arXiv subjects

Julian Osborne

Publications and source records attributed to Julian Osborne.

12 recordsLinked to original sources

arXiv2026

Design and in-orbit calibration of the MXT optics

The Microchannel X-ray Telescope (MXT) is one of four instruments on the Space-based multi-band astronomical Variable Objects Monitor (SVOM) satellite mission, launched on the 22nd June 2024. The MXT is a narrow-field-optimised, lobster eye X-ray focusing telescope, consisting of an array of 25 square MPOs, with a focal length of 1.14 m and working in the energy band 0.2 - 10 keV. The design of the MXT optic (MOP) is optimised to give a 1 degree FoV to match the detector size, but the optic has the unique characteristics of a lobster eye design, with a wide FoV of 6 degree diameter, and a PSF, which is constant over the entire FoV. The MPOs on the Flight Module (FM) MOP have a pore size of 40 um giving the optimum thicknesses across the aperture of 2.4 mm in the centre and 1.2 mm at the edges. Using specific target sources, the in-orbit calibration of the optic is here described, and compared to the extensive on-ground calibration, which was carried out at the PANTER test facility, MPE, Germany. The design and limitations of the electron diverter, situated directly behind the optic, are also discussed.

astro-ph.IM↗

arXiv2016

The nature of the cataclysmic variable PT Per

We present a study of the cataclysmic variable star PT Per based on archival XMM-Newton X-ray data and new optical spectroscopy from the WHT with ISIS. The X-ray data show deep minima which recur at a period of 82 minutes and a hard, unabsorbed X-ray spectrum. The optical spectra of PT Per show a relatively featureless blue continuum. From an analysis of the X-ray and optical data we conclude that PT Per is likely to be a magnetic cataclysmic variable of the polar class in which the minima correspond to those phase intervals when the accretion column rotates out of the field of view of the observer. We suggest that the optical spectrum, obtained around 4 years after the X-ray coverage, is dominated by the white dwarf in the system, implying that PT Per was in a low accretion state at the time of the observations. An analysis of the likely system parameters for PT Per suggests a distance of $\approx90$ pc and a very low-mass secondary, consistent with the idea that PT Per is a "period-bounce" binary.

astro-ph.HE↗

arXiv2015

The On-Site Analysis of the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA) observatory will be one of the largest ground-based very high-energy gamma-ray observatories. The On-Site Analysis will be the first CTA scientific analysis of data acquired from the array of telescopes, in both northern and southern sites. The On-Site Analysis will have two pipelines: the Level-A pipeline (also known as Real-Time Analysis, RTA) and the level-B one. The RTA performs data quality monitoring and must be able to issue automated alerts on variable and transient astrophysical sources within 30 seconds from the last acquired Cherenkov event that contributes to the alert, with a sensitivity not worse than the one achieved by the final pipeline by more than a factor of 3. The Level-B Analysis has a better sensitivity (not be worse than the final one by a factor of 2) and the results should be available within 10 hours from the acquisition of the data: for this reason this analysis could be performed at the end of an observation or next morning. The latency (in particular for the RTA) and the sensitivity requirements are challenging because of the large data rate, a few GByte/s. The remote connection to the CTA candidate site with a rather limited network bandwidth makes the issue of the exported data size extremely critical and prevents any kind of processing in real-time of the data outside the site of the telescopes. For these reasons the analysis will be performed on-site with infrastructures co-located with the telescopes, with limited electrical power availability and with a reduced possibility of human intervention. This means, for example, that the on-site hardware infrastructure should have low-power consumption. A substantial effort towards the optimization of high-throughput computing service is envisioned to provide hardware and software solutions with high-throughput, low-power consumption at a low-cost.

astro-ph.IM↗

arXiv2013

The Hot and Energetic Universe: Luminous extragalactic transients

We discuss the importance and potential contribution of Athena+ to the science questions related to gamma-ray bursts, tidal disruption events and supernova shock break-out. Athena+ will allow breakthrough observations involving high-resolution X-ray spectroscopic observations of high-z gamma-ray bursts, observations of tidal disruption events tailored to determine the mass and potentially the spin of the black hole responsible for the tidal disruption and observations of X-rays from the supernova shock breakout providing a measure of the radius of the exploding star or of the companion in the case of type Ia supernovae. We briefly discuss survey facilities that will provide triggers to these events envisaged to be operational around 2028.

astro-ph.HE↗

arXiv2010

Deriving an X-ray luminosity function of dwarf novae based on parallax measurements

We have derived an X-ray luminosity function using parallax-based distance measurements of a set of 12 dwarf novae, consisting of Suzaku, XMM-Newton and ASCA observations. The shape of the X-ray luminosity function obtained is the most accurate to date, and the luminosities of our sample are concentrated between ~10^{30}-10^{31} erg s^{-1}, lower than previous measurements of X-ray luminosity functions of dwarf novae. Based on the integrated X-ray luminosity function, the sample becomes more incomplete below ~3 x 10^{30} erg s^{-1} than it is above this luminosity limit, and the sample is dominated by X-ray bright dwarf novae. The total integrated luminosity within a radius of 200 pc is 1.48 x 10^{32} erg s^{-1} over the luminosity range of 1 x 10^{28} erg s^{-1} and the maximum luminosity of the sample (1.50 x 10^{32} erg s^{-1}). The total absolute lower limit for the normalised luminosity per solar mass is 1.81 x 10^{26} erg s^{-1} M^{-1}_{solar} which accounts for ~16 per cent of the total X-ray emissivity of CVs as estimated by Sazonov et al. (2006).

astro-ph.SR↗

arXiv2009

Intermediate polars in the Swift/BAT survey: Spectra and white dwarf masses

White dwarf masses in cataclysmic variables are difficult to determine accurately, but are fundamental for understanding binary system parameters, as well as binary evolution. We investigate the X-ray spectral properties of a sample of Intermediate Polars detected above 15 keV to derive the masses of their accreting white dwarfs. We use data from the Swift/BAT instrument which during the first 2.5 yrs of operation has detected 22 known intermediate polars. The X-ray spectra of these sources are used to estimate the mass of the white dwarfs. We are able to produce a mass estimate for 22 out of 29 of the confirmed intermediate polars. Comparison with previous mass measurements shows good agreement. For GK Per, we were able to detect spectral changes due to the changes in the accretion rate. The Swift/BAT detector with its combination of sensitivity and all-sky coverage provides an ideal tool to determine accurate white dwarf masses in intermediate polars.

astro-ph.GA↗

arXiv2007

Redshift Filtering by Swift Apparent X-ray Column Density

We remark on the utility of an observational relation between the absorption column density in excess of the Galactic absorption column density, $ΔN_{\rm H} = N_{\rm H, fit} - N_{\rm H, gal}$, and redshift, z, determined from all 55 Swift-observed long bursts with spectroscopic redshifts as of 2006 December. The absorption column densities, $N_{\rm H, fit}$, are determined from powerlaw fits to the X-ray spectra with the absorption column density left as a free parameter. We find that higher excess absorption column densities with $ΔN_{\rm H} > 2\times 10^{21}$ cm$^{-2}$ are only present in bursts with redshifts z$<$2. Low absorption column densities with $ΔN_{\rm H} < 1\times 10^{21}$ cm$^{-2}$ appear preferentially in high-redshift bursts. Our interpretation is that this relation between redshift and excess column density is an observational effect resulting from the shift of the source rest-frame energy range below 1 keV out of the XRT observable energy range for high redshift bursts. We found a clear anti-correlation between $ΔN_{\rm H}$ and z that can be used to estimate the range of the maximum redshift of an afterglow. A critical application of our finding is that rapid X-ray observations can be used to optimize the instrumentation used for ground-based optical/NIR follow-up observations. Ground-based spectroscopic redshift measurements of as many bursts as possible are crucial for GRB science.

astro-ph↗

arXiv2007

Swift and XMM-Newton Observations of the Extraordinary GRB 060729: An afterglow with a more than 100 days X-ray light curve

We report the results of the Swift and XMM observations of the Swift-discovered long Gamma-Ray Burst GRB 060729 ($T_{90}$=115s). The afterglow of this burst was exceptionally bright in X-rays as well as at UV/Optical wavelengths showing an unusually long slow decay phase ($α$=0.14\plm0.02) suggesting a larger energy injection phase at early times than in other bursts. The X-ray light curve displays a break at about 60 ks after the burst. The X-ray decay slope after the break is $α$=1.29\plm0.03. Up to 125 days after the burst we do not detect a jet break, suggesting that the jet opening angle is larger than 28 degrees. In the first 2 minutes after the burst (rest frame) the X-ray spectrum of the burst changed dramatically from a hard X-ray spectrum to a very soft one. We find that the X-ray spectra at this early phase can all be fitted by an absorbed single power law model or alternatively by a blackbody plus power law model. The power law fits show that the X-ray spectrum becomes steeper while the absorption column density decreases. In Swift's UV/Optical telescope the afterglow was clearly detected up to 9 days after the burst in all 6 filters and even longer in some of the UV filters with the latest detection in the UVW1 31 days after the burst. A break at about 50 ks is clearly detected in all 6 UVOT filters from a shallow decay slope of about 0.3 and a steeper decay slope of 1.3. In addition to the \swift observations we also present and discuss the results from a 61 ks ToO observation by XMM. (Abriviated)

astro-ph↗

arXiv2006

Swift Observations of GRB 050603: An afterglow with a steep late time decay slope

We report the results of Swift observations of the Gamma Ray Burst GRB 050603. With a V magnitude V=18.2 about 10 hours after the burst the optical afterglow was the brightest so far detected by Swift and one of the brightest optical afterglows ever seen. The Burst Alert Telescope (BAT) light curves show three fast-rise-exponential-decay spikes with $T_{90}$=12s and a fluence of 7.6$\times 10^{-6}$ ergs cm$^{-2}$ in the 15-150 keV band. With an $E_{\rm γ, iso} = 1.26 \times 10^{54}$ ergs it was also one of the most energetic bursts of all times. The Swift spacecraft began observing of the afterglow with the narrow-field instruments about 10 hours after the detection of the burst. The burst was bright enough to be detected by the Swift UV/Optical telescope (UVOT) for almost 3 days and by the X-ray Telescope (XRT) for a week after the burst. The X-ray light curve shows a rapidly fading afterglow with a decay index $α$=1.76$^{+0.15}_{-0.07}$. The X-ray energy spectral index was $β_{\rm X}$=0.71\plm0.10 with the column density in agreement with the Galactic value. The spectral analysis does not show an obvious change in the X-ray spectral slope over time. The optical UVOT light curve decays with a slope of $α$=1.8\plm0.2. The steepness and the similarity of the optical and X-ray decay rates suggest that the afterglow was observed after the jet break. We estimate a jet opening angle of about 1-2$^{\circ}$

astro-ph↗

arXiv2005

Gamma-ray bursts: Restarting the Engine

Recent gamma-ray burst observations have revealed late-time, highly energetic events which deviate from the simplest expectations of the standard fireball picture. Instead they may indicate that the central engine is active or restarted at late times. We suggest that fragmentation and subsequent accretion during the collapse of a rapidly rotating stellar core offers a natural mechanism for this.

astro-ph↗

arXiv2000

First XMM-Newton observations of a cataclysmic variable II: the X-ray spectrum of OY Car

We present XMM-Newton X-ray spectra of the disc accreting cataclysmic variable OY Car, which were obtained during the performance verification phase of the mission. These data were taken 4 days after a short outburst. In the EPIC spectra we find strong iron Kalpha emission with weaker iron Kbeta emission together with silicon and sulphur lines. The spectra are best fitted with a three temperature plasma model with a partial covering absorber. Multiple temperature emission is confirmed by the emission lines seen in the RGS spectrum and the H/He like intensity ratio for iron and sulphur which imply temperatures of ~7keV and ~3keV respectively.

astro-ph↗

arXiv2000

First XMM-Newton observations of a Cataclysmic Variable I: Timing studies of OY Car

We present XMM-Newton observations of the eclipsing, disc accreting, cataclysmic variable OY Car which were obtained as part of the performance verification phase of the mission. The star was observed 4 days after an outburst and then again 5 weeks later when it was in a quiescent state. There is a quasi-stable modulation of the X-rays at ~2240 sec, which is most prominent at the lowest energies. We speculate that this may be related to the spin period of the white dwarf. The duration of the eclipse ingress and egress in X-rays is 20--30 sec. This indicates that the bulk of the X-ray emission originates from the boundary layer which has a negligible height above the surface of the white dwarf. The eclipse profile implies a white dwarf of mass M_{1}=0.9-1.1Msun and a secondary star of M_{2}=0.08-0.11Msun.

astro-ph↗