arXiv subjects

Manfred W. Pakull

Publications and source records attributed to Manfred W. Pakull.

13 recordsLinked to original sources

arXiv2026

A Multiwavelength Assessment Disfavoring the X-ray Binary Origin of He III Regions in Metal-Poor Star-Forming Dwarf Galaxies

Recent observations of metal-poor, star-forming dwarf galaxies reveal He III regions, traced by nebular He II 4686 emission that require a strong source of extreme-ultraviolet (EUV) radiation. The origin of this hard ionizing radiation remains poorly understood, as standard stellar populations fail to account for it, posing key implications for the understanding of early galaxy formation. We present a systematic Chandra X-ray study of 21 nearby star-forming galaxies with He II emission but lacking Wolf-Rayet spectral signatures. Using 7 new and 36 archival Chandra X-ray observations combined with optical stellar population synthesis modelling, we constrain the ionizing continuum required to sustain the observed He II line, the ionizing continuum available from X-ray objects, and the properties of the host H II regions. We find that the inferred EUV output from accreting X-ray sources in our sample is systematically lower than what is required to produce the observed He II emission. Our sample is consistent with established empirical scaling relations for X-ray luminosity, indicating that this discrepancy cannot be attributed to an anomalously low number or luminosity of X-ray sources. These results indicate that accreting X-ray sources alone cannot account for the observed He II-ionizing photon budget, pointing to additional or alternative sources of hard EUV radiation in metal-poor star-forming environments. Potential alternative or additional contributors are discussed.

astro-ph.HE↗

arXiv2024

A multiband look at ultraluminous X-ray sources in NGC 7424

We studied the multiband properties of two ultraluminous X-ray sources (2CXO J225728.9-410211 = X-1 and 2CXO J225724.7-410343 = X-2) and their surroundings, in the spiral galaxy NGC 7424. Both sources have approached X-ray luminosities L_{X} ~ 10^{40} erg/s at some epochs. Thanks to a more accurate astrometric solution (based on Australia Telescope Compact Array and Gaia data), we identified the point-like optical counterpart of X-1, which looks like an isolated B8 supergiant (M ~ 9 Msun, age ~ 30 Myr). Instead, X-2 is in a star-forming region (size of about 100 pc x 150 pc), near young clusters and ionized gas. Very Large Telescope long-slit spectra show a spatially extended region of HeII 4686 emission around the X-ray position, displaced by about 50 pc from the brightest star cluster, which corresponds to the peak of lower-ionization line emission. We interpret the HeII 4686 emission as a signature of X-ray photo-ionization from the ULX, while the other optical lines are consistent with UV ionization in an ordinary HeII region. The luminosity of this He^{++} nebula puts it in the same class as other classical photo-ionized ULX nebulae such as those around Holmberg II X-1 and NGC 5408 X-1. We locate a strong (5.5-GHz luminosity nu L_{nu} ~ 10^{35} erg/s), steep-spectrum, unresolved radio source at the peak of the low-ionization lines, and discuss alternative physical scenarios for the radio emission. Finally, we use WISE data to obtain an independent estimate of the reddening of the star-forming clump around X-2.

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arXiv2021

X-raying the galaxy pair Arp 41: no collision in NGC 1232 and three ultraluminous sources in NGC 1232A

We studied the apparent galaxy pair NGC 1232 / NGC 1232A with Chandra, looking for evidence of interactions and collisions. We report that there is no cloud of diffuse emission in NGC 1232, contrary to previous claims in the literature. Instead, we find that the small "companion" galaxy NGC 1232A contains three ultraluminous X-ray sources (ULXs) with peak 0.3-10 keV luminosities above 10^{40} erg/s (assuming a cosmological distance of 93 Mpc for this galaxy). For its mass, morphology, metal abundance and bright ULX population, NGC 1232A is analogous to the more nearby late-type spiral NGC 1313.

astro-ph.GA↗

arXiv2013

Kinematics of the intermediate mass black hole candidate HLX-1

We studied the optical spectrum of HLX-1 during its latest outburst, using the FORS2 spectrograph on the Very Large Telescope. We detect an Halpha emission line centered at lambda = (6718.9 +/- 0.9) Ang and find that its projected radial velocity with respect to the nucleus of ESO243-49 is (424 +/- 27) km/s, while the maximum rotational velocity of the stars in that galaxy is ~209 km/s. This suggests that HLX-1 and its surrounding stars were not formed in situ, but came either from a disrupted dwarf galaxy or from a nuclear recoil. We also find that the Halpha emission line is resolved with full width at half maximum ~400 km/s, suggesting a nebular rather than disk origin for the emission. Its luminosity (L_{Halpha} ~ a few 10^{37} erg/s, equivalent width ~70 Ang) is also consistent with emission from a nebula photo-ionized by HLX-1.

astro-ph.CO↗

arXiv2011

Optical Properties of the Ultraluminous X-ray Source Holmberg IX X-1 and its Stellar Environment

Holmberg IX X-1 is an archetypal ultraluminous X-ray source (ULX). Here we study the properties of the optical counterpart and of its stellar environment using optical data from SUBARU/Faint Object Camera and Spectrograph,GEMINI/GMOS-N and Hubble Space Telescope (HST)/Advanced Camera for Surveys, as well as simultaneous Chandra X-ray data. The V ~ 22.6 spectroscopically identified optical counterpart is part of a loose cluster with an age <~ 20 Myr. Consequently, the mass upper limit on individual stars in the association is about 20 M_sun. The counterpart is more luminous than the other stars of the association, suggesting a non-negligible optical contribution from the accretion disk. An observed UV excess also points to non-stellar light similar to X-ray active low-mass X-ray binaries. A broad HeII4686 emission line identified in the optical spectrum of the ULX further suggests optical light from X-ray reprocessing in the accretion disk. Using stellar evolutionary tracks, we have constrained the mass of the counterpart to be >~ 10 M_sun, even if the accretion disk contributes significantly to the optical luminosity. Comparison of the photometric properties of the counterpart with binary models show that the donor may be more massive, >~ 25 M_sun, with the ULX system likely undergoing case AB mass transfer. Finally, the counterpart exhibits photometric variability of 0.14 mag between two HST observations separated by 50 days which could be due to ellipsoidal variations and/or disk reprocessing of variable X-ray emission.

astro-ph.HE↗

arXiv2010

Radio lobes and X-ray hot spots in the microquasar S26

We have studied the structure and energetics of the powerful microquasar/shock-ionized nebula S26 in NGC 7793, with particular focus on its radio and X-ray properties. Using the Australia Telescope Compact Array, we have resolved for the first time the radio lobe structure and mapped the spectral index of the radio cocoon. The steep spectral index of the radio lobes is consistent with optically-thin synchrotron emission; outside the lobes, the spectral index is flatter, suggesting an additional contribution from free-free emission, and perhaps ongoing ejections near the core. The radio core is not detected, while the X-ray core has a 0.3-8 keV luminosity ~6 x 10^{36} erg/s. The size of the radio cocoon matches that seen in the optical emission lines and diffuse soft X-ray emission. The total 5.5-GHz flux of cocoon and lobes is ~2.1 mJy, which at the assumed distance of 3.9 Mpc corresponds to about 3 times the luminosity of Cas A. The total 9.0-GHz flux is ~1.6 mJy. The X-ray hot spots (combined 0.3-8 keV luminosity ~2 x 10^{37} erg/s) are located ~20 pc outwards of the radio hot spots (ie, downstream along the jet direction), consistent with a different physical origin of X-ray and radio emission (thermal-plasma and synchrotron, respectively). The total particle energy in the bubble is ~10^{53} erg: from the observed radio flux, we estimate that only about a few 10^{50} erg are stored in the relativistic electrons; the rest is in protons, nuclei and non-relativistic electrons. The X-ray-emitting component of the gas in the hot spots contains ~10^{51} erg, and ~10^{52} erg over the whole cocoon. We suggest that S26 provides a clue to understand how the ambient medium is heated by the mechanical power of a black hole near its Eddington accretion rate.

astro-ph.HE↗

arXiv2009

High-excitation nebulae around Magellanic Wolf-Rayet stars

The SMC harbours a class of hot nitrogen-sequence Wolf-Rayet stars (WNE) that display only relatively weak broad emission lines. This indicates low mass-loss rates and makes them also hard to detect. However, such stars are possible emitters of strong He+ Lyman continua that in turn could ionize observable HeIII regions, i.e. highly excited HII regions emitting nebular HeII4686 emission. We here report the discovery of a rare HeIII region in the SMC which is located in the OB association NGC249 around the weak-lined WN star SMC WR10. WR10 is particularly interesting since it is a single star showing the presence of atmospheric hydrogen. While analysing the spectrum in the framework of two popular WR atmosphere models, we found for the same input parameters strongly discrepant predictions (by 1 dex) for the He+ Lyman continuum. A second aspect of the work reported here concerns the beautiful MCELS images which clearly reveal a class of strongly [OIII]5007 emitting (blue-coded) nebulae. Not unexpectedly, most of the 'blue' nebulae are known Wolf-Rayet bubbles, but new bubbles around a few WRs are also detected. Moreover, we report the existence of blue nebulae without associated known WRs and discuss the possibility that they reveal weak-wind WR stars with very faint stellar HeII4686 emission. Alternatively, such nebulae might hint at the hitherto missing population of relatively low-mass, hot He stars predicted by massive binary evolution calculations. Such a binary system is probably responsible for the ionization of the unique HeII4686-emitting nebula N44C.

astro-ph.GA↗

arXiv2009

The ULX NGC 1313 X-2 : an optical study revealing an interesting behavior

We present a summary of our ongoing efforts to study one of the brightest ultraluminous X-ray source, NGC 1313 X-2. Despite a large coverage in the X-rays, much of the information we have about the source and its environment comes from optical wavelenghts. Here, we report on the properties of the stellar environment, and the differences in the optical counterpart between our two observing epochs (2003--2004 and 2007--2008). We summarize our ongoing program designed to look for radial velocity variations in the optical spectra and for photometric variability.

astro-ph.HE↗

arXiv2008

Ultraluminous X-ray Sources: Beambags and Optical Counterparts

A significant fraction of ultraluminous X-ray sources appear to be embedded in observable ionized nebulae that take the form of large, several 100 pc diameter interstellar bubbles. Here we review optical observations of these bubbles, their importance for our understanding of the nature of ULXs, the energetics involved and their formation and evolution. Among the results obtained are new arguments against conventional superbubble scenarios and hypernova remnants, and we present the case in favour of ULX-wind/jet driven bubbles. We report the discovery of new ULXs in very large SNR candidates in nearby galaxies, and finally present an image of a triple X-ray source coincident with the radio-bright bubble S26 in the galaxy NGC 7793 which appears to be a clone of the microquasar SS433/W50 system.

astro-ph↗

arXiv2006

The Ultraluminous X-ray Source IC342 X-1 and its Environment

We present optical observations of a ULX in the nearby spiral galaxy IC 342. This variable source has an average X-ray luminosity of some 10^{40} erg/s. At the position of the source there is an ionized nebula (the "Tooth") having huge dimensions (280 x 130 pc), much larger than normal supernova remnants. Our optical spectra reveal highly supersonic expansion velocities and emission line ratios typical of SNRs. It has been claimed that two [OIII] 5007 bright regions in the nebula might be indicative of excitation by non-isotropic emission from the ULX. However, our continuum subtracted [OIII] 5007 image reveals that O^{++} ions are rather smoothly distributed in the nebula, fully consistent with shock excitation. Within the nebula we find two candidate stars (V=24.0 & 24.6) in the Chandra X-ray error circle. Both are substantially reddened being consistent with the patchy interstellar absorption. We will discuss the nature of this source in the framework of what is currently known about optical counterparts of ULXs.

astro-ph↗

arXiv2006

Ultraluminous X-ray Sources: Bubbles and Optical Counterparts

Optical studies of ultraluminous X-ray sources (ULX) in nearby galaxies have turned out to be instrumental in discriminating between various models including the much advertised intermediate mass black hole hypothesis and various beaming scenarios. Here we report on ESO VLT and SUBARU observations of ULX that have revealed the parent stellar clusters with ages of some 60 million years in two cases. Thus we are able to derive upper limits of about 8 M_sun for the mass donors in these systems. The optical counterparts are dominated by X-ray heated accretion disks, and the discovery of the HeII4686 emission line now allows to derive dynamical masses in these systems. Apparent radial velocity variations of 300 km/s have been detected in NGC 1313 X-2 which, if confirmed by further observations, would exclude the presence of IMBH in these systems.

astro-ph↗

arXiv2006

The Ultraluminous X-ray Source in Holmberg IX and its Environment

We present optical observations of an ultraluminous X-ray source (ULX) in Holmberg IX, a dwarf galaxy near M81. The ULX has an average X-ray luminosity of some 10^{40} erg/s. It is located in a huge (400pc x 300pc) ionized nebula being much larger than normal supernova remnants. From the observed emission lines (widths and ratios) we find that the structure is due to collisional excitation by shocks, rather than by photoionization. We identify the optical counterpart to be a 22.8 mag blue star (M_V=-5.0) belonging to a small stellar cluster. From isochrone fitting of our multi-colour photometry we determine a cluster age of 20 to 50 Myr. We also discovered strong stellar HeII4686 emission (equivalent width of 10 A) which proves the identification with the X-ray source, and which suggests the presence of an X-ray heated accretion disc around the putative black hole.

astro-ph↗

arXiv2002

Optical Counterparts of Ultraluminous X-Ray Sources

Despite much observational and theoretical effort little is presently known about the nature of the luminous non-nuclear X-ray sources which appear to largely surpass the Eddington limit of a few solar masses. Here we present first results of our OHP/ESO/CFHT optical survey of the environments of variable ultraluminous X-ray sources (ULX) in nearby galaxies. At the position of several ULX we find emission nebulae of a few hundred parsecs diameter, and which often show both low and high ionisation emission lines. The gas must therefore be either photoionized by hard XUV continua, or be shock-ionized in the expanding bubbles. The nebulae have kinematic ages of some million years and appear to be directly linked to the highly energetic formation process of the compact ULX or being inflated by ongoing stellar wind/jet activity. The discovery of intense HeII lambda4686 nebular recombination radiation together with comparatively strong [OI] lambda6300 emission around the variable ULX in dwarf galaxy Holmberg II has allowed us to show that the interstellar medium acually 'sees' and reprocesses part of the 10^40 erg/s measured at X-ray wavelengths, if we assume isotropic emission. Strong beaming into our line of sight which has been advocated to avoid such high luminosities can thus be excluded, at least for this source.

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