arXiv subjects

R. Sturm

Publications and source records attributed to R. Sturm.

30 records · Page 2Linked to original sources

arXiv2012

The XMM-Newton survey of the Small Magellanic Cloud: XMMUJ010633.1-731543 and XMMUJ010743.1-715953, two new Be/X-ray binary systems

In the course of the XMM-Newton survey of the Small Magellanic Cloud (SMC), two new bright X-ray sources were discovered exhibiting the spectral characteris- tics of High Mass X-ray Binaries - but revealing only weak evidence for pulsations in just one of the objects(at 153s in XMMUJ010743.1-715953). The accurate X- ray source locations permit the identification of these X-ray source with Be stars, thereby strongly suggesting these systems are new Be/X-ray binaries. From blue spectra the proposed classification for XMMUJ010633.1-731543 is B0.5-1Ve and for XMMUJ010743.1-715953 it is B2IV-Ve.

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arXiv2012

Evolution and Hydrodynamics of the Very-Broad X-ray Line Emission in SN 1987A

Observations of SN 1987A by the Chandra High Energy Transmission Grating (HETG) in 1999 and the XMM-Newton Reflection Grating Spectrometer (RGS) in 2003 show very broad (v-b) lines with a full-width at half-maximum (FWHM) of order 10^4 kms; at these times the blast wave was primarily interacting with the HII region around the progenitor. Since then, the X-ray emission has been increasingly dominated by narrower components as the blast wave encounters dense equatorial ring (ER) material. Even so, continuing v-b emission is seen in the grating spectra suggesting that interaction with HII region material is on-going. Based on the deep HETG 2007 and 2011 data sets, and confirmed by RGS and other HETG observations, the v-b component has a width of 9300 +/-2000 kms FWHM and contributes of order 20% of the current 0.5--2 keV flux. Guided by this result, SN 1987A's X-ray spectra are modeled as the weighted sum of the non-equilibrium-ionization (NEI) emission from two simple 1D hydrodynamic simulations, this "2x1D" model reproduces the observed radii, light curves, and spectra with a minimum of free parameters. The interaction with the HII region (rho_init \sim 130 amu/cc, +/- 15 degrees opening angle) produces the very-broad emission lines and most of the 3-10 keV flux. Our ER hydrodynamics, admittedly a crude approximation to the multi-D reality, gives ER densities of order 10^4 amu/cc, requires dense clumps (x5.5 density enhancement in \sim 30% of the volume), and it predicts that the 0.5-2 keV flux will drop at a rate of \sim 17% per year once no new dense ER material is being shocked.

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arXiv2011

SXP 1062, a young Be X-ray binary pulsar with long spin period; Implications for the neutron star birth spin

(shortened) The SMC is ideally suited to investigating the recent star formation history from X-ray source population studies. It harbours a large number of Be/X-ray binaries, and the supernova remnants can be easily resolved with imaging X-ray instruments. We search for new supernova remnants in the SMC and in particular for composite remnants with a central X-ray source. We study the morphology of newly found candidate supernova remnants using radio, optical and X-ray images and investigate their X-ray spectra. Here we report on the discovery of the new supernova remnant around the recently discovered Be/X-ray binary pulsar SXP 1062 in radio and X-ray images. The Be/X-ray binary system is found near the centre of the supernova remnant, which is located at the outer edge of the eastern wing of the SMC. The remnant is oxygen-rich, indicating that it developed from a type Ib event. From XMM-Newton observations we find that the neutron star with a spin period of 1062 s shows a very high average spin-down rate of 0.26 s per day over the observing period of 18 days. From the currently accepted models, our estimated age of around 10000-25000 years for the supernova remnant is not long enough to spin down the neutron star from a few 10 ms to its current value. Assuming an upper limit of 25000 years for the age of the neutron star and the extreme case that the neutron star was spun down by the accretion torque that we have measured during the XMM-Newton observations since its birth, a lower limit of 0.5 s for the birth spin period is inferred. For more realistic, smaller long-term average accretion torques our results suggest that the neutron star was born with a correspondingly longer spin period. This implies that neutron stars in Be/X-ray binaries with long spin periods can be much younger than currently anticipated.

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arXiv2011

A new super-soft X-ray source in the Small Magellanic Cloud: Discovery of the first Be/white dwarf system in the SMC?

The Small Magellanic Cloud (SMC) hosts a large number of Be/X-ray binaries, however no Be/white dwarf system is known so far, although population synthesis calculations predict that they might be more frequent than Be/neutron star systems. XMMUJ010147.5-715550 was found as a new faint super-soft X-ray source (SSS) with a likely Be star optical counterpart. We investigate the nature of this system and search for further high-absorbed candidates in the SMC. We analysed the XMM-Newton X-ray spectrum and light curve, optical photometry, and the I-band OGLE III light curve. The X-ray spectrum is well represented by black-body and white dwarf atmosphere models with highly model-dependent temperature between 20 and 100 eV. The likely optical counterpart AzV 281 showed low near infrared emission during X-ray activity, followed by a brightening in the I-band afterwards. We find further candidates for high-absorbed SSSs with a blue star as counterpart. We discuss XMMUJ010147.5-715550 as the first candidate for a Be/white dwarf binary system in the SMC.

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arXiv2011

Highly absorbed X-ray binaries in the Small Magellanic Cloud

Many of the high mass X-ray binaries (HMXRBs) discovered in recent years in our Galaxy are characterized by a high absorption, most likely intrinsic to the system, which hampers their detection at the softest X-ray energies. We have undertaken a search for highly-absorbed X-ray sources in the Small Magellanic Cloud (SMC) with a systematic analysis of 62 XMM-Newton SMC observations. We obtained a sample of 30 sources showing evidence for an equivalent hydrogen column density larger than 3x10^23 cm^-2. Five of these sources are clearly identified as HMXRBs: four were already known (including three X-ray pulsars) and one, XMM J005605.8-720012, reported here for the first time. For the latter, we present optical spectroscopy confirming the association with a Be star in the SMC. The other sources in our sample have optical counterparts fainter than magnitude ~16 in the V band, and many of them have possible NIR counterparts consistent with highly reddened early type stars in the SMC. While their number is broadly consistent with the expected population of background highly-absorbed active galactic nuclei, a few of them could be HMXRBs in which an early type companion is severely reddened by local material.

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arXiv2011

IKT 16: A Composite SNR in the SMC

Aims: IKT 16 is an X-ray and radio-faint supernova remnant (SNR) in the Small Magellanic Cloud (SMC). A previous X-ray study of this SNR found a hard X-ray source near its centre. Using all available archival and proprietary XMM-Newton data, alongside new multi-frequency radio-continuum surveys and optical observations at H-alpha and forbidden [SII] and [OIII] lines, we aim to constrain the properties of the SNR and discover the nature of the hard source within. Methods: We combine XMM-Newton datasets to produce the highest quality X-ray image of IKT 16 to date. We use this, in combination with radio and optical images, to conduct a multi-wavelength morphological analysis of the remnant. We extract separate spectra from the SNR and the bright source near its centre, and conduct spectral fitting of both regions. Results: We find IKT 16 to have a radius of 37+-3 pc, with the bright source located 8+-2 pc from the centre. This is the largest known SNR in the SMC. The large size of the remnant suggests it is likely in the Sedov-adiabatic phase of evolution. Using a Sedov model to fit the SNR spectrum, we find an electron temperature kT of 1.03+-0.12 keV and an age of 14700 yr. The absorption found requires the remnant to be located deep within the SMC. The bright source is fit with a power law with index 1.58+-0.07, and is associated with diffuse radio emission extending towards the centre of the SNR. We argue that this source is likely to be the neutron star remnant of the supernova explosion, and infer its transverse kick velocity to be 580+-100 km/s. The X-ray and radio properties of this source strongly favour a pulsar wind nebula (PWN) origin.

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arXiv2011

The XMM-Newton survey of the Small Magellanic Cloud: XMMUJ005011.2-730026 = SXP214, a Be/X-ray binary pulsar

In the course of the XMM-Newton survey of the Small Magellanic Cloud (SMC), a region to the east of the emission nebula N19 was observed in November 2009. To search for new candidates for high mass X-ray binaries the EPIC PN and MOS data of the detected point sources were investigated and their spectral and temporal characteristics identified. A new transient (XMMUJ005011.2-730026= SXP214) with a pulse period of 214.05 s was discovered; the source had a hard X-ray spectrum with power-law index of ~0.65. The accurate X-ray source location permits the identification of the X-ray source with a ~15th magnitude Be star, thereby confirming this system as a new Be/X-ray binary.

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arXiv2010

The XMM-Newton survey of the Small Magellanic Cloud: Discovery of the 11.866 s Be/X-ray binary pulsar XMMUJ004814.0-732204 (SXP11.87)

One of the goals of the XMM-Newton survey of the Small Magellanic Cloud is the study of the Be/X-ray binary population. During one of our first survey observations a bright new transient - XMMUJ004814.0-732204 - was discovered. We present the analysis of the EPIC X-ray data together with optical observations, to investigate the spectral and temporal characteristics of XMMUJ004814.0-732204. We found coherent X-ray pulsations in the EPIC data with a period of (11.86642 +/- 0.00017) s. The X-ray spectrum can be modelled by an absorbed power-law with indication for a soft excess. Depending on the modelling of the soft X-ray spectrum, the photon index ranges between 0.53 and 0.66. We identify the optical counterpart as a B = 14.9mag star which was monitored during the MACHO and OGLE-III projects. The optical light curves show regular outbursts by ~0.5 mag in B and R and up to 0.9 mag in I which repeat with a time scale of about 1000 days. The OGLE-III optical colours of the star are consistent with an early B spectral type. An optical spectrum obtained at the 1.9 m telescope of the South African Astronomical Observatory in December 2009 shows H_alpha emission with an equivalent width of 3.5 +/- 0.6 A. The X-ray spectrum and the detection of pulsations suggest that XMMUJ004814.0-732204 is a new high mass X-ray binary pulsar in the SMC. The long term variability and the H_alpha emission line in the spectrum of the optical counterpart identify it as a Be/X-ray binary system.

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arXiv2010

The XMM-Newton survey of the Small Magellanic Cloud: A new X-ray view of the symbiotic binary SMC3

The XMM-Newton survey of the Small Magellanic Cloud (SMC) was performed to study the population of X-ray sources in this neighbouring galaxy. During one of the observations, the symbiotic binary SMC3 was found at its highest X-ray luminosity observed until now. In SMC3 wind accretion from a giant donor star onto a white dwarf is believed to cause steady hydrogen burning on the white dwarf surface, making such systems candidates for supernova type Ia progenitors. It was suggested that the X-ray source is eclipsed every ~4.5 years by the companion star and its stellar wind to explain the large X-ray variability seen in ROSAT data. We use the available X-ray data to test this scenario. We present the ~20 year X-ray light curve of SMC3 and study the spectral evolution as seen with XMM-Newton/EPIC-pn to investigate possible scenarios which can reproduce the high X-ray variability. We did not find significant variations in the photo-electric absorption, as it would be expected during eclipse ingress and egress. Instead, the X-ray spectra from different intensity levels, when modelled by black-body emission, can be better explained by variations either in normalisation (by a factor of ~50) or in temperature (kT between 24 eV and 34 eV). The light curve shows maxima and minima with slow transitions between them. To explain the gradual variations in the X-ray light curve and to avoid changes in absorption by neutral gas, a predominant part of the stellar wind must be ionised by the X-ray source. Compton scattering with variable electron column density (of the order of 5 x 10^24 cm^-2) along the line of sight could then be responsible for the intensity changes. The X-ray variability of SMC3 could also be caused by temperature changes in the hydrogen burning envelope of the white dwarf, an effect which could even dominate if the stellar wind density is not sufficiently high.

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arXiv2010

The Orbital Solution and Spectral Classification of the High-Mass X-Ray Binary IGR J01054-7253 in the Small Magellanic Cloud

We present X-ray and optical data on the Be/X-ray binary (BeXRB) pulsar IGR J01054-7253 = SXP11.5 in the Small Magellanic Cloud (SMC). Rossi X-ray Timing Explorer (RXTE) observations of this source in a large X-ray outburst reveal an 11.483 +/- 0.002s pulse period and show both the accretion driven spin-up of the neutron star and the motion of the neutron star around the companion through Doppler shifting of the spin period. Model fits to these data suggest an orbital period of 36.3 +/- 0.4d and Pdot of (4.7 +/- 0.3) x 10^{-10} ss^{-1}. We present an orbital solution for this system, making it one of the best described BeXRB systems in the SMC. The observed pulse period, spin-up and X-ray luminosity of SXP11.5 in this outburst are found to agree with the predictions of neutron star accretion theory. Timing analysis of the long-term optical light curve reveals a periodicity of 36.70 +/- 0.03d, in agreement with the orbital period found from the model fit to the X-ray data. Using blue-end spectroscopic observations we determine the spectral type of the counterpart to be O9.5-B0 IV-V. This luminosity class is supported by the observed V-band magnitude. Using optical and near-infrared photometry and spectroscopy, we study the circumstellar environment of the counterpart in the months after the X-ray outburst.

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arXiv2010

An XMM-Newton view of Planetary Nebulae in the Small Magellanic Cloud. The X-ray luminous central star of SMP SMC 22

During an X-ray survey of the Small Magellanic Cloud, carried out with the XMM-Newton satellite, we detected significant soft X-ray emission from the central star of the high-excitation planetary nebula SMP SMC 22. Its very soft spectrum is well fit with a non local thermodynamical equilibrium model atmosphere composed of H, He, C, N, and O, with abundances equal to those inferred from studies of its nebular lines. The derived effective temperature of 1.5x10^5 K is in good agreement with that found from the optical/UV data. The unabsorbed flux in the 0.1-0.5 keV range is about 3x10^{-11} erg cm^-2 s^-1, corresponding to a luminosity of 1.2x10^37 erg/s at the distance of 60 kpc. We also searched for X-ray emission from a large number of SMC planetary nebulae, confirming the previous detection of SMP SMC 25 with a luminosity of (0.2-6)x10^35 erg/s (0.1-1 keV). For the remaining objects that were not detected, we derived flux upper limits corresponding to luminosity values from several tens to hundreds times smaller than that of SMP SMC 22. The exceptionally high X-ray luminosity of SMP SMC 22 is probably due to the high mass of its central star, quickly evolving toward the white dwarf's cooling branch, and to a small intrinsic absorption in the nebula itself.

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arXiv2010

High Resolution X-Ray Spectroscopy of SN 1987A: Monitoring with XMM-Newton

We report the results of our XMM-Newton monitoring of SN 1987A. The ongoing propagation of the supernova blast wave through the inner circumstellar ring caused a drastic increase in X-ray luminosity during the last years, enabling detailed high resolution X-ray spectroscopy with the Reflection Grating Spectrometer. The observations can be used to follow the detailed evolution of the arising supernova remnant. The fluxes and broadening of the numerous emission lines seen in the dispersed spectra provide information on the evolution of the X-ray emitting plasma and its dynamics. These were analyzed in combination with the EPIC-pn spectra, which allow a precise determination of the higher temperature plasma. We modeled individual emission lines and fitted plasma emission models. Especially from the observations between 2003 and 2007 we can see a significant evolution of the plasma parameters and a deceleration of the radial velocity of the lower temperature plasma regions. We found an indication (3-sigma-level) of an iron K feature in the co-added EPIC-pn spectra. The comparison with Chandra grating observations in 2004 yields a clear temporal coherence of the spectral evolution and the sudden deceleration of the expansion velocity seen in X-ray images ~6100 days after the explosion.

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