We present the first detailed spectral and timing analysis of the High Mass X-ray Binary (HMXB) 4U 1909+07 with INTEGRAL and RXTE. 4U 1909+07 is detected in the ISGRI 20-40 keV energy band with an average countrate of 2.6 cps. The pulse period of ~604 sec is not stable, but changing erratically on timescales of years. The pulse profile is strongly energy dependent: it shows a double peaked structure at low energies, the secondary pulse decreases rapidly with increasing energy and above 20 keV only the primary pulse is visible. This evolution is consistent between PCA, HEXTE, and ISGRI. The phase averaged spectrum can be well described by the sum of a photoabsorbed power law with a cutoff at high energies and a blackbody component. To investigate the pulse profile, we performed phase resolved spectral analysis. We find that the changing spectrum can be best described with a variation of the folding energy. We rule out a correlation between the black body component and the continuum variation and discuss possible accretion geometries.
The accreting neutron star X-ray binary 4U 1909+07 was first mentioned in the 3 rd Uhuru catalog as 3U 1912+07 (Giacconi et al., 1974). The position and name of the source were later refined to 4U 1909+07 in the 4 th Uhuru catalog (Forman et al., 1978). Many other X-ray missions, such as OSO 7, Ariel 5, and EXOSAT, among others, also detected a source close to these coordinates. Wen et al. (2000) showed that these detections are likely all originating from the same source and refer to it as X1908+075. We will use the name 4U 1909+07, to honor the original discovery.
Despite the regular detections, the first dedicated paper to discuss 4U 1909+07 was published in 2000 when Wen et al. (2000) analyzed RXTE-ASM data and found a stable period of 4.4 days. This period is interpreted as the orbital period of a binary system. Due to the high photoabsorption, however, no optical counterpart could be identified. In RXTE-PCA data, Levine et al. (2004) found a second, shorter period of ∼605 s in the X-ray flux, explained as the pulse period of a slowly rotating neutron star. Using Doppler delay curves, they could also refine the binary orbit parameters and estimated the mass of the companion star to be M ⋆ = 9-31 M ⊙ and the radius of the companion star to be R ⋆ ≤ 22 R ⊙ , adopting a canonical mass of 1.4 M ⊙ for the neutron star. One year later, Morel & Grosdidier (2005) detected an OB star in the near infrared at the location of the X-ray source, thus confirming that the system is a High Mass Xray Binary (HMXB). The distance of the system was estimated to be 7 kpc (Morel & Grosdidier, 2005). Prior to this discovery, Levine et al. (2004) argued that the companion star could be a Wolf-Rayet star, which would make this system a possible progenitor system to a neutron star-black hole binary. With the identification of the companion, however, this intriguing possibility can be ruled out. The X-ray luminosity L of 4U 1909+07 is around 2.8 × 10 36 erg s -1 for 4.5-200 keV. Although the system shows no eclipse, the X-ray flux is still strongly orbital phase dependent (Levine et al., 2004). Levine et al. (2004) also analyzed orbital phase resolved PCA spectra and found that around orbital phase φ orb = 1 the photoabsorption increases by a factor of 2 or more to N H ≥ 30 × 10 22 cm -2 , explaining the decreased ASM flux. This increase in absorption can be very well described by a spherical wind model and an inclination of 54 • ≤ i ≤ 70 • , depending on the parameters of the wind model.
In archival Chandra data, Torrejón et al. (2010) found that the soft energy spectrum of 4U 1909+07 shows evidence for a Compton shoulder on the soft energy edge of the iron Kα line. This feature has so far only been seen in GX 301-2 but in no other HMXB with Chandra. Torrejón et al. (2010) showed that the Compton shoulder is consistent with the X-ray source being embedded in a Compton thick medium, which is also responsible for the observed photo absorption. Additionally these authors have shown that 4U 1909+07 is an exception from the correlation between N H and the equivalent width of the iron line found in most other HXMB (Inoue, 1985). The origin of this exception is still a mystery and can only be investigated with more high-resolution spectra.
Although there was renewed interest in 4U 1909+07 in recent years, many basic parameters are still unknown. For example, despite the detection of the pulse period, its evolution with time was not yet studied, even though it can give insight on the prevailing accretion mechanism (Bildsten et al., 1997). Additionally, no detailed analysis of the high energy spectrum has been carried out so far. RXTE data provide enough statistics to even perform pulse phase resolved spectroscopy, allowing us to obtain a better understanding of the accretion region and mechanism together with the energy dependence of the pulse profile. In this article we are aiming at improving our knowl- edge in these points, using data from INTEGRAL and RXTE.
In Sect. 2 we present the data and reduction methods. The pulse period evolution and the pulse profiles at different energies are analyzed in Sect. 3. In Sect. 4 we perform phase averaged and phase resolved spectroscopy. We summarize and discuss our results in Sect. 5.
For our study of the 4U 1909+07 system we used all available public data from the X-ray missions INTERnational Gamma-Ray Astrophysics Laboratory (INTEGRAL, Winkler et al., 2003) and Rossi X-Ray Timing Explorer (RXTE, Bradt et al., 1993). One of the main detectors of INTEGRAL is ISGRI, a coded mask instrument sensitive in the 15 keV -1 MeV energy range and part of the Imager on Board the Integral Satellite (IBIS; Ubertini et al., 2003;Lebrun et al., 2003). IBIS/ISGRI was the first instrument to produce high resolution images of the X-ray sky above 20 keV. Even though no pointed observations on 4U 1909+07 were performed with INTEGRAL, thanks to IBIS/ISGRI’s large field of view of almost 30 • × 30 • , there are m
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