arXiv subjects

F. Haberl

Publications and source records attributed to F. Haberl.

At least 181 records · Page 10Linked to original sources

arXiv2009

Recent discoveries of supersoft X-ray sources in M 31

Classical novae (CNe) have recently been reported to represent the major class of supersoft X-ray sources (SSSs) in the central area of our neighbouring galaxy M 31. This paper presents a review of results from recent X-ray observations of M 31 with XMM-Newton and Chandra. We carried out a dedicated optical and X-ray monitoring program of CNe and SSSs in the central area of M 31. We discovered the first SSSs in M 31 globular clusters (GCs) and their connection to the very first discovered CN in a M 31 GC. This result may have an impact on the CN rate in GCs. Furthermore, in our optical and X-ray monitoring data we discovered the CN M31N 2007-11a, which shows a very short SSS phase of 29 - 52 days. Short SSS states (durations < 100 days) of CNe indicate massive white dwarfs (WDs) that are candidate progenitors of supernovae type Ia. In the case of M31N 2007-11a, the optical and X-ray light curves suggest a binary containing a WD with M_WD > 1.0 M_sun. Finally, we present the discovery of the SSS counterpart of the CN M31N 2006-04a. The X-ray light curve of M31N 2006-04a shows short-time variability, which might indicate an orbital period of about 2 hours.

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arXiv2009

Supersoft sources in M 31: Comparing the XMM-Newton Deep Survey, ROSAT and Chandra catalogues

To investigate the transient nature of supersoft sources (SSSs) in M 31, we compared SSS candidates of the XMM-Newton Deep Survey, ROSAT PSPC surveys and the Chandra catalogues in the same field. We found 40 SSSs in the XMM-Newton observations. While 12 of the XMM-Newton sources were brighter than the limiting flux of the ROSAT PSPC survey, only two were detected with ROSAT ~10 yr earlier. Five correlate with recent optical novae which explains why they were not detected by ROSAT. The remaining 28 XMM-Newton SSSs have fluxes below the ROSAT detection threshold. Nevertheless we found one correlation with a ROSAT source, which had significantly larger fluxes than during the XMM-Newton observations. Ten of the XMM-Newton SSSs were detected by Chandra with <1-~6 yr between the observations. Five were also classified as SSSs by Chandra. Of the 30 ROSAT SSSs three were confirmed with XMM-Newton, while for 11 sources other classifications are suggested. Of the remaining 16 sources one correlates with an optical nova. Of the 42 Chandra very-soft sources five are classified as XMM-Newton SSSs, while for 22 we suggest other classifications. Of the remaining 15 sources, nine are classified as transient by Chandra, one of them correlates with an optical nova. These findings underlined the high variability of the sources of this class and the connection between SSSs and optical novae. Only three sources, were detected by all three missions as SSSs. Thus they are visible for more than a decade, despite their variability.

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arXiv2009

Chandra ACIS Survey of M33 (ChASeM33): The enigmatic X-ray emission from IC131

We present the first X-ray analysis of the diffuse hot ionized gas and the point sources in IC131, after NGC604 the second most X-ray luminous giant HII region in M33. The X-ray emission is detected only in the south eastern part of IC131 (named IC131-se) and is limited to an elliptical region of ~200pc in extent. This region appears to be confined towards the west by a hemispherical shell of warm ionized gas and only fills about half that volume. Although the corresponding X-ray spectrum has 1215 counts, it cannot conclusively be told whether the extended X-ray emission is thermal, non-thermal, or a combination of both. A thermal plasma model of kT_e=4.3keV or a single power law of Gamma=2.1 fit the spectrum equally well. If the spectrum is purely thermal (non-thermal), the total unabsorbed X-ray luminosity in the 0.35-8keV energy band amounts to L_X = 6.8(8.7)x10^35erg/s. Among other known HII regions IC131-se seems to be extreme regarding the combination of its large extent of the X-ray plasma, the lack of massive O stars, its unusually high electron temperature (if thermal), and the large fraction of L_X emitted above 2keV (~40-53%). A thermal plasma of ~4keV poses serious challenges to theoretical models, as it is not clear how high electron temperatures can be produced in HII regions in view of mass-proportional and collisionless heating. If the gas is non-thermal or has non-thermal contributions, synchrotron emission would clearly dominate over inverse Compton emission. It is not clear if the same mechanisms which create non-thermal X-rays or accelerate CRs in SNRs can be applied to much larger scales of 200pc. In both cases the existing theoretical models for giant HII regions and superbubbles do not explain the hardness and extent of the X-ray emission in IC131-se.

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arXiv2009

VLT optical observations of the isolated neutron star RX J0420.0-5022

X-ray observations performed with ROSAT led to the discovery of 7 radio-silent isolated neutron stars (INSs) which are detected only through the relatively dim and purely thermal X-ray emission from the cooling star surface. A few of these INSs (a.k.a. X-ray Dim INSs, or XDINSs) have been also detected at optical wavelengths where they seem to feature thermal spectra. Optical studies of XDINSs thus play a crucial role in mapping the temperature distribution on the neutron star surface and in investigating the existence of an atmosphere around the neutron star. The aim of this work is to investigate the optical identification of the XDINS RX J0420.0-5022, tentatively proposed by Haberl et al. (2004) based on VLT observations. We re-analysed the original observations of Haberl et al. to assess the detection significance of the proposed counterpart and we performed deeper VLT observations aiming at a higher confidence detection. With a ~2 sigma detection significance and a re-computed flux of B=27.52+/-0.61, we can not rule out that the proposed counterpart was spurious and produced by the halo of a very bright nearby star. While we could not detect the proposed counterpart in our deeper VLT observations, we found evidence for a marginally significant (~3.9 sigma) detection of a similarly faint object (B= 27.5+/-0.3), ~ 0.5" north of it and coincident with the updated Chandra position of RX J0420.0-5022. Interestingly, the angular separation is consistent with the upper limit on the RX J0420.0-5022 proper motion (Motch et al. 2009), which suggests that we might have actually detected the Haberl et al. proposed counterpart. From the flux of the putative RX J0420.0-5022 counterpart we can rule out a > 7 optical excess with respect to the extrapolation of the XMM-Newton spectrum.

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arXiv2009

The first two transient supersoft X-ray sources in M 31 globular clusters and the connection to classical novae

Classical novae (CNe) have been found to represent the major class of supersoft X-ray sources (SSS) in our neighbour galaxy M 31. We determine properties and evolution of the two first SSSs ever discovered in the M 31 globular cluster (GC) system. We have used XMM-Newton, Chandra and Swift observations of the centre region of M 31 to discover both SSS and to determine their X-ray light curves and spectra. We performed detailed analysis of XMM-Newton EPIC PN spectra of the source in Bol 111 (SS1) using blackbody and NLTE white dwarf (WD) atmosphere models. For the SSS in Bol 194 (SS2) we used optical monitoring data to search for an optical counterpart. Both GC X-ray sources were classified as SSS. We identify SS1 with the CN M31N 2007-06b recently discovered in the M 31 GC Bol 111. For SS2 we did not find evidence for a recent nova outburst and can only provide useful constraints on the time of the outburst of a hypothetical nova. The only known CN in a M 31 GC can be identified with the first SSS found in a M31 GC. We discuss the impact of our observations on the nova rate for the M 31 GC system.

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arXiv2009

The very short supersoft X-ray state of the classical nova M31N 2007-11a

Short supersoft X-ray source (SSS) states (durations < 100 days) of classical novae (CNe) indicate massive white dwarfs that are candidate progenitors of supernovae type Ia. We carry out a dedicated optical and X-ray monitoring program of CNe in the bulge of M 31. We discovered M31N 2007-11a and determined its optical and X-ray light curve. We used the robotic Super-LOTIS telescope to obtain the optical data and XMM-Newton and Chandra observations to discover an X-ray counterpart to that nova. Nova M31N 2007-11a is a very fast CN, exhibiting a very short SSS state with an appearance time of 6-16 days after outburst and a turn-off time of 45-58 days after outburst. The optical and X-ray light curves of M31N 2007-11a suggest a binary containing a white dwarf with a mass greater than one solar mass.

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arXiv2009

Spectral and temporal variations of the isolated neutron star RX J0720.4-3125: new XMM-Newton observations

In the past, the isolated, radio-quiet neutron star RX J0720.4-3125 showed variations in the spectral parameters (apparent radius, temperature of the emitting area and equivalent width of the absorption feature) seen in the X-ray spectra, not only during the spin period of 8.39s, but also over time scales of years. New X-ray observations of RX J0720.4-3125 with XMM Newton extend the coverage to about 7.5 years with the latest pointing performed in November 2007. Out of a total of fourteen available EPIC-pn datasets, eleven have been obtained with an identical instrumental setup (full frame read-out mode with thin filter), and are best suited for a comparative investigations of the spectral and timing properties of this enigmatic X-ray pulsar. We analysed the new XMM Newton observations together with archival data in order to follow the spectral and temporal evolution of RX J0720.4-3125 All XMM-Newton data were reduced with the standard XMM-SAS software package. A systematic and consistent data reduction of all these observations was emphasised in order to reduce systematic errors as far as possible. We investigate the phase residuals derived from data from different energy bands using different timing solutions for the spin period evolution and confirm the phase lag between hard and soft photons. The phase shift in the X-ray pulses between hard and soft photons varies with time and changes sign around MJD=52800 days, regardless of the chosen timing solution. The phase residuals[abridge]

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arXiv2009

XMM-Newton RGS spectrum of RX J0720.4-3125: An absorption feature at 0.57 keV

We found a narrow absorption feature at 0.57keV in the co-added RGS spectrum of the isolated neutron star RX J0720.4-3125 with an equivalent width of 1.35+/-0.3eV and FWHM ~6.0eV. The feature was identified with an absorption line of highly ionized oxygen OVII, most probably originating in the ambient medium of RX J0720.4-3125. An extensive investigation with the photo-ionization code CLOUDY indicates the possibility that the optical flux excess observed in the spectrum of RX J0720.4-3125 at least partially originates in a relatively dense (e.g. nH~10^8 cm^-3) slab, located in the vicinity of the neutron star (e.g. ~10^10cm).

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arXiv2009

Proper motions of thermally emitting isolated neutron stars measured with Chandra

The remarkable astrometric capabilities of Chandra offer the possibility to measure proper motions of X-ray sources with an unprecedented accuracy in this wavelength range. We recently completed a proper motion survey of three of the seven thermally emitting radio-quiet isolated neutron stars (INSs) discovered in the ROSAT all-sky survey. These INSs (RX J0420.0-5022, RX J0806.4-4123, and RX J1308.6+2127) either lack an optical counterpart or have one so faint that ground based or space born optical observations push the current possibilities of the instrumentation to the limit. Pairs of ACIS observations were acquired 3 to 5 years apart to measure the displacement of the sources on the X-ray sky using as reference the background of extragalactic or remote Galactic X-ray sources. We derive 2 sigma upper limits of 123 mas/yr and 86 mas/yr on the proper motion of RX J0420.0-5022 and RX J0806.4-4123, respectively. RX J1308.6+2127 exhibits a very significant displacement (~ 9 sigma) yielding mu = 220 +/- 25 mas/yr, the second fastest measured among all ROSAT discovered INSs. The source is probably moving away rapidly from the Galactic plane at a speed which precludes any significant accretion of matter from the interstellar medium. Its transverse velocity of ~ 740 (d/700pc) km/s might be the largest of all ROSAT INSs and its corresponding spatial velocity stands among the fastest recorded for neutron stars. RX J1308.6+2127 is thus a middle-aged (age ~ 1 My) high velocity cooling neutron star. We investigate its possible origin in nearby OB associations or from a field OB star. In most cases, the flight time from birth place appears significantly shorter than the characteristic age derived from spin down rate. The distribution in transverse velocity of ROSAT INSs is not statistically different from that of normal radio pulsars.

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arXiv2008

Detection of the second eclipsing high mass X-ray binary in M 33

Chandra data of the X-ray source [PMH2004] 47 were obtained in the ACIS Survey of M 33 (ChASeM33) in 2006. During one of the observations, the source varied from a high state to a low state and back, in two other observations it varied from a low state to respectively intermediate states. These transitions are interpreted as eclipse ingress and egresses of a compact object in a high mass X-ray binary system. The phase of mid eclipse is given by HJD 2453997.476+-0.006, the eclipse half angle is 30.6+-1.2 degree. Adding XMM-Newton observations of [PMH2004] 47 in 2001 we determine the binary period to be 1.732479+-0.000027 d. This period is also consistent with ROSAT HRI observations of the source in 1994. No short term periodicity compatible with a rotation period of the compact object is detected. There are indications for a long term variability similar to that detected for Her X-1. During the high state the spectrum of the source is hard (power law spectrum with photon index ~0.85) with an unabsorbed luminosity of 2E37 erg/cm2/s (0.2-4.5 keV). We identify as an optical counterpart a V ~ 21.0mag star with T_eff > 19000 K, log(g) > 2.5. CFHT optical light curves for this star show an ellipsoidal variation with the same period as the X-ray light curve. The optical light curve together with the X-ray eclipse can be modeled by a compact object with a mass consistent with a neutron star or a black hole in a high mass X-ray binary. However, the hard power law X-ray spectrum favors a neutron star as the compact object in this second eclipsing X-ray binary in M 33. Assuming a neutron star with a canonical mass of 1.4 M_sun and the best fit companion temperature of 33000 K, a system inclination i = 72 degree and a companion mass of 10.9 M_sun are implied.

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arXiv2008

Searching for substellar companions of young isolated neutron stars

Context: Only two planetary systems around old ms-pulsars are currently known. Young radio pulsars and radio-quiet neutron stars cannot be covered by the usually-applied radio pulse timing technique. However, finding substellar companions around these neutron stars would be of great interest -- not only because of the companion's possible exotic formation but also due to the potential access to neutron star physics. Aims: We investigate the closest young neutron stars to search for substellar companions around them. Methods: Young, thus warm substellar companions are visible in the Near Infrared while the neutron star itself is much fainter. Four young neutron stars are moving fast enough to enable a common proper motion search for substellar companions within few years. Results. For Geminga, RX J0720.4-3125, RX J1856.6-3754, and PSR J1932+1059 we did not find any co-moving companion down to 12, 15, 11, 42 Jupiter masses for assumed ages of 1, 1, 1, 3.1 Myrs and distances of 250, 361, 167, 361 pc, respectively. Near Infrared limits are presented for these four as well as five other neutron stars for which we currently have only observations at one epoch. Conclusions: We conclude that young isolated neutron stars rarely have brown dwarf companions.

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arXiv2008

High mass X-ray binary SXP18.3 undergoes the longest Type II outburst ever seen in the Small Magellanic Cloud

On 30th August 2006 SXP18.3 a high mass X-ray binary (HMXB) in the Small Magellanic Cloud (SMC) with an 18.3s pulse period was observed by Rossi X-ray Timing Explorer (RXTE). The source was seen continuously for the following 36 weeks. This is the longest Type II outburst ever seen from a HMXB in the SMC. During the outburst SXP18.3 was located from serendipitous XMM-Newton observations. The identification of the optical counterpart has allowed SXP18.3 to be classified as a Be/X-ray binary. This paper will report on the analysis of the optical and weekly RXTE X-ray data that span the last 10 years. The extreme length of this outburst has for the first time enabled us to perform an extensive study of the pulse timing of an SMC Be/X-ray binary. We present a possible full orbital solution from the pulse timing data. An orbital period of 17.79d is proposed from the analysis of the OGLE III light curve placing SXP18.3 on the boundary of known sources in the Corbet diagram.

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arXiv2008

New XMM-Newton analysis of three bright X-ray sources in M31 globular clusters, including a new black hole candidate

We present detailed analysis of three globular cluster X-ray sources in the XMM-Newton extended survey of M31. The X-ray counterpart to the M31 globular cluster Bo 45 (XBo 45) was observed with XMM-Newton on 2006 December 26. Its combined pn+MOS 0.3--10 keV lightcurve exhibited a r.m.s variability of ~10%, and its 0.3--7.0 keV emission spectrum was well described by an absorbed power law with photon index 1.44$\pm$0.12. Its variability and emission is characteristic of low mass X-ray binaries (LMXBs) in the low-hard state, whether the accretor is a neutron star or black hole. Such behaviour is typically observed at luminosities $\la$10% Eddington. However, XBo 45 exhibited this behaviour at an unabsorbed, 0.3--10 keV luminosity of 2.5$\pm0.2\times 10^{38}$ erg s$^{-1}$, or{~140%} Eddington for a 1.4 $M_{\odot}$ neutron star accreting hydrogen. Hence, we identify XBo 45 as a new candidate black hole LMXB. XBo 45 appears to have been consistently bright for ~30 years, consistent with theoretical prediction for a globular cluster black hole binary formed via tidal capture. Bo 375 was observed in the 2007, January 2 XMM-Newton observation, and has a two-component spectrum that is typical for a bright neutron star LMXB. Bo 135 was observed in the same field as Bo 45, and could contain either a black hole or neutron star.

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arXiv2008

Three X-ray transients in M31 observed with Swift

The purpose of this study is to find transient X-ray sources in M31, and to investigate and classify their nature. Three X-ray transients were observed with Swift. For each of the three X-ray transients we use the Swift X-ray and optical data together with observations from XMM-Newton and Chandra to investigate the lightcurves and the spectra of the outburst, and thereby to identify the source types. The outburst of XMMU J004215.8+411924 lasted for about one month. The source had a hard power-law spectrum with a photon index of 1.6. It was previously identified as a Be/X-ray binary based on the optical identification with a star. However, we show that with improved source coordinates it is clear that the optical source is not the counterpart to the X-ray source. The source SWIFT J004217.3+411532 had a bright outburst, after which it slowly decayed over half a year. The spectrum was soft, corresponding to a thermal accretion disk with innermost temperature of 250-600 eV. The source was not seen in the optical, and the soft spectrum indicates that the source is most likely a black hole low mass X-ray binary. M31N 2006-11a is a nova that was previously observed in the optical. We detected it both in X-rays and UV with Swift half a year after the optical maximum, after which it decayed below the Swift detection threshold within a month. The spectrum of the X-ray transient can be modelled by a black-body with a temperature of 50 eV. We use catalogues of X-ray transients in M31 to estimate their rate, and we find a lower limit of 9/yr.

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arXiv2008

XMM-Newton observations of the Small Magellanic Cloud: Be/X-ray binary pulsars active between October 2006 and June 2007

(abridged) We analysed eight XMM-Newton observations toward the Small Magellanic Cloud (SMC), performed between October 2006 and June 2007, to investigate high mass X-ray binary systems. We found new X-ray binary pulsars with periods of 202 s (XMMU J005929.0-723703), 342 s (XMMU J005403.8-722632), 645 s (XMMU J005535.2-722906) and 325 s (XMMU J005252.1-721715), in the latter case confirming the independent discovery in Chandra data. In addition we detected sixteen known Be/X-ray binary pulsars and six ROSAT-classified candidate high mass X-ray binaries. From one of the candidates, RX J0058.2-7231, we discovered X-ray pulsations with a period of 291 s which makes it the likely counterpart of XTE J0051-727. From the known pulsars, we revise the pulse period of CXOU J010206.6-714115 to 967 s, and we detected the 18.37 s pulsar XTE J0055-727 (= XMM J004911.4-724939) in outburst, which allowed us to localise the source. The pulse profiles of the X-ray pulsars show a large variety of shapes from smooth to highly structured patterns and differing energy dependence. For all the candidate high mass X-ray binaries optical counterparts can be identified with magnitudes and colours consistent with Be stars. Twenty of the Be/X-ray binaries were detected with X-ray luminosities in the range 1.5x10^35 erg/s - 5.5x10^36 erg/s. The majority of the spectra is well represented by an absorbed power-law with an average power-law index of 0.93. The absorption (in addition to the Galactic foreground value) varies over a wide range between a few 10^20 H cm^-2 and several 10^22 H cm^-2. An overall correlation of the absorption with the total SMC HI column density suggests that the absorption seen in the X-ray spectra is often largely caused by interstellar gas.

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arXiv2008

XMM-Newton Observations of Radio Pulsars B0834+06 and B0826-34 and Implications for Pulsar Inner Accelerator

We report the X-ray observations of two radio pulsars with drifting subpulses: B0834 + 06 and B0826 - 34 using \xmm\. PSR B0834 + 06 was detected with a total of 70 counts from the three EPIC instruments over 50 ks exposure time. Its spectrum was best described as that of a blackbody (BB) with temperature $T_s=(2.0^{+2.0}_{-0.9}) \times 10^6$ K and bolometric luminosity of $L_b=(8.6^{+14.2}_{-4.4}) \times 10^{28}$ erg s$^{-1}$. As it is typical in pulsars with BB thermal components in their X-ray spectra, the hot spot surface area is much smaller than that of the canonical polar cap, implying a non-dipolar surface magnetic field much stronger than the dipolar component derived from the pulsar spin-down (in this case about 50 times smaller and stronger, respectively). The second pulsar PSR B0826 - 34 was not detected over 50 ks exposure time, giving an upper limit for the bolometric luminosity $L_b \leq 1.4 \times 10^{29}$ erg s$^{-1}$. We use these data as well as the radio emission data concerned with drifting subpulses to test the Partially Screened Gap (PSG) model of the inner accelerator in pulsars.

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arXiv2008

New XMM-Newton observations of SNRs in the SMC

A complete overview of the supernova remnant (SNR) population is required to investigate their evolution and interaction with the surrounding interstellar medium in the Small Magellanic Cloud (SMC). Recent XMM-Newton observations of the SMC cover three known SNRs (DEM S5, SNR B0050-72.8, and SNR B0058-71.8), which are poorly studied and are X-ray faint. We used new multi-frequency radio-continuum surveys and new optical observations at Ha, [SII], and [OIII] wavelengths, in combination with the X-ray data, to investigate their properties and to search for new SNRs in the SMC. We used X-ray source selection criteria and found one SMC object with typical SNR characteristics (HFPK 334), that was initially detected by ROSAT. We analysed the X-ray spectra and present multi-wavelength morphological studies of the three SNRs and the new candidate. Using a non-equilibrium ionisation collisional plasma model, we find temperatures kT around 0.18 keV for the three known remnants and 0.69 keV for the candidate. The low temperature, low surface brightness, and large extent of the three remnants indicates relatively large ages. The emission from the new candidate (HFPK 334) is more centrally peaked and the higher temperature suggests a younger remnant. Our new radio images indicate that a pulsar wind nebulae (PWN) is possibly associated with this object. The SNRs known in the SMC show a variety of morphological structures that are relatively uncorrelated in the different wavelength bands, probably caused by the different conditions in the surrounding medium with which the remnant interacts.

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arXiv2008

An Optical Counterpart Candidate for the Isolated Neutron Star RBS1774

Multiwavelength studies of the seven identified X-ray dim isolated neutron stars (XDINSs) offer a unique opportunity to investigate their surface thermal and magnetic structure and the matter-radiation interaction in presence of strong gravitational and magnetic fields. As a part of an ongoing campaign aimed at a complete identification and spectral characterization of XDINSs in the optical band, we performed deep imaging with the ESO Very Large Telescope (VLT) of the field of the XDINS RBS1774 (1RXS J214303.7 +065419). The recently upgraded FORS1 instrument mounted on the VLT provided the very first detection of a candidate optical counterpart in the B band. The identification is based on a very good positional coincidence with the X-ray source (chance probability ~2E-3). The source has B=27.4 +/- 0.2 (1 sigma confidence level), and the optical flux exceeds the extrapolation of the X-ray blackbody at optical wavelengths by a factor ~35 (+/- 20 at 3sigma confidence level). This is barely compatible with thermal emission from the neutron star surface, unless the source distance is d~200-300 pc, and the star is an almost aligned rotator or its spin axis is nearly aligned with the line of sight. At the same time, such a large optical excess appears difficult to reconcile with rotation-powered magnetospheric emission, unless the source has an extremely large optical emission efficiency. The implications and possible similarities with the optical spectra of other isolated NSs are discussed.

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