband H alpha imaging, most likely associated with the X-ray binary. A nebula like this has never been observed before in association with a LMXB, and as such provides a unique laboratory to study the energetics of accretion and jets. We put forward different models to explain the possible ways the LMXB may form this nebulosity, and outline how they can be confirmed observationally.

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arXiv2009

Statistical evaluation of the flux cross-calibration of the XMM-Newton EPIC cameras

The second XMM-Newton serendipitous source catalogue, 2XMM, provides the ideal data base for performing a statistical evaluation of the flux cross-calibration of the XMM-Newton European Photon Imaging Cameras (EPIC). We aim to evaluate the status of the relative flux calibration of the EPIC cameras on board XMM-Newton (MOS1, MOS2, and pn) and investigate the dependence of the calibration on energy, position in the field of view of the X-ray detectors, and lifetime of the mission. We compiled the distribution of flux percentage differences for large samples of 'good quality' objects detected with at least two of the EPIC cameras. The mean offset of the fluxes and dispersion of the distributions was then found by Gaussian fitting. Count rate to flux conversion was performed with a fixed spectral model. The impact on the results of varying this model was investigated. Excellent agreement was found between the two EPIC MOS cameras to better than 4% from 0.2 keV to 12.0 keV. MOS cameras register 7-9% higher flux than pn below 4.5 keV and 10-13% flux excess above 4.5 keV. No evolution of the flux ratios is seen with time, except at energies below 0.5 keV, where we found a strong decrease in the MOS to pn flux ratio with time. This effect is known to be due to a gradually degrading MOS redistribution function. The flux ratios show some dependence on distance from the optical axis in the sense that the MOS to pn flux excess increases with off-axis angle. Furthermore, in the 4.5-12.0 keV band there is a strong dependence of the MOS to pn excess flux on the azimuthal-angle. These results strongly suggest that the calibration of the Reflection Grating Array (RGA) blocking factors is incorrect at high energies. Finally, we recommend ways to improve the calculation of fluxes in future versions of XMM-Newton source catalogues.

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arXiv2008

High precision X-ray logN-logS distributions: implications for the obscured AGN population

We have constrained the extragalactic source count distributions over a broad range of X-ray fluxes and in various energy bands to test whether the predictions from X-ray background synthesis models agree with the observational constraints provided by our measurements. We have used 1129 XMM-Newton observations at |b|>20 deg covering a sky area of 132.3 deg^2 to compile the largest complete samples of X-ray objects to date in the 0.5-1 keV, 1-2 keV, 2-4.5 keV, 4.5-10 keV, 0.5-2 keV and 2-10 keV energy bands. Our survey includes in excess of 30,000 sources down to ~10^-15 erg/cm^2/s below 2 keV and down to ~10^{-14} erg/cm^2/s above 2 keV. A break in the source count distributions was detected in all energy bands except the 4.5-10 keV band. An analytical model comprising 2 power-law components cannot adequately describe the curvature seen in the source count distributions. The shape of the logN(>S)-logS is strongly dependent on the energy band with a general steepening apparent as we move to higher energies. This is due to non-AGN populations, comprised mainly of stars and clusters of galaxies, contribute up to 30% of the source population at energies <2 keV and at fluxes >10^{-13} erg/cm^2/s, and these populations of objects have significantly flatter source count distributions than AGN. We find a substantial increase in the relative fraction of hard X-ray sources at higher energies, from >55% below 2 keV to >77% above 2 keV. However the majority of sources detected above 4.5 keV still have significant flux below 2 keV. Comparison with predictions from the synthesis models suggest that the models might be overpredicting the number of faint absorbed AGN, which would call for fine adjustment of some model parameters such as the obscured to unobscured AGN ratio and/or the distribution of column densities at intermediate obscuration.

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arXiv2008

Evolution of tidal disruption candidates discovered by XMM-Newton

It has been demonstrated that active galactic nuclei are powered by gas accretion onto supermassive black holes located at their centres. The paradigm that the nuclei of inactive galaxies are also occupied by black holes was predicted long ago by theory. In the last decade, this conjecture was confirmed by the discovery of giant-amplitude, non-recurrent X-ray flares from such inactive galaxies and explained in terms of outburst radiation from stars tidally disrupted by a dormant supermassive black hole at the nuclei of those galaxies. Due to the scarcity of detected tidal disruption events, the confirmation and follow-up of each new candidate is needed to strengthen the theory through observational data, as well as to shed new light on the characteristics of this type of events. Two tidal disruption candidates have been detected with XMM-Newton during slew observations. Optical and X-ray follow-up, post-outburst observations were performed on these highly variable objects in order to further study their classification and temporal evolution. We show that the detected low-state X-ray emission for these two candidates has properties such that it must still be related to the flare. The X-ray luminosity of the objects decreases according to theoretical predictions for tidal disruption events. At present, optical spectra of the sources do not present any evident signature of the disruption event. In addition, the tidal disruption rate as derived from the XMM-Newton slew survey has been computed and agrees with previous studies.

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arXiv2008

XMM-Newton slew survey discovery of the nova XMMSL1 J070542.7-381442 (V598 Pup)

In an attempt to catch new X-ray transients while they are still bright, the data taken by XMM-Newton as it slews between targets is being processed and cross-correlated with other X-ray observations as soon as the slew data appears in the XMM-Newton archive. A bright source, XMMSL1 J070542.7-381442, was detected on 9 Oct 2007 at a position where no previous X-ray source had been seen. The XMM slew data and optical data acquired with the Magellan Clay 6.5m telescope were used to classify the new object. No XMM slew X-ray counts are detected above 1keV and the source is seen to be ~750 times brighter than the ROSAT All-Sky Survey upper limit at that position. The normally m(V)~16 star, USNO-A2.0 0450-03360039, which lies 3.5" from the X-ray position, was seen in our Magellan data to be very much enhanced in brightness. Our optical spectrum showed emission lines which identified the source as a nova in the auroral phase. Hence this optical source is undoubtedly the progenitor of the X-ray source - a new nova (now also known as V598 Pup). The X-ray spectrum indicates that the nova was in a super-soft state (with kT(eff)~35eV). We estimate the distance to the nova to be ~3kpc. Analysis of archival robotic optical survey data shows a rapid decline light curve consistent with that expected for a very fast nova. The XMM-Newton slew data present a powerful opportunity to find new X-ray transient objects while they are still bright. Here we present the first such source discovered by the analysis of near real-time slew data.

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arXiv2008

The first XMM-Newton slew survey catalogue: XMMSL1

We report on the production of a large area, shallow, sky survey, from XMM-Newton slews. The great collecting area of the mirrors coupled with the high quantum efficiency of the EPIC detectors have made XMM-Newton the most sensitive X-ray observatory flown to date. We use data taken with the EPIC-pn camera during slewing manoeuvres to perform an X-ray survey of the sky. Data from 218 slews have been subdivided into small images and source searched. This has been done in three distinct energy bands; a soft (0.2-2 keV) band, a hard (2-12 keV) band and a total XMM-Newton band (0.2-12 keV). Detected sources, have been quality controlled to remove artifacts and a catalogue has been drawn from the remaining sources. A 'full' catalogue, containing 4710 detections and a 'clean' catalogue containing 2692 sources have been produced, from 14% of the sky. In the hard X-ray band (2-12 keV) 257 sources are detected in the clean catalogue to a flux limit of 4x10^-12 ergs/s/cm2. The flux limit for the soft (0.2-2 keV) band is 6x10^-13 ergs/s/cm2 and for the total (0.2-12 keV) band is 1.2x10^-12 ergs/s/cm2. The source positions are shown to have an uncertainty of 8" (1-sigma confidence).

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arXiv2006

Candidate tidal disruption events from the XMM-Newton Slew Survey

In recent years, giant amplitude X-ray flares have been observed from a handful of non-active galaxies. The most plausible scenario of these unusual phenomena is tidal disruption of a star by a quiescent supermassive black hole at the centre of the galaxy. Comparing the XMM-Newton Slew Survey Source Catalogue with the ROSAT PSPC All-Sky Survey five galaxies have been detected a factor of up to 88 brighter in XMM-Newton with respect to ROSAT PSPC upper limits and presenting a soft X-ray colour. X-ray luminosities of these sources derived from slew observations have been found in the range 10^41-10^44 erg s^-1, fully consistent with the tidal disruption model. This model predicts that during the peak of the outburst, flares reach X-ray luminosities up to 10^45 erg s^-1, which is close to the Eddington luminosity of the black hole, and afterwards a decay of the flux on a time scale of months to years is expected. Multi-wavelength follow-up observations have been performed on these highly variable objects in order to disentangle their nature and to investigate their dynamical evolution. Here we present sources coming from the XMM-Newton Slew Survey that could fit in the paradigm of tidal disruption events. X-ray and optical observations revealed that two of these objects are in full agreement with that scenario and three other sources that, showing signs of optical activity, need further investigation within the transient galactic nuclei phenomena.

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arXiv2005

The XMM-Newton Slew Survey: a wide-angle survey in the 0.2 - 12 keV band

The scientific data collected during slews of the XMM-Newton satellite are used to construct a slew survey catalogue. This comprises of the order of 4000 sources detected in the EPIC-pn 0.2-12 keV band with exposures of less than 15s and a sky coverage of about 6300 square degrees (source density ~0.65 per square degree). Below 2 keV the sensitivity limit is comparable to the ROSAT PSPC All-Sky Survey and the XMM-Newton slew survey offers long-term variablity studies. Above 2 keV the survey will be a factor of 10 more sensitive than all previous all-sky X-ray surveys. The slew survey is almost complementary to the serendipitous survey compiled from pointed XMM-Newton observations. It is aimed to release the first source catalogue by the end of 2005. Later slew observations and detections will continuously be added. This paper discusses the XMM-Newton slew survey also in a historical context.

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arXiv2005

The XMM-Newton Slew Survey: towards the XMMSL1 catalogue

The XMM-Newton satellite is the most sensitive X-ray observatory flown to date due to the great collecting area of its mirrors coupled with the high quantum efficiency of the EPIC detectors. It performs slewing manoeuvers between observation targets tracking almost circular orbits through the ecliptic poles due to the Sun constraint. Slews are made with the EPIC cameras open and the other instruments closed, operating with the observing mode set to the one of the previous pointed observation and the medium filter in place. Slew observations from the EPIC-pn camera in FF, eFF and LW modes provide data, resulting in a maximum of 15 seconds of on-source time. These data can be used to give a uniform survey of the X-ray sky, at great sensitivity in the hard band compared with other X-ray all-sky surveys.

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arXiv2005

Status of the XMM-Newton cross-calibration with SASv6.5.0

Further achievements of the XMM-Newton cross-calibration - XMM internal as well as with other X-ray missions - are presented. We explain the major changes in the new version SASv6.5 of the XMM-Newton science analysis system. The current status of the cross-calibration of the three EPIC cameras is shown. Using a large sample of blazars, the pn energy redistribution at low energy could be further calibrated, correcting the overestimation of fluxes in the lowest energy regime. In the central CCDs of the MOSs, patches were identified at the bore-sight positions, leading to an underestimation of the low energy fluxes. The further improvement in the understanding of the cameras resulted in a good agreement of the EPIC instruments down to lowest energies. The latest release of the SAS software package already includes corrections for both effects as shown in several examples of different types of sources. Finally the XMM internal cross-calibration is completed by the presentation of the current cross-calibration status between EPIC and RGS instruments. Major efforts have been made in cross-calibrations with other X-ray missions, most importantly with Chandra, of course, but also with currently observing satellites like Swift.

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arXiv2005

The XMM-Newton Slew Survey

XMM-Newton, with the huge collecting area of its mirrors and the high quantum efficiency of its EPIC detectors, is the most sensitive X-ray observatory ever flown. This is strikingly evident during slew exposures, which, while yielding only at most 14 seconds of on-source exposure time, actually constitute a 2-10 keV survey ten times deeper than all other "all-sky" surveys. The current (April 2005) XMM archive contains 374 slew exposures which give a uniform coverage over around 10,000 square degrees (approx. 25% of the sky). Here we describe the results of pilot studies, the current status of the XMM-Newton Slew Survey, up-to-date results and our progress towards constructing a catalogue of slew detections in the full 0.2-12 keV energy band.

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arXiv2004

XMM-Newton (cross)-calibration

ESA's large X-ray space observatory XMM-Newton is in its fifth year of operations. We give a general overview of the status of calibration of the five X-ray instruments and the Optical Monitor. A main point of interest in the last year became the cross-calibration between the instruments. A cross-calibration campaign started at the XMM-Newton Science Operation Centre at the European Space Astronomy Centre in collaboration with the Instrument Principle Investigators provides a first systematic comparison of the X-ray instruments EPIC and RGS for various kind of sources making also an initial assessment in cross calibration with other X-ray observatories.

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