arXiv subjects

B. Aschenbach

Publications and source records attributed to B. Aschenbach.

At least 19 recordsLinked to original sources

arXiv2015

Reverse-Shock in Tycho's Supernova Remnant

Thermal X-ray emission from young supernova remnants (SNRs) is usually dominated by the emission lines of the supernova (SN) ejecta, which are widely believed being crossed and thus heated by the inwards propagating reverse shock (RS). Previous works using imaging X-ray data have shown that the ejecta are heated by the RS by locating the peak emission region of the most recently ionized matter, which is found well separated towards the inside from the outermost boundary. Here we report the discovery of a systematic increase of the Sulfur (S) to Silicon (Si) K$α$ line flux ratio with radius in Tycho's SNR. This allows us, for the first time, to present continuous radial profiles of the ionization age and, furthermore, the elapsed ionization time since the onset of the ionization, which tells the propagation history of the ionization front into the SNR ejecta.

astro-ph.HE↗

arXiv2010

XMM-Newton observation of the eclipsing binary Algol

We present an {\sl XMM-Newton} observation of the eclipsing binary Algol which contains an X-ray dark B8V primary and an X-ray bright K2IV secondary. The observation covered the optical secondary eclipse and captured an X-ray flare that was eclipsed by the B star. The EPIC and RGS spectra of Algol in its quiescent state are described by a two-temperature plasma model. The cool component has a temperature around 6.4$\times 10^{6}$ K while that of the hot component ranges from 2 to 4.0$\times 10^{7}$ K. Coronal abundances of C, N, O, Ne, Mg, Si and Fe were obtained for each component for both the quiescent and the flare phases, with generally upper limits for S and Ar, and C, N, and O for the hot component. F-tests show that the abundances need not to be different between the cool and the hot component and between the quiescent and the flare phase with the exception of Fe. Whereas the Fe abundance of the cool component remains constant at $\sim$0.14, the hot component shows an Fe abundance of $\sim$0.28, which increases to $\sim$0.44 during the flare. This increase is expected from the chromospheric evaporation model. The absorbing column density $N_H$ of the quiescent emission is 2.5$\times10^{20}$ cm$^{-2}$, while that of the flare-only emission is significantly lower and consistent with the column density of the interstellar medium. This observation substantiates earlier suggestions of the presence of X-ray absorbing material in the Algol system.

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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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arXiv2008

X-ray hiccups from SgrA* observed by XMM-Newton. The second brightest flare and three moderate flares caught in half a day

[truncated] In Spring 2007, we observed SgrA* with XMM with a total exposure of ~230ks. We have performed timing and spectral analysis of the new X-ray flares detected during this campaign. To study the range of flare spectral properties, in a consistent manner, we have also reprocessed, using the same analysis procedure and the latest calibration, archived XMM data of previously reported rapid flares. The dust scattering was taken into account during the spectral fitting. We also used Chandra archived observations of the quiescent state of SgrA* for comparison. On April 4, 2007, we observed for the first time within a time interval of ~1/2 day, an enhanced incidence rate of X-ray flaring, with a bright flare followed by three flares of more moderate amplitude. The former event represents the second brightest X-ray flare from Sgr A* on record. This new bright flare exhibits similar light-curve shape (nearly symmetrical), duration (~3ks) and spectral characteristics to the very bright flare observed in October 3, 2002. The measured spectral parameters of the new bright flare, assuming an absorbed power law model taken into account dust scattering effect, are N_H=12.3(+2.1,-1.8)e22 cm-2 and Gamma~2.3+/-0.3 calculated at the 90% c.l. The spectral parameter fits of the sum of the three following moderate flares, while lower, are compatible within the error bars with those of the bright flares. The column density found, for a power-law, during the flares is at least two times higher than the value expected from the (dust) visual extinction toward SgrA* (AV~25 mag). However, our fitting of the SgrA* quiescent spectra obtained with Chandra shows that an excess of column density is already present during the non-flaring phase. The two brightest X-ray flares observed so far from SgrA* exhibited similar soft spectra.

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arXiv2007

Measurement of Mass and Spin of Black Holes with QPOs

There are now four low mass X-ray binaries with black holes which show twin resonant-like HFQPOs. Similar QPOs might have been found in Sgr A*. I review the power spectral density distributions of the three X-ray flares and the six NIR flares published for Sgr A* so far, in order to look for more similarities than just the frequencies between the microquasar black holes and Sgr A*. The three X-ray flares of Sgr A* are re-analysed in an identical way and white noise probabilities from their power density distributions are given for the periods reported around 1100 s. Progress of the resonant theory using the anomalous orbital velocity effect is summarized.

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arXiv2007

Multi-wavelength study of a new Galactic SNR G332.5-5.6

We present compelling evidence for confirmation of a Galactic supernova remnant (SNR) candidate, G332.5-5.6, based initially on identification of new, filamentary, optical emission line nebulosity seen in the arcsecond resolution images from the AAO/UKST HAlpha survey. The extant radio observations and X-ray data which we have independently re-reduced, together with new optical spectroscopy of the large-scale fragmented nebulosity, confirms the identification. Optical spectra, taken across five different, widely separated nebula regions of the remnant as seen on the HAlpha images, show average ratios of [NII]/HAlpha =2.42, [SII]/HAlpha = 2.10, and [SII] 6717/6731 = 1.23, as well as strong [OI] 6300, 6364A and [OII] 3727A emission. These ratios are firmly within those typical of SNRs. Here, we also present the radio-continuum detection of the SNR at 20/13cm from observations with the Australia Telescope Compact Array (ATCA). Radio emission is also seen at 4850 MHz, in the PMN survey (Griffith and Wright 1993) and at 843 MHz from the SUMSS survey (Bock, Large and Sadler 1999). We estimate an angular diameter of ~30 arcmin and obtain an average radio spectral index of alpha = -0.6 +- 0.1 which indicates the non-thermal nature of G332.5-5.6. Fresh analysis of existing ROSAT X-ray data in the vicinity also confirms the existence of the SNR. The distance to G332.5-5.6 has been independently estimated by Reynoso and Green (2007) as 3.4 kpc based on measurements of the HI lambda21 cm line seen in absorption against the continuum emission. Our cruder estimates via assumptions on the height of the dust layer (3.1 kpc) and using the Sigma-D relation (4 kpc) are in good agreement.

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arXiv2007

Multiwavelength appearance of Vela Jr.: Is it up to expectations?

Vela Jr. is one of the youngest and likely nearest among the known galactic supernova remnants (SNRs). Discovered in 1997 it has been studied since then at quite a few wavelengths, that spread over almost 20 decades in energy. Here we present and discuss Vela Jr. properties revealed by these multiwavelength observations, and confront them with the SNR model expectations. Questions that remained unanswered at the time of publication of the paper of Iyudin et al. (2005), e.g. what is the nature of the SNR's proposed central compact source CXOU J085201.4-461753, and why is the ISM absorption column density apparently associated with RX J0852.0-4622 much greater than the typical column of the Vela SNR, can be addressed using the latest radio and X-ray observations of Vela Jr.. These, and other related questions are addressed in the following.

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arXiv2006

XMM-Newton observations of SN1987A

(abridged) We report on XMM-Newton observations of SN1987A in the Large Magellanic Cloud. The large collecting area telescopes together with the European Photon Imaging Cameras (EPIC) provide X-ray spectra with unprecedented statistical quality and make it possible to investigate the spectral evolution during the brightening observed since the discovery in X-rays. High resolution spectra from the Reflection Grating Spectrometers yield a complementary view and allow us to perform more detailed investigations of prominent emission lines. The X-ray spectra were modeled with two-temperature emission components from a hot plasma in collisional ionization equilibrium and in non-equilibrium (NEI). We find a temperature for the equilibrium component of 0.24+/-0.02 keV in January 2000 and April 2001 which increased to 0.30+/-0.02 keV in May 2003 and also an indication for a temperature increase in the hot NEI component from \~2 keV to ~3 keV. Emission line ratios inferred from the RGS spectra suggest temperatures as low as 100 eV and an increase in the ionization state of oxygen and neon consistent with the observed temperature increases. The fast readout of the EPIC-pn instrument yields X-ray fluxes free of CCD pile-up effects which we used to normalize pile-up corrections for the published Chandra fluxes. The corrected X-ray light curve of SN1987A in the 0.5-2.0 keV energy band is best represented by a linear increase up to about day 4000 after the explosion and an exponential rise afterwards until the last published Chandra observation on day 6716. Modeling the light curve by emission from the inner ring which is approximated by a circular torus a central density n_H = 1.15x10^4 cm^-3 is found. In this model the forward shock has just passed the center of the torus.

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arXiv2006

Exploring the Central Compact Object in the RX J0852.0-4622 Supernova Remnant with XMM-Newton

The properties of the presumably young galactic supernova remnant (SNR) RX J0852.0-4622, discovered by ROSAT, are still uncertain. The data concerning the distance to the SNR, its age, and the presence of a compact remnant remain controversial. We report the results of several XMM-Newton observations of CXOU J085201.4-461753, the central compact source in RX J0852.0-4622. The currently prefered interpretation of CXOU J085201.4-461753 being a neutron star is in line with our analysis. The Chandra candidate pulsation periods are not confirmed; actually no period was found down to a 3-sigma upper limit for any pulsed fraction. The spectrum of CXOU J085201.4-461753 is best described by either a two blackbody spectrum or a single blackbody spectrum with a high energy power law tail. The two blackbody temperatures of 4 MK and 6.6 MK along with the small size of the emitting regions with radii of 0.36 and 0.06 km invalidate the interpretation that the thermal radiation is cooling emission from the entire neutron star surface. The double blackbody model suggests emission from the neutron star's hot polar regions. No X-ray lines, including the emission feature previously claimed to be present in Chandra data, were found.

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arXiv2006

Mass and Angular Momentum of Black Holes: An Overlooked Effect of General Relativity Applied to the Galactic Center Black Hole Sgr A*

I report the discovery of a new effect of General Relativity which is important to understand very rapidly rotating (Kerr) black holes. The orbital velocity of a test particle is no longer a monotonic function of the orbit radius when the spin of the black hole is >0.9953, but displays a local minimum-maximum structure for radii smaller than 1.8 gravitational radii. There the rate of change of the orbital velocity per radius unit equals the radial epicyclic frequency and is exactly one third of the polar epicyclic frequency, suggesting a 3:1 resonant oscillatory motion of the particle. If associated with the observed quasi-periods - including the most recent 2005/2006 data - the mass of the supermassive black hole Sgr A* in the centre of the our Galaxy is determined to 3.3 million solar masses and the spin is 0.99616.

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arXiv2004

Discovery of a bright X-ray transient in the Galactic Center with XMM-Newton

We report the discovery of a bright X-ray transient object, XMMU J174554.4-285456, observed in outburst with XMM-Newton on October 3, 2002,and located at 6.3' from SgrA*, the supermassive black hole at the Galactic center.This object exhibits a very large X-ray luminosity variability of a factor of about 1300 between two X-ray observations separated by four months. The X-ray spectrum is best fitted by a power-law with a photon index of 1.6+/-0.2 and absorption column density of 14.1 (+1.6,-1.4) x 10^22 cm^-2. This large absorption suggests this source is located at the distance of the Galactic center, i.e., 8 kpc. The 2-10 keV luminosity is about 1.0 x 10^35(d/8kpc)^2 erg/s. A pulsation period of about 172 s is hinted by the timing analysis. The X-ray properties strongly suggest a binary system with either a black hole or a neutron star for the compact object.

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arXiv2004

XMM-Newton observations of the supernova remnant RX J0852.0-4622/GRO J0852-4642

RX J0852.0-4622 is a shell type supernova remnant with a power law spectrum in the south-eastern corner of the Vela supernova remnant. We report XMM-Newton observations of three fields centered on the rim of the remnant and confirm the power law shape of the spectra with no prominent emission lines as previously obtained with ASCA. The emission line feature at ~4.1 keV indicated in the ASCA/SIS spectrum of the northern field is detected and found in the southern and western fields, as well, with a total significance of slightly more than 4 sigmas. The line position is improved and the feature is centered on 4.45 +/- 0.05 keV. We suggest this line feature to be emission from Ti and Sc. The measured X-ray flux is consistent with the Ti gamma-ray flux previously measured with COMPTEL. We discuss various physical processes which may lead to the creation of these lines, and we discuss the implications for the progenitor and the supernova type.

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arXiv2004

Mass and Angular Momentum of Sgr A*

For very fast spinning black holes with a>0.9953 the orbital velocity of a test particle unexpectly shows a positive radial gradient for low orbits, an effect of GR which has been overlooked so far. For a=0.99616 the maximum radial change of the orbital velocity equals the radial epicyclic frequency, which in turn is in a 1:3 resonance with the vertical epicyclic frequency. Application to Sgr A* using the quasi-periodic oscillations recently measured suggest that the mass of Sgr A* is 3.28 +/- 0.13 million solar masses.

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arXiv2004

Measuring mass and angular momentum of black holes with high-frequency quasi-periodic oscillations

3:2 and/or 3:1 twin high frequency quasi-periodic oscillations (HFQPOs) have been measured for the three microquasars GRO J1655-40, XTE J1550-564 and GRS 1915+105. For a test particle orbiting a rotating black on a stable circular orbit there exist two different orbits at which the vertical and radial epicyclic oscillations are in either a 3:1 or 3:2 parametric resonance for any choice of the black hole angular momentum $a$. If the two orbits are required to be frequency commensurable Keplerian orbits there is only one solution for the two orbit radii and $a$. This model predicts that the microquasars have the same $a$, and it predicts their black hole masses on the basis of the measured HFQPOs in agreement with the dynamically determined masses. Application of this model to the Galactic Center black hole Sgr A* using the recently measured QPOs (Genzel et al., 2003, Aschenbach et al., 2004) leads to a black hole mass of (3.28 +/- 0.13) x 10^6 M_odot, and the same $a$ as for the microquasars. The possibility that all four sources have $a$=0.99616 suggests that this value is the upper limit of $a$ imposed by general relativity. The same value for the lower orbit radius and the same value for $a$ are also suggested by an analysis of the general relativistic expression for the radial gradient of the orbital velocity, which changes sign in a narrow annular region around the lower orbit when $a$>0.9953.

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arXiv2004

X-ray flares reveal mass and angular momentum of the Galactic Center black hole

We have analysed the light curve of the two brightest X-ray flares from the Galactic Center black hole, one flare observed by XMM-Newton on October 3, 2002 (Porquet et al. 2003), and the other flare observed by Chandra on October 26, 2000 (Baganoff et al. 2001). The power density spectra show five distinct peaks at periods of ~ 100 s, 219 s, 700 s, 1150 s, and 2250 s common to both observations within their estimated measurement uncertainties. The power density spectrum of the recently reported infrared flare of June 16, 2003 (Genzel et al. 2003) shows distinct peaks at two, if not three, periods (including the 1008 +/- 120 s infrared period), which are consistent with the X-ray periods. The remaining two periods could not be covered by the infrared measurements. Each period can be identified with one of the characteristic gravitational cyclic modes associated with accretion disks, i.e. either Lense-Thirring precession, Kepler orbital motion and the vertical and radial epicyclic oscillation modes, in such a way that a consistent value for the black hole mass of M_BH = 2.72^{+0.12}_{-0.19} x 10^6 M_odot and the angular momentum $a$ = 0.9939^{+0.0026}_{-0.0074} is obtained. The available data on M_BH derived from studies of the orbital motion of the S2 (S0-2) star (Schoedel et al. 2002, Ghez et al. 2003) agree with our findings. Finally we discuss some implications of the fairly high value for the angular momentum.

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arXiv2003

Multi-wavelength study of the G 82.2+5.3 supernova remnant

We present the first CCD flux-calibrated images of the snr G 82.2+5.3 in major optical emission lines. The [O III] 5007 line provides the first direct evidence of optical emission originating from G 82.2+5.3. Filamentary emission is detected in the west and east areas, roughly defining an ellipsoidal shell. The [O III] emission is rather well correlated with the radio emission suggesting their association, while typical fluxes are found in the range of 20-30x10^{-17} erg s^{-1} cm^{-2} arcsec^{-2}. Deep long-slit spectra taken at specific positions of the remnant verify that the filamentary emission originates from shock heated gas with shock velocities ~100 km/s and low electron densities, while the diffuse [O III] emission in the south results from photoionization processes. The X-ray surface brightness is quite patchy, missing obvious limb brightening and is dominated by a bright bar-like emission region which is off-set from the geometric center by ~9'. The X-ray emission is thermal and requires two temperatures of 0.2 keV and 0.63 keV. The bright bar region shows overabundant Mg, Si and Fe, which might indicate still radiating ejecta matter. The azimuthally averaged radial surface profile is consistent with the matter density changing with distance r from the center ~e^{-r/r_0} with a characteristic angular length of 36', or, alternatively, with an r^{-1/2} density profile. The matter inside the remnant is quite likely structured like a porous cloudy medium. The average matter density is ~0.04 x d_{1.6}^{-0.5}. Because of the low density and the long cooling times involved the remnant is more likely to be in the adiabatic phase, which is consistent with the densities derived for the X-ray plasma and the optical line emission, but it is not excluded that is has reached the radiating phase.

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arXiv2003

XMM-Newton Witness of M86 X-ray Metamorphosis

The environmental influence of cluster media on its member galaxies, known as Butcher--Oemler effect, has recently been subject to revision due to numerous observations of strong morphological transformations occurring outside the cluster virial radii, caused by some unidentified gas removal processes. In this context we present new XMM-Newton observations of M86 group. The unique combination of high spatial and spectral resolution and large field of view of XMM-Newton allows an in-depth investigation of the processes involved in the spectacular disruption of this object. We identify a possible shock with Mach number of ~1.4 in the process of crushing the galaxy in the North-East direction. The latter is ascribed to the presence of a dense X-ray emitting filament, previously revealed in the RASS data. The shock is not associated with other previously identified features of M86 X-ray emission, such as the plume, the north-eastern arm and the southern extension, which are found to have low entropy, similar to the inner 2 kpc of M86. Finally, mere existence of the large scale gas halo around the M86 group, suggests that the disruptions of M86's X-ray halo may be caused by small-scale types of interactions such as galaxy-galaxy collisions.

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arXiv2003

Near-infrared flares from accreting gas around the supermassive black hole at the Galactic Centre

Recent measurements of stellar orbits provide compelling evidence that the compact radio source Sagittarius A* at the Galactic Centre is a 3.6-million-solar-mass black hole. Sgr A* is remarkably faint in all wavebands other than the radio region, however, which challenges current theories of matter accretion and radiation surrounding black holes. The black hole's rotation rate is not known, and therefore neither is the structure of space-time around it.Here we report high-resolution infrared observations of Sgr A* that reveal 'quiescent' emission and several flares. The infrared emission originates from within a few milliarcseconds of the black hole, and traces very energetic electrons or moderately hot gas within the innermost accretion region. Two flares exhibit a 17-minute quasi-periodic variability. If the periodicity arises from relativistic modulation of orbiting gas, the emission must come from just outside the event horizon, and the black hole must be rotating at about half of the maximum possible rate.

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