Publication archive

Publication archive

We investigate long-term X-ray behaviors from the Sgr B2 complex using archival data of the X-ray satellites Suzaku, XMM-Newton, Chandra and ASCA. The observed region of the Sgr B2 complex includes two prominent spots in the Fe I K-alpha line at 6.40 keV, a giant molecular cloud M0.66-0.02 known as the "Sgr B2 cloud" and an unusual X-ray source G0.570-0.018. Although these 6.40 keV spots have spatial extensions of a few pc scale, the morphology and flux of the 6.40 keV line has been time variable for 10 years, in contrast to the constant flux of the Fe XXV K-alpha line at 6.67 keV in the Galactic diffuse X-ray emission. This time variation is mostly due to M0.66-0.02; the 6.40 keV line flux declined in 2001 and decreased to 60% in the time span 1994-2005. The other spot G0.570-0.018 is found to be conspicuous only in the Chandra observation in 2000. From the long-term time variability (~10 years) of the Sgr B2 complex, we infer that the Galactic Center black hole Sgr A* was X-ray bright in the past 300 year and exhibited a time variability with a period of a few years.
Published: 16 April 2008
We have selected a sample of broad absorption line (BAL) quasars which show significant radio variations, indicating the presence of polar BAL outflows. We obtained snapshot XMM observations of four polar BAL QSOs, to check whether strong X-ray absorption, one of the most prominent characteristics of most BAL QSOs, also exists in polar outflows. Two of the sources are detected in the X-ray. Spectral fittings show that they are X-ray normal with no intrinsic X-ray absorption, suggesting the X-ray shielding gas might be absent in polar BAL outflows. Comparing to non-BAL QSOs, one of two X-ray-nondetected sources remains consistent with X-ray normal, while the other one, which is an iron low-ionization BAL (FeLoBAL), shows an X-ray weakness factor of >19, suggesting strong intrinsic X-ray absorption. Alternative explanations for the nondetection of strong X-ray absorption in the two X-ray-detected sources are that (1) the absorption is more complex than a simple neutral absorber, such as partial covering absorption or ionized absorption; (2) there might be significant jet contribution to the detected X-ray emission. Current data are insufficient to test these possibilities, and further observations are required to understand the X-ray nature of polar BAL outflows.
Published: 02 March 2008
V5116 Sgr (Nova Sgr 2005 No. 2), discovered on 2005 July 4, was observed with XMM-Newton in 2007 March, 20 months after the optical outburst. The X-ray spectrum shows that the nova had evolved to a pure supersoft X-ray source, with no significant emission at energies above 1 keV. The X-ray light curve shows abrupt decreases and increases of the flux by a factor ~8. It is consistent with a periodicity of 2.97 hr, the orbital period suggested by Dobrotka and coworkers, although the observation lasted just a little more than a whole period. We estimate the distance to V5116 Sgr to be 11±3 kpc. A simple blackbody model does not fit correctly the EPIC spectra, with Chi²>4. In contrast, ONe-rich white dwarf atmosphere models provide a good fit, with NH=(1.3±0.1)x1021 cm-2, T=(6.1±0.1)x105 K, and L=(3.9±0.8)x1037 (D/10 kpc)² erg s-1 (during the high-flux periods). This is consistent with residual hydrogen burning in the white dwarf envelope. The white dwarf atmosphere temperature is the same both in the low- and the high-flux periods, ruling out an intrinsic variation of the X-ray source as the origin of the flux changes. We speculate that the X-ray light curve may result from a partial coverage by an asymmetric accretion disk in a high-inclination system.
Published: 10 March 2008
We report the successful identification of the type of the supernova responsible for the supernova remnant SNR 0509-675 in the Large Magellanic Cloud (LMC) using Gemini spectra of surrounding light echoes. The ability to classify outbursts associated with centuries-old remnants provides a new window into several aspects of supernova research and is likely to be successful in providing new constraints on additional LMC supernovae as well as their historical counterparts in the Milky Way Galaxy (MWG). The combined spectrum of echo light from SNR 0509-675 shows broad emission and absorption lines consistent with a supernova (SN) spectrum. We create a spectral library consisting of 28 SNe Ia and 6 SN Ib/c that are time-integrated, dust-scattered by LMC dust, and reddened by the LMC and MWG. We fit these SN templates to the observed light echo spectrum using chi² minimization as well as correlation techniques, and we find that overluminous 91T-like SNe Ia with delta m15<0.9 match the observed spectrum best.
Published: 01 February 2008
We examine the dynamics and X-ray spectrum of the young Type Ia supernova remnant 0509-67.5 in the context of the recent results obtained from the optical spectroscopy of its light echo. Our goal is to estimate the kinetic energy of the supernova explosion using Chandra and XMM-Newton observations of the supernova remnant, thus placing the birth event of 0509-67.5 in the sequence of dim to bright Type Ia supernovae. We base our analysis on a standard grid of one-dimensional delayed detonation explosion models, together with hydrodynamic and X-ray spectral calculations of the supernova remnant evolution. From the remnant dynamics and the properties of the O, Si, S, and Fe emission in its X-ray spectrum we conclude that 0509-67.5 was originated ~400 years ago by a bright, highly energetic Type Ia explosion similar to SN 1991T. Our best model has a kinetic energy of 1.4x1051 erg and synthesizes 0.97 Msun of 56Ni. These results are in excellent agreement with the age estimate and spectroscopy from the light echo. We have thus established the first connection between a Type Ia supernova and its supernova remnant based on a detailed quantitative analysis of both objects.
Published: 01 February 2008
Most stars form as members of large associations within dense, very cold (10 to 100 Kelvin) molecular clouds. The nearby giant molecular cloud in Orion hosts several thousand stars of ages less than a few million years, many of which are located in or around the famous Orion Nebula, a prominent gas structure illuminated and ionized by a small group of massive stars (the Trapezium). We present x-ray observations obtained with the X-ray Multi-Mirror satellite XMM-Newton, revealing that a hot plasma with a temperature of 1.7 to 2.1 million kelvin pervades the southwest extension of the nebula. The plasma flows into the adjacent interstellar medium. This x-ray outflow phenomenon must be widespread throughout our Galaxy.
Published: 18 January 2008
Most stars form as members of large associations within dense, very cold (10-100 K) molecular clouds. The nearby giant molecular cloud in Orion hosts several thousand stars of ages less than a few million years, many of which are located in or around the famous Orion Nebula, a prominent gas structure illuminated and ionized by a small group of massive stars (the Trapezium). We present X-ray observations obtained with the X-ray Multi-Mirror satellite XMM-Newton revealing that a hot plasma with a temperature of 1.7-2.1 million K pervades the southwest extension of the nebula. The plasma flows into the adjacent interstellar medium. This X-ray outflow phenomenon must be widespread throughout our Galaxy
Published: 29 November 2007
Context. 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.
Aims. Only a small number of these type of events have been observed and confirmed to date. The discovery of more cases would allow a number of fundamental conclusions to be drawn about properties such as the frequency of tidal disruption events, the distribution of quiescent black hole masses and their influence in the context of galaxy/AGN formation and evolution among others.
Methods. 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 1041-1044 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 1045 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.
Results. 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.
Published: 08 February 2007
E-print arXiv:astro-ph/0612311, for ApJS COSMOS Special Issue, 2007 in press

We present the first set of XMM-Newton EPIC observations in the 2 square degree COSMOS field. The strength of the COSMOS project is the unprecedented combination of a large solid angle and sensitivity over the whole multiwavelength spectrum. The XMM-Newton observations are very efficient in localizing and identifying active galactic nuclei (AGN) and clusters as well as groups of galaxies. One of the primary goals of the XMM-Newton Cosmos survey is to study the co-evolution of active galactic nuclei as a function of their environment in the Cosmic web. Here we present the log of observations, images and a summary of first research highlights for the first pass of 25 XMM-Newton pointings across the field. In the existing dataset we have detected 1416 new X-ray sources in the 0.5-2, 2-4.5 and 4.5-10 keV bands to an equivalent 0.5-2 keV flux limit of 7x10-16 erg cm-2 s-1. The number of sources is expected to grow to almost 2000 in the final coverage of the survey. From an X-ray color color analysis we identify a population of heavily obscured, partially leaky or reflecting absorbers, most of which are likely to be nearby, Compton-thick AGN.

Published: 12 December 2006
XMM-Newton is a major X-ray observatory of the European Space Agency (ESA). Its observing time is open to astronomers from the whole scientific community on a peer reviewed competitive basis. The Science Operations Centre, located at ESAs premises in Villafranca del Castillo, Spain, is responsible for the instrument operations, as well as for all the tasks related to facilitating the scientific exploitation of the data which the mission has been producing since its launch in December 1999.
Published: 12 December 2006
We present details of the discovery of XLSS J022303.0-043622, a z = 1.2 cluster of galaxies. This cluster was identified from its X-ray properties and selected as a z>1 candidate from its optical/near-infrared (IR) characteristics in the XMM Large-Scale Structure Survey (XMM-LSS). It is the most distant system discovered in the survey to date. We present ground-based optical and near-IR observations of the system carried out as part of the XMM-LSS survey. The cluster has a bolometric X-ray luminosity of 1.1 +/- 0.7 × erg, fainter than most other known z >1 X-ray selected clusters. In the optical it has a remarkably compact core, with at least a dozen galaxies inside a 125 kpc radius circle centred on the X-ray position. Most of the galaxies within the core, and those spectroscopically confirmed to be cluster members, have stellar masses similar to those of massive cluster galaxies at low redshift. They have colours comparable to those of galaxies in other z >1 clusters, consistent with showing little sign of strong ongoing star formation. The bulk of the star formation within the galaxies appears to have ceased at least 1.5 Gyr before the observed epoch. Our results are consistent with massive cluster galaxies forming at z >1 and passively evolving thereafter. We also show that the system is straightforwardly identified in Spitzer/IRAC 3.6- and 4.5-mm data obtained by the Spitzer Wide-area Infrared Extragalactic (SWIRE) survey emphasizing the power and utility of joint XMM and Spitzer searches for the most distant clusters.
Published: 02 August 2006
We present the results of a five-day monitoring campaign with xmm of six X-ray bright Young Stellar Objects in the star-forming complex L1551, in Taurus. All stars present significant variability on the five-day time scale. Modulation of the light curve on time scales comparable with the stars rotational period appear to be present in the case of one WTTS. The CTTS XZ~Tau is the only star showing significant spectral variations between the 2000 and the 2004 observations: a hot plasma component which was present in the X-ray spectrum in 2000 has disappeared. As XZ~Tau was undergoing a strong (EXor-like) optical outburst in 2000, which has since then terminated, we speculate on the possible relationship between episodic, burst accretion and X-ray heating. The transition object HL~Tau undergoes a strong flare with a complex temperature evolution, indicative of an event confined within a very large magnetic structure (few stellar radii), similar to the ones found in YSOs in the Orion Nebula Cluster.
Published: 02 June 2006
We present here the results of a deep (130 ks) XMM-Newton observation of the cluster of galaxies 2A 0335+096. The deep exposure allows us to study in detail its temperature structure and its elemental abundances. We fit three different thermal models and find that the multi-temperature wdem model fits our data best. We find that the abundance structure of the cluster is consistent with a scenario where the relative number of type Ia supernovae contributing to the enrichment of the intra-cluster medium is ~25%, while the relative number of core collapse supernovae is ~75%. Comparison of the observed abundances to the supernova yields does not allow us to put any constrains on the contribution of Pop III stars to the enrichment of the ICM. Radial abundance profiles show a strong central peak of both type Ia and core collapse supernova products. Both the temperature and iron abundance maps show an asymmetry in the direction of the elongated morphology of the surface brightness. In particular the temperature map shows a sharp change over a brightness edge on the southern side of the core, which was identified as a cold front in the Chandra data. This suggests that the cluster is in the process of a merger with a subcluster. Moreover, we find that the blobs or filaments discovered in the core of the cluster by Chandra are, contrary to the previous results, colder than the ambient gas and they appear to be in pressure equilibrium with their environment.
Published: 16 April 2006
Using radio observations by FIRST and NVSS, we build a sample of 151 radio-variable quasars selected from the Sloan Digital Sky Survey Data Release 3 (SDSS DR3). Six (and probably another two) of these objects are classified as broad absorption line (BAL) quasars and show radio flux variations of a few tens of percent within 1.5-5 yr. Such large amplitudes of variations imply brightness temperatures much higher than the inverse Compton limits (1012 K) in all these BAL quasars, suggesting the presence of relativistic jets beaming toward the observer. The angles between the outflow and the jet are constrained to be less than ~20°. Such BAL quasars with polar outflows are beyond the simple unification models of BAL and non-BAL quasars, which hypothesize BAL quasars as otherwise normal quasars seen nearly edge-on.
Published: 10 March 2006
We present results from three XMM-Newton observations of the M31 low mass X-ray binary (LMXB) XMMU J004314.4+410726.3 (Bo 158), spaced over 3 days in 2004 July. Bo 158 was the first dipping LMXB to be discovered in M31. Periodic intensity dips were previously seen to occur on a 2.78-h period, due to absorption in material that is raised out of the plane of the accretion disc. The report of these observations stated that the dip depth was anticorrelated with source intensity. In light of the 2004 XMM-Newton observations of Bo 158, we suggest that the dip variation is due to precession of the accretion disc. This is to be expected in LMXBs with a mass ratio 0.3 (period 4 h), as the disc reaches the 3:1 resonance with the binary companion, causing elongation and precession of the disc. A smoothed particle hydrodynamics simulation of the disc in this system shows retrograde rotation of a disc warp on a period of 11Porb, and prograde disc precession on a period of 29 ± 1Porb. This is consistent with the observed variation in the depth of the dips. We find that the dipping behaviour is most likely to be modified by the disc precession, hence we predict that the dipping behaviour repeats on an 81 ± 3 h cycle.
Published: 15 February 2006
We present the results of an XMM-Newton observation of the young (~2-4 Myr) cluster around the hot star sigma Orionis. In a previous paper we presented the analysis of the RGS spectrum of the central hot star; here we discuss the results of the analysis of the full EPIC field. We have detected 175 X-ray sources, 88 of which have been identified with cluster members, including very low-mass stars down to the substellar limit. We detected also eleven new possible candidate members from the 2MASS catalogue. We find that late-type stars have a median log L_X/L_bol ~ -3.3, i.e. very close to the saturation limit. We detected significant variability in ~40% of late-type members or candidates, including 10 flaring sources; rotational modulation is detected in one K-type star and possibly in other 3 or 4 stars. Spectral analysis of the brightest sources shows typical quiescent temperatures in the range T_1~ 0.3-0.8 keV and T_2~ 1-3 keV, with subsolar abundances Z~ 0.1-0.3 Z_sol, similarly to what is found in other star-forming regions and associations. We find no significant difference in the spectral properties of classical and weak-lined T Tauri stars, although classical T Tauri stars tend to be less X-ray luminous than weak-lined T~Tauri stars.
Published: 02 February 2006
The eclipsing active binary SV Cam (G0V/K6V, Porb=0.593071 d) was observed with XMM-Newton during two campaigns in 2001 and 2003. No eclipses in the quiescent emission are clearly identified, but a flare was eclipsed during the 2001 campaign, allowing us to strongly constrain, from purely geometrical considerations, the position and size of the event: the flare is compact and it is formed at a latitude below 65deg. The size, temperature and Emission Measure of the flare imply an electron density of log n_e (cm^-3)~10.6-13.3 and a magnetic field of ~65-1400 G in order to confine the plasma, consistently with the measurements that are obtained from density-sensitive line ratios in other similar active stars. Average emission seems to come from either extended or polar regions because of lack of eclipses. The Emission Measure Distribution, coronal abundances, and characteristics of variability are very similar to other active stars such as AB~Dor (K1V).
Published: 11 January 2006
Very long (172 ks effective exposure time) observations of the BALQSO LBQS 2212-1759 with XMM-Newton yield a stringent upper-limit on its 0.2-10 keV (rest- frame 0.64-32.2 keV) flux, F < 6 E-17 erg/cm2/s, while simultaneous UV and optical observations reveal a rather blue spectrum extending to 650 A in the source rest frame. These results are used to set a tight upper-limit on its optical to X-ray spectral index alpha_{ox} < -2.56. Given the HI-BAL nature of LBQS 212-1759, its X-ray weakness is most likely due to intrinsic absorption. If this is the case, and assuming that the intrinsic alpha_{ox} of LBQS 2212-1759 is -1.63 - a value appropriate for a radio-quiet quasar of this luminosity - one can set a lower limit on the X-ray absorbing column N_{H} > 3.4 E25 cm-2. Such a large column has a Thomson optical depth to electron scattering tau > 23, sufficient to extinguish the optical and UV emission. The problem only gets worse if the gas is neutral since the opacity in the Lyman continuum becomes extremely large, > 2 E8, conflicting with the source detection below 912 A. This apparent contradiction probably means that our lines-of-sight to the X-ray and to the UV emitting regions are different, such that the gas covers completely the compact X-ray source but only partially the more extended source of ultraviolet photons. An extended (~ 1) X-ray source is detected 2 to the south-east of the QSO. Given its thermal spectrum and temperature (1.5 < T < 3.0 keV}, it is probably a foreground (0.29 < z < 0.46) cluster of galaxies.
Published: 04 January 2006
X-ray observations have revealed that many microquasars and low-mass X-ray binaries (LMXBs) exhibit narrow absorption features identified with resonant absorption from FeXXV and FeXXVI and other abundant ions. In many well studied systems there is evidence for blue-shifts, indicating outflowing plasmas. The changes in both the X-ray continuum and the Fe absorption features during dips from the LMXB XB 1323-619 have been modeled as resulting primarily from an increase in column density and a decrease in the ionization state of a highly-ionized absorber. We successfully fit the same ionized absorber model to the persistent and dipping emission from all the other bright dipping LMXBs observed by XMM-Newton (EXO 0748-676, XB 1254-690, X 1624-490, MXB 1659-298, 4U 1746-371 and XB 1916-053) and find similar changes in the properties of the ionized absorber as for XB 1323-619. This implies that the complex spectral changes in the X-ray continua observed from the dip sources as a class can be most simply explained primarily by changes in the highly ionized absorbers present in these systems. There is no need to invoke unusual abundances or partial covering of extended emission regions. Outside of the dips, the absorption line properties do not vary strongly with orbital phase. This implies that the ionized plasma has a cylindrical geometry with a maximum column density close to the plane of the accretion disk. Since dipping sources are simply normal LMXBs viewed from close to the orbital plane this implies that ionized plasmas are a common feature of LMXBs.
Published: 02 March 2006
The light from historical supernovae could in principle still be visible as scattered-light echoes centuries after the explosion. The detection of light echoes could allow us to pinpoint the supernova event both in position and age and, most importantly, permit the acquisition of spectra to determine the 'type' of the supernova centuries after the direct light from the explosion first reached Earth. Although echoes have been discovered around some nearby extragalactic supernovae, targeted searches have not found any echoes in the regions of historical Galactic supernovae. Here we report three faint variable-surface-brightness complexes with high apparent proper motions pointing back to three of the six smallest (and probably youngest) previously catalogued supernova remnants in the Large Magellanic Cloud, which are believed to have been thermonuclear (type Ia) supernovae. Using the distance and apparent proper motions of these echo arcs, we estimate ages of 610 and 410 years for two of them.
Published: 22 December 2005
8-Dec-2021 06:48 UT

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