X-ray emission from hot subdwarfs with compact companions

We review the X-ray observations of hot subdwarf stars. While no X-ray emission has been detected yet from binaries containing B-type subdwarfs, interesting results have been obtained in the case of the two luminous O-type subdwarfs HD 49798 and BD +37 442. Both of them are members of binary systems in which the X-ray luminosity is powered by accretion onto a compact object: a rapidly spinning (13.2 s) and massive (1.28 M_sun) white dwarf in the case of HD 49798 and most likely a neutron star, spinning at 19.2 s, in the case of BD +37 442. Their study can shed light on the poorly known processes taking place during common envelope evolutionary phases and on the properties of wind mass loss from hot subdwarfs.


INTRODUCTION
Hot subdwarfs are evolved low-mass stars that lost most of their hydrogen envelopes and are now in the stage of helium core burning.A possible mechanism responsible for the loss of the massive H envelopes necessary to form hot subdwarfs is mass transfer in a binary.Indeed, many hot subdwarfs are members of binary systems [18,22], supporting the idea that non-conservative mass transfer played a role in the formation of these stars.Evolutionary models predict that the most of the subdwarf companions in binaries with orbital period shorter than ∼10 days should fall into two main groups: late type main sequence stars and white dwarfs [9].
X-rays can be used to identify hot subdwarfs with compact companions: they can originate from surface thermal emission of neutron stars or sufficiently hot white dwarfs or can be produced if the compact object accretes mass from the subdwarf at a sufficiently high rate.This is well illustrated by our recent results on the sdO binaries discussed below: HD 49798 and BD +37 • 442.Furthermore, accreting compact objects can be used as probes to investigate the poorly known properties of the stellar winds of hot subdwarfs.

A MASSIVE WHITE DWARF WITH A SDO COMPANION
The peculiar properties of the bright blue star HD 49798 (B = 8, B-V = −0.27)attracted the attention of many astronomers since the sixties, when it was included in the (then small) group of early type subdwarfs [12].Its first spectroscopic observations showed a dominance of He and N lines and radial velocity variations, pointing to a binary nature, that was later confirmed with the discovery of the orbital period of 1.5477 days [25].In the following years several studies concentrated on a detailed modeling of the star's atmosphere.HD 49798 was classified as a subdwarf of spectral type O6, with effective temperature T eff = 47, 500 K, and surface gravity log g = 4.25 ± 0.2 [15].The overabundance of He and N, and the low abundances of C and O, indicated that HD 49798 is the stripped core of an initially a e-mail: sandro@iasf-milano.inaf.itThis is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.much more massive and larger star.The optical mass function could be measured with great precision (f OP T = 0.263 ± 0.004 M , [24]), but all the attempts to reveal the companion star, outshined in the optical/UV by the very luminous sdO (L ∼ 10 4 L ), were unsuccessful.
The nature of the "invisible" companion of HD 49798 was partially clarified only in 1996, thanks to the ROSAT discovery of soft X-rays showing periodic pulsations at 13 s [11].This clearly pointed to the presence of a collapsed object.However, with the poorly constrained spectrum obtained with ROSAT, it was impossible to distinguish between a neutron star or a white dwarf.In fact, depending on the spectral assumptions, the derived X-ray luminosity resulted in the range from ∼10 32 erg s −1 up to > ∼ 10 35 erg s −1 , compatible with both possibilities.
To further advance in the understanding of this system we had to await for a long XMM-Newton pointing carried out in 2008.We strategically scheduled this observation at the orbital phase of the expected X-ray eclipse, that was never covered in previous X-ray observations.Our main objectives were to exploit the regular X-ray periodicity, which makes this system equivalent to a double spectroscopic binary, to constrain the masses of the two stars, and to get a better estimate of the source spectral parameters and luminosity.The measurement of the X-ray pulse delays induced by the orbital motion, together with the discovery of an X-ray eclipse lasting ∼1.3 hours, allowed us to derive the X-ray mass function as well as the system's inclination.This information, coupled to the already known optical mass function, gives the masses of the two stars: M sd = 1.50 ± 0.05 M for the subdwarf and M W D = 1.28 ± 0.05 M for its companion (Fig. 1).Furthermore, the high quality spectrum obtained with the XMM-Newton instrument showed that the total luminosity is only ∼10 32 erg s −1 , much smaller than that expected from a neutron star accreting in the stellar wind of HD 49798 .We thus concluded that this system most likely contains one of the most massive white dwarfs with a dynamical mass measurement, which is also the one with the shortest spin period (P = 13 s) [19].
The radius of HD 49798 (1.45 ± 0.25 R ) is much smaller than that of its Roche-lobe, so accretion onto the white dwarf must occur through stellar wind capture.In fact, HD 49798 is one of the few O-type subdwarfs for which evidence for a relatively strong stellar wind has been obtained from At the end of the current He-burning phase, HD 49798 will expand again and transfer He-rich matter through Roche-lobe overflow during a semi-detached phase [10], but the fraction of mass that is retained on the white dwarf is rather uncertain.Recent computations, performed assuming the mass accumulation efficiency that takes into account the wind mass loss triggered by the He-shell flashes [14], indicate that a mass of 1.4 M can be reached after only a few 10 4 years [28].However, there are other critical factors which influence the final fate of the white dwarf, such as, e.g., its composition and rotational velocity.
If HD 49798 hosts a CO white dwarf, it could be the progenitor of an over-luminous type Ia supernova, since the fast rotation can increase the mass stability limit above the value for non-rotating stars.Massive white dwarfs are expected to have an ONe composition, but again the high spin might play a role here, since it can lead to the formation of CO white dwarfs even for high masses.The fact that this system originated from a pair of relatively massive stars (∼8-9 M ) might suggest that it could be the progenitor of a type Ia supernova with a short delay time.However, the delay time might be considerably longer if the explosion has to await that the white dwarf spins down [2].
Alternatively, if the companion of HD 49798 is an ONe white dwarf, an accretion-induced collapse might occur, leading to the formation of a neutron star.The high spin rate and low magnetic field make this white dwarf an ideal progenitor of a millisecond pulsar.The evolution of systems like this one could be a promising scenario for the direct formation of millisecond pulsars, i.e. one not involving the recycling of old pulsars in accreting low-mass X-ray binaries.
Besides their obvious interest in the context of the binary evolutionary modes, these X-ray observations are interesting for the study of the sdO star itself.It is remarkable that X-rays have been detected also when the white dwarf is eclipsed by the much larger sdO companion (Fig. 2).The observed emission, with a luminosity of ∼ 2 × 10 30 erg s −1 , could be the first detection of a sdO star in the X-ray band [21].The observed ratio of X-ray to bolometric luminosity of a few 10 −7 is in accordance to that of main sequence and supergiant OB stars.Alternatively, the X-rays seen during the eclipse could be due to the reprocessing of the white dwarf emission in the stellar wind.Our most recent XMM-Newton The top panels show the light curves in the range 0.2-0.5 keV folded at the periods of 13.2 s and 19.2 s, respectively for HD 49798 and BD +37 • 442.The bottom panels show the spectra with the best fit blackbody plus power-law models.Note that the flux of BD +37 • 442 is much lower than that of HD 49798.Hence its spectral parameters are not well constrained.This implies a large uncertainty on its total X-ray luminosity.
observations showing the presence of emission lines of N and Ne in the eclipse spectrum seem to favor this interpretation.

AN EXTREME HE STAR WITH A NEUTRON STAR OR WHITE DWARF COMPANION
Prompted by the possible detection of X-ray emission from HD 49798 itself during the white dwarf eclipse, we requested XMM-Newton observations of a supposedly single hot subdwarf.This led to the discovery of soft X-rays with a periodicity of 19.2 s from the luminous sdO BD +37 • 442, indicating also in this case the presence of a compact object [17].This was quite surprising because all the data reported in the literature for BD +37 • 442 did not show any evidence of a binary companion: infrared observations did not show any excess emission [26], and no signatures of a binary nature were seen in spectroscopic or photometric data [4,5,16].On the other hand, the few radial velocity measurements found in the literature give inconsistent values (V r = −156.4± 1.1 km s −1 [23], V r = −94 ± 1 km s −1 [3]), which could be caused by binary motion.Further spectroscopic observations are clearly needed to search for an orbital period in BD +37 • 442.
The X-ray emission from BD +37 • 442 has a soft spectrum well described by a blackbody with temperature ∼45 eV plus a weak power-law component.This is very similar to the spectrum of HD 49798 (Fig. 3).The best fit X-ray luminosity is ∼ 10 33 erg s −1 (for a distance of 2 kpc, [1]), but, due to the uncertainties in the X-ray blackbody temperature, the bolometric X-ray luminosity could be between ∼ 10 32 and ∼ 10 35 erg s −1 .
The observed pulsations clearly indicate the presence of a white dwarf or a neutron star, most likely powered by accretion from the stellar wind of BD +37 • 442, which is loosing mass at a rate of ∼ 3 × 10 −9 M yr −1 and with a wind terminal velocity of v ∞ = 2, 000 km s −1 [13].Simple computations, assuming a canonical velocity law for the wind and Bondi-Hoyle accretion, show that a neutron star orbiting BD +37 • 442 with a period of a few days would accrete from the wind at a sufficiently high rate to produce the observed X-rays.Alternatively, the accreting companion could be a white dwarf, but this would require a larger accretion rate that would only be possible if the accretion is due to Roche lobe overflow.

SDB WITH CANDIDATE COMPACT COMPANIONS
We observed with the Swift satellite a few sdB binaries selected from the MUCHFUSS project, which aims at finding hot subdwarfs with compact companions by means of radial and rotational velocity measurements in the optical band [7].None of the observed targets was detected in the X-ray band, and we could set upper limits on their luminosity in the range L X ∼ 10 30 -10 31 erg s −1 [20].Although this negative result does not allow us to confirm the presence of compact stars inferred from the optical data, the upper limits on L X can be used to constrain the poorly known properties of the sdB stellar winds 1 .
The luminosity upper limits can be converted into limits on the mass loss rates ṀW from the sdB stars, assuming for simplicity Bondi-Hoyle accretion from the stellar wind onto the compact objects.This is justified by the fact that in these systems the sdB stars do not fill their Roche lobes.Our assumptions lead to conservative upper limits on ṀW , since most wind-accreting neutron stars in highmass X-ray binaries show an X-ray luminosity larger than that predicted by the simple Bondi-Hoyle theory.
The resulting limits on ṀW are plotted in Fig. 4, where different symbols are used to indicate the assumed compact object.For the systems likely hosting white dwarfs (triangles) the limits are above

Figure 1 .
Figure 1.Masses of HD 49798 (X axis) and of its white dwarf companion (Y axis) in units of solar masses.The curved lines give, for different inclination angles, the constraints from the optical mass function.The straight lines indicate the mass ratio interval derived combining the X-ray and optical mass function.The eclipse duration constrains the inclination in the range 79-84 • .

Figure 2 .
Figure 2. X-ray light curve of the May 2008 observation of HD 49798 .An eclipse lasting 1.3 hours is clearly visible.The image accumulated during the eclipse time interval shows the presence of significant emission in the 0.2-10 keV band.

04003-p. 3 EPJFigure 3 .
Figure 3.Comparison of the X-ray light curves and spectra of HD 49798 (left) and BD +37 • 442 (right).The top panels show the light curves in the range 0.2-0.5 keV folded at the periods of 13.2 s and 19.2 s, respectively for HD 49798 and BD +37 • 442.The bottom panels show the spectra with the best fit blackbody plus power-law models.Note that the flux of BD +37 • 442 is much lower than that of HD 49798.Hence its spectral parameters are not well constrained.This implies a large uncertainty on its total X-ray luminosity.

Figure 4 .
Figure 4. Upper limits (3 ) on the sdB mass loss rates in the case of a white dwarf (triangles), a neutron star (squares), or a black hole (circle) companion.