X–ray and optical monitoring of the peculiar source 4U 1700+24/V934 Her

Abstract

Abstract. We report on ASCA and BeppoSAX X-ray broad band observations of the galactic low-luminosity X-ray source 4U 1700+24 performed on 1995 and 1998, respectively, and on (quasi-)simultaneous ground observations of its optical counterpart, V934 Her, from the Loiano 1.5-meter telescope. In order to better understand the nature of the source we also analyze public archival ROSAT and RXTE data as well as the RXTE ASM light curve of 4U 1700+24; we also re-analyze a 1985 EXOSAT pointing. The optical spectra are typical of a M2 III star; this allows us to determine a revised distance to the object of ∼400 pc. While these spectra do not show either any spectral change between the two epochs or any peculiar feature apart from those observed in normal red giants, the spectroscopic measurements carried out in X-rays reveal a complex and long-term variable spectrum, with a clear soft excess. The X-ray spectral properties of the source are best described by a thermal Comptonization spectrum plus a soft energy (<1 keV) excess, which can be modeled in the form of a blackbody emission with kTBB ∼ 1 keV; the latter component is not detected at the lowest source flux levels. The ratio between the two components varies substantially with the source flux. The X-ray emission from the object appears to become harder as its luminosity increases: indeed, the RXTE data acquired during an outburst occurred in October-November 1997 display a hard tail, detected up to 100 keV and modeled with a comptonizing cloud which is hotter and less opaque than that seen in the low intensity state. Apart from erratic shot-noise variability on timescales of tens to thousands of seconds, no significant properties (such as pulsations or QPOs) are found from the timing analysis of the X-ray light curves extracted from the observations presented here. With the new distance determination, the 2-10 keV X-ray luminosity range spanned in the considered observations lies between ∼2×10 32 and ∼1×10 34 erg s −1 . All this information, combined with the findings by other authors, allows us to suggest that the scenario which best describes the object consists of a wide binary system in which a neutron star accretes matter from the wind of a M-type giant star. Implications of such a model are discussed.

M-typegiants as optical counterparts of X-ray sources 4U 1700+24 and 4U 1954+319

“…The latter characteristic is due to the low X-ray luminosity when compared to the bolometric luminosity of a M4 III star (∼2 ×10 36 erg s −1 ; Lang 1992); so, reprocessing of variable X-ray emission is completely lost into the glare of the M-type giant companion. Thus, a scenario similar to that described by Masetti et al (2002) for 4U 1700+24 may also be applicable to the system 4U 1954+319.…”

“…Garcia et al (1983), using the X-ray position afforded by the HEAO-1 satellite, identified this source in the optical with a "normal" M-type giant star, HD 154791, located at ∼800 pc from Earth. Masetti et al (2002) gave a more accurate spectral classification (M2−3 III) to this putative optical companion and, in turn, a new distance estimate (d = 420 pc). The latter authors moreover explained the absence of the features typical of accreting systems in the optical spectrum of HD 154791 as due to the fact that the luminosity of the giant star is ∼200 times larger that any possible optical emission from the accreting stream, and so the latter is overwhelmed by the former.…”

“…This corresponds to a 2−10 keV flux of ∼2×10 −10 erg cm −2 s −1 assuming a Comptonization spectrum as described in Masetti et al (2002) for the BeppoSAX data. This value converts into a 2−10 keV luminosity of ∼4 × 10 33 erg s −1 using the above distance estimate from Masetti et al (2002).…”

“…This corresponds to a 2−10 keV flux of ∼2×10 −10 erg cm −2 s −1 assuming a Comptonization spectrum as described in Masetti et al (2002) for the BeppoSAX data. This value converts into a 2−10 keV luminosity of ∼4 × 10 33 erg s −1 using the above distance estimate from Masetti et al (2002). The comparison of this value with the long-term monitoring of this source reported in Masetti et al (2002) indicates that 4U 1700+24 was in an intermediate X-ray activity state during the Chandra observation.…”

A puzzling event during the X-ray emission of the binary system GX 1+4

“…Among the large X-ray binary zoo, we know so far only another system hosting a neutron star (NS) with a red giant companion: 4U 1700+24 ( [16,9]). GX 1+4 shows an unpredictably variable X-ray flux on timescales from hours to decades.…”