-
M. J. Page,
C. Brindle,
A. Talavera,
M. Still,
S. R. Rosen,
V. N. Yershov,
H. Ziaeepour, K. O. Mason,
M. S. Cropper,
A. A. Breeveld,
N. Loiseau,
R. Mignani,
A. Smith,
P. Murdin
[show abstract]
[hide abstract]
ABSTRACT: The XMM-Newton Serendipitous Ultraviolet Source Survey (XMM-SUSS) is a
catalogue of ultraviolet (UV) sources detected serendipitously by the Optical
Monitor (XMM-OM) on-board the XMM-Newton observatory. The catalogue contains
ultraviolet-detected sources collected from 2,417 XMM-OM observations in 1-6
broad band UV and optical filters, made between 24 February 2000 and 29 March
2007. The primary contents of the catalogue are source positions, magnitudes
and fluxes in 1 to 6 passbands, and these are accompanied by profile
diagnostics and variability statistics. The XMM-SUSS is populated by 753,578 UV
source detections above a 3 sigma signal-to-noise threshold limit which relate
to 624,049 unique objects. Taking account of substantial overlaps between
observations, the net sky area covered is 29-54 square degrees, depending on UV
filter. The magnitude distributions peak at 20.2, 20.9 and 21.2 in UVW2, UVM2
and UVW1 respectively. More than 10 per cent of sources have been visited more
than once using the same filter during XMM-Newton operation, and > 20 per cent
of sources are observed more than once per filter during an individual visit.
Consequently, the scope for science based on temporal source variability on
timescales of hours to years is broad. By comparison with other astrophysical
catalogues we test the accuracy of the source measurements and define the
nature of the serendipitous UV XMM-OM source sample. The distributions of
source colours in the UV and optical filters are shown together with the
expected loci of stars and galaxies, and indicate that sources which are
detected in multiple UV bands are predominantly star-forming galaxies and stars
of type G or earlier.
07/2012;
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A. A. Breeveld,
P. A. Curran,
E. A. Hoversten,
S. Koch,
W. Landsman,
F. E. Marshall,
M. J. Page,
T. S. Poole,
P. Roming,
P. J. Smith, [......],
S. Immler,
M. Ivanushkina,
T. Kennedy, K. O. Mason,
A. N. Morgan,
S. Oates,
M. De Pasquale,
P. Schady,
M. Siegel,
D. Vanden Berk
[show abstract]
[hide abstract]
ABSTRACT: The Ultraviolet/Optical Telescope (UVOT) is one of three instruments onboard the Swift observatory. The photometric calibration has been published, and this paper follows up with details on other aspects of the calibration including a measurement of the point spread function with an assessment of the orbital variation and the effect on photometry. A correction for large-scale variations in sensitivity over the field of view is described, as well as a model of the coincidence loss which is used to assess the coincidence correction in extended regions. We have provided a correction for the detector distortion and measured the resulting internal astrometric accuracy of the UVOT, also giving the absolute accuracy with respect to the International Celestial Reference System. We have compiled statistics on the background count rates, and discuss the sources of the background, including instrumental scattered light. In each case, we describe any impact on UVOT measurements, whether any correction is applied in the standard pipeline data processing or whether further steps are recommended.
Monthly Notices of the Royal Astronomical Society 06/2010; 406(3):1687 - 1700. · 4.90 Impact Factor
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A. A. Breeveld,
P. A. Curran,
E. A. Hoversten,
S. Koch,
W. Landsman,
F. E. Marshall,
M. J. Page,
T. S. Poole,
P. Roming,
P. J. Smith, [......],
S. Immler,
M. Ivanushkina,
T. Kennedy, K. O. Mason,
A. N. Morgan,
S. Oates,
M. De Pasquale,
P. Schady,
M. Siegel,
D. Vanden Berk
[show abstract]
[hide abstract]
ABSTRACT: The Ultraviolet/Optical Telescope (UVOT) is one of three instruments onboard the Swift observatory. The photometric calibration has been published, and this paper follows up with details on other aspects of the calibration including a measurement of the point spread function with an assessment of the orbital variation and the effect on photometry. A correction for large scale variations in sensitivity over the field of view is described, as well as a model of the coincidence loss which is used to assess the coincidence correction in extended regions. We have provided a correction for the detector distortion and measured the resulting internal astrometric accuracy of the UVOT, also giving the absolute accuracy with respect to the International Celestial Reference System. We have compiled statistics on the background count rates, and discuss the sources of the background, including instrumental scattered light. In each case we describe any impact on UVOT measurements, whether any correction is applied in the standard pipeline data processing or whether further steps are recommended. Comment: Accepted for publication in MNRAS. 15 pages, 21 figures, 4 tables
04/2010;
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[show abstract]
[hide abstract]
ABSTRACT: We present XMM-Newton Reflection Grating Spectrometer observations of the soft X-ray spectrum of NGC 4051 and explore their implications for the inner accretion disk and ionized outflow in the active galactic nucleus. We fit the soft X-ray excess with a relativistically broadened O VIII recombination spectrum, including the entire line series and recombination continuum. This plus an underlying power-law continuum provides a much better fit to the soft X-ray spectrum than a single temperature or disk blackbody plus power law. The emission-line profiles, computed for a Kerr metric around a maximally rotating black hole, reveal a sharply peaked disk emissivity law and inner radius smaller than 1.7RG. The spectrum also includes narrow absorption and emission lines from C, N, O, Ne, and Fe in an ionized outflow. Outflow column densities are relatively low and do not create significant edges in the spectrum. The small amount of absorption bolsters confidence in the detection of relativistic emission-line features. The narrow-line emitter has a large (76%) global covering fraction, leading to strong forbidden lines and filling in of the resonance absorption lines. We also find broad C VI Lyα and very broad O VII emission from the broad-line region. The narrow- and broad-line regions span large ranges in ionization parameter and may arise in a disk outflow. The ionized absorber has a large ionization range, which is inconsistent with pressure equilibrium in a multiphase medium. The mass outflow rate exceeds the accretion rate by a factor of 1000.
The Astrophysical Journal 12/2008; 606(1):151. · 6.02 Impact Factor
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K. O. Mason,
A. J. Blustin,
P. Boyd,
S. T. Holland,
M. J. Page,
P. Roming,
M. Still,
B. Zhang,
A. Breeveld,
M. de Pasquale, [......],
C. Gronwall,
S. Hunsberger,
M. Ivanushkina,
W. Landsman,
K. McGowan,
J. Nousek,
T. Poole,
J. Rhoads,
S. Rosen,
and P. Schady
[show abstract]
[hide abstract]
ABSTRACT: The UVOT telescope on the Swift observatory has detected optical afterglow emission from GRB 050319. The flux declined with a power-law slope of α = -0.57 between the start of observations some 230 s after the burst onset (90 s after the burst trigger) until it faded below the sensitivity threshold of the instrument after ~5 × 104 s. There is no evidence for the rapidly declining component in the early light curve that is seen at the same time in the X-ray band. The afterglow is not detected in UVOT shortward of the B band, suggesting a redshift of about 3.5. The optical V-band emission lies on the extension of the X-ray spectrum, with an optical-to-X-ray slope of β = -0.8. The relatively flat decay rate of the burst suggests that the central engine continues to inject energy into the fireball for as long as a few × 104 s after the burst.
The Astrophysical Journal 12/2008; 639(1):311. · 6.02 Impact Factor
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R. Willingale,
P. T. O'Brien,
S. W. H. Cowley,
G. H. Jones,
D. J. McComas, K. O. Mason,
J. P. Osborne,
A. Wells,
M. Chester,
S. Hunsberger,
D. N. Burrows,
N. Gehrels,
J. A. Nousek,
L. Angelini,
L. R. Cominsky,
S. L. Snowden,
and G. Chincarini
[show abstract]
[hide abstract]
ABSTRACT: Comet 9P/Tempel 1 was observed by the Swift X-Ray Telescope (XRT) for a total of 250,024 s. Soft X-ray emission, 0.2-1.0 keV, was seen as a diffuse extended halo with an FWHM of 1.03 × 105 km centered on the comet's nucleus. The X-ray light curve indicates that the comet exhibited a prolonged soft X-ray outburst just after impact of the NASA Deep Impact (DI) spacecraft and enhanced X-ray activity lasted for 12 days. The radial brightness distribution and X-ray spectrum are in excellent agreement with a model of X-ray production in which highly charged minor heavy ion species in the solar wind undergo charge exchange reactions with water group or carbon dioxide group molecules in the neutral coma of the comet. Using this model, we derive a simple expression for the X-ray emission and show that the X-ray flare is, in part, due to an increase in solar wind flux at the comet but is largely due to an enhanced molecule production rate. Assuming that the main outgassing constituent was water, the comet produced (2.9 ± 0.4) × 108 kg over the 12 day period postimpact. The quiescent water production was expected to inject ~1.0 × 108 kg into the coma over the same period so the observed X-ray flux indicates that an additional (1.9 ± 0.4) × 108 kg of water or, alternatively, (3.9 ± 0.5) × 108 kg of carbon dioxide were liberated by the DI impact.
The Astrophysical Journal 12/2008; 649(1):541. · 6.02 Impact Factor
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K. O. Mason,
G. Branduardi-Raymont,
P. M. Ogle,
M. J. Page,
E. M. Puchnarewicz,
E. Behar,
F. A. Córdova,
S. Davis,
L. Maraschi,
I. M. McHardy,
P. T. O'Brien,
W. C. Priedhorsky,
and T. P. Sasseen
[show abstract]
[hide abstract]
ABSTRACT: Competing models for broad spectral features in the soft X-ray spectrum of the Seyfert I galaxy Mrk 766 are tested against data from a 130 ks XMM-Newton observation. A model including relativistically broadened Lyα emission lines of O VIII, N VII, and C VI is a better fit to 0.3-2 keV XMM RGS data than a dusty warm absorber. Moreover, the measured depth of neutral iron absorption lines in the spectrum is inconsistent with the magnitude of the iron edge required to produce the continuum break at 17-18 Å in the dusty warm absorber model. The relativistic emission line model can reproduce the broadband (0.1-12 keV) XMM EPIC data with the addition of a fourth line to represent emission from ionized iron at 6.7 keV and an excess due to reflection at energies above the iron line. The profile of the 6.7 keV iron line is consistent with that measured for the low-energy lines. There is evidence in the RGS data, at the 3 σ level, of spectral features that vary with source flux. The covering fraction of warm absorber gas is estimated to be 12%. Iron in the warm absorber is found to be overabundant with respect to CNO, compared to solar values.
The Astrophysical Journal 12/2008; 582(1):95. · 6.02 Impact Factor
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S. Campana,
L. A. Antonelli,
G. Chincarini,
S. Covino,
G. Cusumano,
D. Malesani,
V. Mangano,
A. Moretti,
C. Pagani,
P. Romano, [......],
A. F. Abbey,
A. P. Beardmore,
A. Breeveld,
M. R. Goad,
O. Godet, K. O. Mason,
J. P. Osborne,
K. L. Page,
T. Poole,
and N. Gehrels
[show abstract]
[hide abstract]
ABSTRACT: Swift discovered GRB 050128 with the Burst Alert Telescope and promptly pointed its narrow field instruments to monitor the afterglow. X-ray observations started 108 s after the trigger time. The early decay of the afterglow is relatively flat, with a temporal decay modeled with a power-law index of ~-0.3. A steepening occurs at later times (~1500 s) with a power-law index of ~-1.3. During this transition, the observed X-ray spectrum does not change. We interpret this behavior as either an early jet break or evidence of a transition from the fast cooling regime to the slow cooling regime in a wind environment.
The Astrophysical Journal 12/2008; 625(1):L23. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We discuss a 1.5 day long observation of the bright variable Seyfert 1 galaxy NGC 4051 that was obtained with the X-ray and ultraviolet telescopes on XMM-Newton and extended in the X-ray band using the Rossi X-Ray Timing Explorer. The ultraviolet (2900 Å) and X-ray (2-10 keV) light curves varied with peak-to-peak amplitudes of 5% and a factor of ~4, respectively. A simple interband cross-correlation function shows three broad features: a positive correlation with coefficient rM = +0.63 at a lag (Topt - TX-ray) τ ~ +0.2 days, an anticorrelation with rM = -0.65 at τ ~ +0.7 days, and a broad but slightly weaker feature with rM ≥ 0.53 over τ = 1.2-1.4 days. Because of the asymmetric sampling of the optical and X-ray data, only positive lags could be probed. Monte Carlo simulations based on synthetic uncorrelated light curves derived from the power spectrum of the real data give an ~85% confidence in features at this level. If the X-ray and UV light curves are indeed correlated, the lag is highly significant (i.e., not consistent with zero), which is suggestive of a scenario in which X-rays are being reprocessed into UV light at a distance from the X-ray source. A simple model in which the reprocessor is an inclined optically thick ring at a radius of 0.14 lt-days from the X-ray source reproduces the UV data well. A comparison of the modulated fractions in the ultraviolet and X-ray bands suggests that 5% of the 2900 Å flux is produced in this way. The reprocessing site is close enough to the X-ray emitter for X-ray heating from a source with the luminosity of NGC 4051 to produce a brightening in the UV band, and the amount of UV flux emitted suggests that the covering fraction of the reprocessor is about 6%. We briefly discuss the possible physical nature of the reprocessing ring structure.
The Astrophysical Journal 12/2008; 580(2):L117. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We present an analysis of a 50 ks XMM-Newton observation of the merging galaxy cluster Cl J0152.7-1357 at z = 0.83. In addition to the two main subclusters and an infalling group detected in an earlier Chandra observation of the system, XMM-Newton detects another group of galaxies possibly associated with the cluster. This group may be connected to the northern subcluster by a filament of cool (1.4 keV) X-ray-emitting gas and lies outside the estimated virial radius of the northern subcluster. The X-ray morphology agrees well with the projected galaxy distribution in new K-band imaging data presented herein. We use detailed spectral and imaging analysis of the X-ray data to probe the dynamics of the system and find evidence that another subcluster or group has recently passed through the northern subcluster. Cl J0152.7-1357 is an extremely dynamically active system, with mergers at different stages occurring along two perpendicular merger axes.
The Astrophysical Journal 12/2008; 640(1):219. · 6.02 Impact Factor
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P. J. Brown,
S. T. Holland,
C. James,
P. Milne,
P. W. A. Roming, K. O. Mason,
K. L. Page,
A. P. Beardmore,
D. Burrows,
A. Morgan, [......],
A. Breeveld,
M. de Pasquale,
S. Hunsberger,
M. Ivanushkina,
W. Landsman,
K. McGowan,
T. Poole,
S. Rosen,
P. Schady,
and N. Gehrels
[show abstract]
[hide abstract]
ABSTRACT: We present ultraviolet and optical light curves in six broadband filters and grism spectra obtained by Swift's Ultraviolet/Optical Telescope for the Type Ia supernova SN 2005am. The data were collected beginning about 4 days before the B-band maximum, with excellent coverage of the rapid decline phase and later observations extending out to 69 days after the peak. The optical and near-UV light curve match well those of SN 1992A. The other UV observations constitute the first set of light curves shorter than 2500 Å and allow us to compare the light curve evolution in three UV bands. One interesting feature is that the decay in the intermediate UVM2 band is shallower than in the filters on either side and may result from the bump in the interstellar extinction curve. The UV behavior of this and other low-redshift supernovae can be used to constrain theories of progenitor evolution or to interpret optical light curves of high-redshift supernovae. Using Swift's X-Ray Telescope, we also report the upper limit to SN 2005am's X-ray luminosity to be 6 × 1039 ergs s-1 in the 0.3-10 keV. This result is derived from 58 ks of exposure time spread out over 7 weeks beginning 4 days before the B-band maximum.
The Astrophysical Journal 12/2008; 635(2):1192. · 6.02 Impact Factor
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M. Still,
P. W. A. Roming, K. O. Mason,
A. Blustin,
P. Boyd,
A. Breeveld,
P. Brown,
M. De Pasquale,
C. Gronwall,
S. T. Holland, [......],
H. Krimm,
T. Sakamoto,
P. Giommi,
M. R. Goad,
V. Mangano,
K. Page,
M. Perri,
D. N. Burrows,
N. Gehrels,
and J. Nousek
[show abstract]
[hide abstract]
ABSTRACT: We present observations of GRB 050318 by the Ultraviolet/Optical Telescope (UVOT) on board the Swift observatory. The data are the first detections of a gamma-ray burst (GRB) afterglow decay by the UVOT instrument, launched specifically to open a new window on these transient sources. We showcase UVOT's ability to provide multicolor photometry and the advantages of combining UVOT data with simultaneous and contemporaneous observations from the high-energy detectors on the Swift spacecraft. Multiple filters covering 1800-6000 Å reveal a red source with a spectral slope steeper than the simultaneous X-ray continuum. Spectral fits indicate that the UVOT colors are consistent with dust extinction by systems at z = 1.2037 and 1.4436, redshifts where absorption systems have been preidentified. However, the data can be most easily reproduced with models containing a foreground system of neutral gas redshifted by z = 2.8 ± 0.3. For both of the above scenarios, spectral and decay slopes are, for the most part, consistent with fireball expansion into a uniform medium, provided a cooling break occurs between the energy ranges of the UVOT and Swift's X-ray instrumentation.
The Astrophysical Journal 12/2008; 635(2):1187. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: Broad absorption line quasars (commonly termed BALQSOs) contain the most dramatic examples of AGN-driven winds. The high absorbing columns in these winds, ~10^24 cm^-2, ensure that BALQSOs are generally X-ray faint. This high X-ray absorption means that almost all BALQSOs have been discovered through optical surveys, and so what little we know about their X-ray properties is derived from very bright optically-selected sources. A small number of X-ray selected BALQSOs (XBALQSOs) have, however, recently been found in deep X-ray survey fields. In this paper we investigate the X-ray and rest-frame UV properties of five XBALQSOs for which we have obtained XMM-Newton EPIC X-ray spectra and deep optical imaging and spectroscopy. We find that, although the XBALQSOs have an alpha_ox steeper by ~0.5 than normal QSOs, their median alpha_ox is nevertheless flatter by 0.30 than that of a comparable sample of optically selected BALQSOs (OBALQSOs). We rule out the possibility that the higher X-ray to optical flux ratio is due to intrinsic optical extinction. We find that the amount of X-ray and UV absorption due to the wind in XBALQSOs is similar, or perhaps greater than, the corresponding wind absorption in OBALQSOs, so the flatter alpha_ox cannot be a result of weaker wind absorption. We conclude that these XBALQSOs have intrinsically higher X-ray to optical flux ratios than the OBALQSO sample with which we compare them.
09/2008;
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X. Barcons,
F. J. Carrera,
M. T. Ceballos,
M. J. Page,
J. Bussons-Gordo,
A. Corral,
J. Ebrero,
S. Mateos,
J. A. Tedds,
M. G. Watson, [......],
G. C. Stewart,
G. Szokoly,
A. Ullan,
M. J. Ward,
R. S. Warwick,
P. J. Wheatley,
N. A. Webb,
D. Worrall,
W Yuan,
H. Ziaeepour
[show abstract]
[hide abstract]
ABSTRACT: We present the XMM-Newton Medium sensitivity Survey (XMS), including a total of 318 X-ray sources found among the serendipitous content of 25 XMM-Newton target fields. The XMS comprises four largely overlapping source samples selected at soft (0.5-2 keV), intermediate (0.5-4.5 keV), hard (2-10 keV) and ultra-hard (4.5-7.5 keV) bands, the first three of them being flux-limited. We report on the optical identification of the XMS samples, complete to 85-95%. At the intermediate flux levels sampled by the XMS we find that the X-ray sky is largely dominated by Active Galactic Nuclei. The fraction of stars in soft X-ray selected samples is below 10%, and only a few per cent for hard selected samples. We find that the fraction of optically obscured objects in the AGN population stays constant at around 15-20% for soft and intermediate band selected X-ray sources, over 2 decades of flux. The fraction of obscured objects amongst the AGN population is larger (~35-45%) in the hard or ultra-hard selected samples, and constant across a similarly wide flux range. The distribution in X-ray-to-optical flux ratio is a strong function of the selection band, with a larger fraction of sources with high values in hard selected samples. Sources with X-ray-to-optical flux ratios in excess of 10 are dominated by obscured AGN, but with a significant contribution from unobscured AGN. Comment: A&A, accepted
10/2007;
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P. Schady,
M. De Pasquale,
M. J. Page,
L. Vetere,
S. B. Pandey,
X. Y. Wang,
J. Cummings,
B. Zhang,
S. Zane,
A. Breeveld, [......],
C. Markwardt, K. O. Mason,
P. Mészáros,
J. P. Norris,
S. R. Oates,
C. Pagani,
T. S. Poole,
P. W. A. Roming,
P. J. Smith,
D. E. Vanden Berk
[show abstract]
[hide abstract]
ABSTRACT: ABSTRACTGRB 061007 was the brightest gamma-ray burst (GRB) to be detected by Swift and was accompanied by an exceptionally luminous afterglow that had a V-band magnitude <11.1 at 80 s after the prompt emission. From the start of the Swift observations the afterglow decayed as a power law with a slope of αX= 1.66 ± 0.01 in the X-ray and αopt= 1.64 ± 0.01 in the UV/optical, up to the point that it was no longer detected above background in the optical or X-ray bands. The brightness of this GRB and the similarity in the decay rate of the X-ray, optical and γ-ray emission from 100 s after the trigger distinguish this burst from others and present a challenge to the fireball model. The lack of a cooling or jet break in the afterglow up to ∼105 s constrains any model that can produce the large luminosity observed in GRB 061007, which we found to require either an excessively large kinetic energy or highly collimated outflow. Analysis of the multiwavelength spectral and high-resolution temporal data taken with Swift suggests an early time jet break to be a more plausible scenario than a highly energetic GRB. This must have occurred within 80 s of the prompt emission, which places an upper limit on the jet opening angle of θj= 0.8°. Such a highly collimated outflow resolves the energy budget problem presented in a spherical emission model, reducing the isotropic-equivalent energy of this burst to Ecorrγ= 1050 erg, consistent with other GRBs.
Monthly Notices of the Royal Astronomical Society 09/2007; 380(3):1041 - 1052. · 4.90 Impact Factor
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M. De Pasquale,
S. R. Oates,
A. Beardmore,
K. L. Page,
A. J. Blustin,
S. Zane,
M. J. Page, K. O’Mason,
D. N. Burrows,
D. Palmer,
N. Gehrels,
J. Nousek,
P. Roming,
P. Price,
B. Zhang
[show abstract]
[hide abstract]
ABSTRACT: Swift very fast follow up observations of Gamma‐Ray Bursts have allowed us to discover a new feature of GRB afterglows: a phase of shallow decay, usually attributed to energy injection in the burst ejecta, which can last for several thousands of seconds both in the X‐ray and optical bands. Here I shall discuss this phase in the cases of Swift GRBs 050401, 050801 and 050802. © 2007 American Institute of Physics
AIP Conference Proceedings. 08/2007; 924(1):437-440.
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T. S. Poole,
A. A. Breeveld,
M. J. Page,
W. Landsman,
S. T. Holland,
P. Roming,
N. P. M. Kuin,
P. J. Brown,
C. Gronwall,
S. Hunsberger, [......],
H. Huckle,
S. Immler,
M. Ivanushkina,
T. Kennedy,
F. Marshall,
A. Morgan,
S. Pandey,
M. De Pasquale,
P. J. Smith,
M. Still
[show abstract]
[hide abstract]
ABSTRACT: We present the photometric calibration of the Swift UltraViolet/Optical Telescope (UVOT) which includes: optimum photometric and background apertures, effective area curves, colour transformations, conversion factors for count rates to flux, and the photometric zero points (which are accurate to better than 4 per cent) for each of the seven UVOT broadband filters. The calibration was performed with observations of standard stars and standard star fields that represent a wide range of spectral star types. The calibration results include the position dependent uniformity, and instrument response over the 1600-8000A operational range. Because the UVOT is a photon counting instrument, we also discuss the effect of coincidence loss on the calibration results. We provide practical guidelines for using the calibration in UVOT data analysis. The results presented here supersede previous calibration results. Comment: Minor improvements after referees report. Accepted for publication in MNRAS
08/2007;
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S. R. Oates,
M. De Pasquale,
M. J. Page,
A. J. Blustin,
S. Zane,
K. McGowan, K. O. Mason,
T. S. Poole,
P. Schady,
P. W. A. Roming,
K. L. Page,
A Falcone,
N. Gehrels
[show abstract]
[hide abstract]
ABSTRACT: This paper investigates GRB 050802, one of the best examples of a it Swift gamma-ray burst afterglow that shows a break in the X-ray lightcurve, while the optical counterpart decays as a single power-law. This burst has an optically bright afterglow of 16.5 magnitude, detected throughout the 170-650nm spectral range of the UVOT on-board Swift. Observations began with the XRT and UVOT telescopes 286s after the initial trigger and continued for 1.2 x 10^6s. The X-ray lightcurve consists of three power-law segments: a rise until 420s, followed by a slow decay with alpha_2 = 0.63 +/- 0.03 until 5000s, after which, the lightcurve decays faster with a slope of alpha_3 = 1.59 +/- 0.03. The optical lightcurve decays as a single power-law with alpha_O = 0.82 +/- 0.03 throughout the observation. The X-ray data on their own are consistent with the break at 5000s being due to the end of energy injection. Modelling the optical to X-ray spectral energy distribution, we find that the optical afterglow can not be produced by the same component as the X-ray emission at late times, ruling out a single component afterglow. We therefore considered two-component jet models and find that the X-ray and optical emission is best reproduced by a model in which both components are energy injected for the duration of the observed afterglow and the X-ray break at 5000s is due to a jet break in the narrow component. This bright, well-observed burst is likely a guide for interpreting the surprising finding of Swift that bursts seldom display achromatic jet breaks. Comment: 13 pages, 5 figures, accepted MNRAS
06/2007;
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Massimiliano De Pasquale,
S. R. Oates,
M. J. Page,
D. N. Burrows,
A. J. Blustin,
S. Zane, K. O. Mason,
P. W. A. Roming,
D Palmer,
N. Gehrels,
B Zhang
[show abstract]
[hide abstract]
ABSTRACT: We present results of Swift optical, UV and X-ray observations of the afterglow of GRB 050801. The source is visible over the full optical, UV and X-ray energy range of the Swift UVOT and XRT instruments.Both optical and X-ray lightcurves exhibit a broad plateau (\Delta t/t ~ 1) during the first few hundred seconds after the gamma-ray event. We investigate the multiwavelength spectral and timing properties of the afterglow, and we suggest that the behaviour at early times is compatible with an energy injection by a newly born magnetar with a period of a few tenths of a millisecond, which keeps the forward shock refreshed over this short interval by irradiation. Reverse shock emission is not observed. Its suppression might be due to GRB ejecta being permeated by high magnetic fields, as expected for outflows powered by a magnetar.Finally, the multiwavelength study allows a determination of the burst redshift, z=1.56. Comment: 13 pages, 7 figs, Accepted for publication in MNRAS
03/2007;
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ABSTRACT: Using a sample of gamma-ray burst (GRB) afterglows detected by both the X-Ray and the UV/Optical Telescopes (XRT and UVOT) on Swift, we modelled the spectral energy distributions (SEDs) to determine gas column densities and dust extinction in the GRB local environment. In six out of seven cases we find an X-ray absorber associated with the GRB host galaxy with column density (assuming solar abundances) ranging from (0.8 - 7.7)x10^{21}cm^{-2}. We determine the rest-frame visual extinction A_V using the SMC, LMC and Galactic extinction curves to model the dust in the GRB host galaxy, and this ranges from A_V = 0.12\pm 0.04 to A_V = 0.65^{+0.08}_{-0.07}. The afterglow SEDs were typically best fit by a model with an SMC extinction curve. In only one case was the GRB afterglow better modelled by a Galactic extinction curve, which has a prominent absorption feature at 2175angstrom. We investigate the selection effects present in our sample and how these might distort the true distribution of A_V in GRB host galaxies. We estimate that GRBs with no afterglow detected blueward of 5500angstrom have average rest-frame visual extinctions almost eight times those observed in the optically bright population of GRBs. This may help account for the ~1/3 of GRBs observed by Swift that have no afterglow detected by UVOT.
03/2007;
