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INTEGRAL OBSERVATIONS OF THE VELA REGION FOCUSING ON VELA X-1
StÈphane Schanne(1,*), Diego GÆtz(1), Lucie GÈrard(1), Patrick Sizun(1), Maurizio Falanga(1), Clarisse Hamadache(1), Bertand Cordier (1), Andreas von Kienlin
(1) (2) (2)

CEA-Saclay, DAPNIA/Service d'Astrophysique, F-91191 Gif sur Yvette, France Max-Planck-Institut fÝr extraterrestrische Physik, D-85740 Garching, Germany (*) corresponding author: schanne{at}hep.saclay.cea.fr

ABSTRACT The Vela region has been observed for 1.7 Ms in November 2005 by the INTEGRAL satellite. We present preliminary spectral and temporal results of Vela X-1, an eclipsing neutron star hosted in a windaccreting high-mass X-ray binary system. Using data from ISGRI, SPI and JEM-X, we firmly confirm the existence of cyclotron resonant scattering features (CRSF) at ~27 keV and ~54 keV, implying a neutronstar magnetic field of 3â1012 Gauss, and the presence of an iron emission line at ~6.5 keV. During two strong flares those parameters remained unchanged. Furthermore we measure the neutron-star spin period of 283.6 s, indicating a still constant trend. 1. INTRODUCTION

sources found comprise four sources (Tab. 1) ­all of them known­ which are detected with significance above 6 , considered to be the limit for firm source detection.

The Vela region has been observed by INTEGRAL [1], ESA's gamma-ray satellite, in the frame of our accepted AO-3 open-time proposal. Data were taken from November 4 to December 4, 2005, during 1.7 Ms, from INTEGRAL revolution n°373 to 383, in 473 different pointings. The key target is the detection of rays emitted by the decay of radioactive isotopes produced in stellar nucleosynthesis processes as e.g. 1809 keV emission from 26Al decays. Results of the 26 Al studies are presented in [2]. Besides nucleosynthesis studies, the region is also of interest for the presence of several point sources, among which Vela X-1, for which we present preliminary results in this paper. In the soft -ray band the brightest point source in the region is Vela X-1, an eclipsing wind accreting high-mass X-ray binary system (HMXB) consisting of a neutron star (NS) and a massive, 23 solar masses (M ), donor star (HD 77581) classified as a B0.5 super-giant [3]. The system has an orbital period of 8.964 days [4]. The NS is deeply embedded in the intense stellar wind of the companion, whose inferred mass-loss rate is of the order of 10-7 M /yr [5]. The NS shows a spin period of ~283 s in X-rays (e.g. [6]). 2. SEARCH FOR SOURCES IN VELA REGION the full IBIS/I s in the Vela of the region most suited SGRI data region, by (Fig. 1) in for new set, we search for point constructing a mosaic the 18-60 keV energy source detection. The


Fig. 1. Image of the Vela region by ISGRI (18-60 keV).

Source Vela X-1 Vela Pulsar H 0918-548 IGR J09026-4812 IGR J07597-3842 GRO J1008-57 Fairall 1146 ESO 209-12 IGR J10109-5746

-level
13 4 1 1 5 5 5 5 5 16.3 8 .2 3 .9 2 .0 .7 7 .4 6 .4 5 .1 1 .0 8

Tab. 1. Sources found by ISGRI in the Vela region (18-60 keV energy band, 1.7 Ms exposure). Only the first four sources can be considered firmly detected.

Using source image range,

A similar search has been performed with the SPI data. We use SPIROS imaging in the 20-40 keV energy band. The net exposure time was 1 Ms after selecting the best 271 pointings (2<10 in SPIROS). The SPIROS mode used was a blind search for 3 sources. The background model used empty fields close in time to the Vela data taking period, however the saturating Ge-detector background model yields similar results. We obtain a Vela region image (Fig. 2) and the results are summarized in Tab. 2. Two sources, Vela X-1 and

_____________________________________________________________________ Proc. 6th INTEGRAL Workshop `The Obscured Universe', Moscow, 3-7 July 2006 (ESA SP-622)


Vela Pulsar, are firmly detected, while the 3rd source might be associated with IGR J07597-3842 or IGR J07565-4139, both located within the SPI angular resolution at a distance<2° from the detected position.

SPI Vela X-1 light curve

Fig. 4. SPI light curve of Vela X-1 (20-40 keV) in ph/cm2/s.

4.

VELA X-1 FOLDED LIGHTCURVE

Fig. 2. SPIROS significance image of the Vela region (20-40 keV) obtained in a blind search for three sources.

Source 1 2 3

Sigma 2 3 5 .1 1 1 .6 9 .7

Cat Source Vela X1 Vela Pulsar IGR J07597-3842 IGR J07565-4139

Distance 0° ~0 . 5 ° ~1 ° ~2 °

Using the barycenter-corrected ISGRI light-curves, spanning over a single pointing (science window) of ~1 h duration, we find the NS spin period to be 283.6±0.1 s, consistent with previous findings [13]. The pulse profiles in two ISGRI energy bands (20-40 keV and 40-100 keV) are shown in Fig. 5. A deeper analysis is ongoing.

Tab. 2. Sources found by SPIROS (20-40 keV) in a blind search for three sources. The distances of those sources to known sources in the region are listed.

3.

VELA X-1 LIGHTCURVES

ISGRI Vela X-1 light curve
Flares

Fig. 5. ISGRI light curve of Vela X-1 folded at the best period of 283.6 s. Two periods are shown for clarity.

5.

VELA X-1 PERSISTENT AVERAGE SEPCTRA

Eclipses
Fig. 3. ISGRI light curve of Vela X-1. Energy 18-60 keV, time bin 1h, time measured in days since January 1st, 2000 (IJD).

The Vela X ISGRI and instruments typical for eclipses by flares (one 3 8 2 ).

-1 light curves, observed respectively with SPI are shown in Fig. 3 and Fig. 4. Both detect the same Vela X-1 variability, wind accreting pulsars, namely three NS the companion star, as well as two strong in revolution 373 and one in revolution

The presence of two cyclotron resonant scattering features (CRSF) in the average spectrum of Vela X-1 has been debated since a long time in the literature [715]. With our analysis we assess the presence of two CRSF with fundamental energy at ~27 keV and the 1st harmonic at ~54 keV in agreement with the RXTE observations by Kreykenbohm et al. (2002) [11], and at variance with the single CRSF at ~55 keV reported by La Barbera et al. (2003) [12] using BeppoSAX data. We confirm the real nature of the debated ~27 keV feature, seen independently by the two different instruments ISGRI and SPI. We derive the persistent average spectrum of Vela X-1, using JEM-X and ISGRI, by removing the time intervals of the two strong flares and the eclipses (80 ks


exposure available for JEM-X, 737 ks for IBIS). The fit with a simple cutoff power law (Fig. 6) displays the presence in the data of residuals at ~6.5 keV, ~27 keV and ~54 keV. We model the low-energy excess with a Gaussian emission line, and describe the two highenergy dips with the cyclotron absorption model cyclabs of XSPEC [16]. This yields a fit with a reduced 2 of 1.2 applying 2% systematics (Fig. 7). We associate the Gaussian emission feature found at 6.44 keV with a Fe emission line. The low-energy cyclotron absorption line is found to be located at 27.0±0.3 keV. Note that the ISGRI/JEM-X inter-calibration factor is around 1.3. Including the SPI data (Fig. 8), the same model yields a fit with a reduced 2 of 1.2 applying 3% systematics, a Fe emission line at 6.56 keV and the low energy cyclotron line located at 27.2 keV.

D(WE / Ei ) 2 ( E ) = exp - RSF 2 2 ( E - Ei ) + W The result of the fit is shown in Tab. 3 (1st data column). The fit using the same model applied to both SPI and ISGRI data, leads to compatible results, shown in Tab. 3 (2nd data column). FC

Fig. 8. JEM-X, ISRGI and SPI persistent average spectrum of Vela X-1 (combined fit). Persist. avr. S PI 27.44 ± 0.43 6.73 ± 1.6 0.17 ± 0.052 8.84 ± 1.96 1.01 ± 0.12 1.03 71 Persit. avr. SPI + ISGRI 27.88 ± 0.14 12.09 ± 0.94 0.28 ± 0.078 10.86 ± 1.05 1.1 ± 0.07 1 .9 90 In flares S PI 27.5 ± 0. 5.58 ± 1. 0.16 ± 0. 8.45 ± 2. 0.76 ± 0. 1.17 71

Fig. 6. INTEGRAL JEM-X and ISRGI persistent average spectrum of Vela X-1, fit with a simple cutoff power law. The upper panel shows the data and the best fit model (solid line); the lower panel shows the residuals.

E1=E2/2 W1 D1 W2 D2 2 ndf

6 69 049 96 15

Tab.3. Results of fits to the SPI and SPI+ISRGI persistent average spectra, as well as the SPI spectrum during flares, using the model "constant, cutoff power-law, 2 CRSF (with fixed energy ratios)".

Fig. 7. INTEGRAL JEM-X and ISRGI unfolded persistent average spectrum of Vela X-1.

Fig. 9. SPI persistent average spectrum of Vela X-1.

The Vela X-1 persistent average SPI spectrum u SPIROS is shown in Fig. 9. This spectrum is mod using constant, cutoff power-law, two CRSF feat with energies fixed by E1=E2/2 and fluxes modeled

sing eled ures by:

6.

VELA X-1 SEPCTRA DURING FLARES

For the Vela X-1 flare in revolution 373, one pointing is used to derive the combined JEM-X and ISGRI spectrum, shown in Fig. 10. In this limited dataset, the


8.

REFERENCES

Fig. 10. JEM-X and ISRGI spectrum of Vela X-1 during flare in rev. 373, fitted with a simple cutoff power law, showing the presence of residuals at ~6.5 keV, ~26 keV and ~52 keV.

Fig. 11. SPI spectrum of Vela X-1 during flare, fitted with a cutoff power-law and two CRSF.

7.

CONCLUSIONS

The INTEGRAL satellite observed the Vela region in the X-ray and gamma-ray energy bands during 1.7 Ms between 4th November and 4th December 2005, from revolution 373 to 383, in the context of our accepted AO-3 INTEGRAL open-time observation. We present preliminary results of the analysis of the ISGRI, SPI and JEM-X data. We focus on a spectral and temporal study of the eclipsing high-mass X-ray binary Vela X1. Thanks to this long-term observation we derive the time-averaged spectrum of the source and firmly confirm the existence of cyclotron resonant scattering

[1] Schanne, S., Observing with a space-borne gamma-ray telescope: selected results from INTEGRAL, IOP Journ. Phys. 41 (2006) 46, and references herein. [2] Schanne, S., et al, Evidence for 1809 keV gamma-ray emission from 26Al decays in the Vela region with INTEGRAL/SPI, ESA SP-622 (2006). [3] Nagase, F., Accretion-powered X-ray pulsars Astron. Soc. Japan (ISSN 0004-6264), vol. 41, no. 1, (1989) p. 1-79 [4] van Kerkwijk et al, A&A 303 (1995) p. 483. [5] Sako, M. et al., X-Ray Spectrum and Global Structure of the Stellar Wind in Vela X-1, ApJ 525 (1999) p. 921S. [6] McClintocket et al., ApJ 525 (1976) p. 978. [7] Makishima, K., Magnetic Field of Binary X-Ray Pulsars, in "Structure and Evolution of Neutron Stars" (1992) p. 86. [8] Choi et al., Pulse phase-dependent Spectroscopic Study of Vela X-1, ApJ 471 (1996) 447. [9] Kretschmar, P. et al., Phase resolved X-ray spectra of Vela X-1, A&A 325 (1997) 623. [10] Kreykenbohm, I. et al., Vela X-1 as seen by RXTE, A&A 341 (1999) 141. [11] Kreykenbohm, I. et al., Confirmation of two cyclotron lines in Vela X-1, A&A 395 (2002) 129. [12] La Barbera et al., Pulse phase spectroscopic study of Vela X-1 in the 8-100 keV band, A&A 400 (2003) p. 993. [13] Kretschmar, P. et al, INTEGRAL Broadband Spectroscopy of Vela X-1, ESA SP-552 (2004) 267 [14] Staubert, R. et al, INTEGRAL/IBIS Observations of Vela X-1 in a Flaring State, ESA SP-552 (2004) 259 [15] AttiÈ, D., PhD, CEA-Saclay, UniversitÈ Pierre et Marie Curie (Paris VI), 2005. [16] Arnaud, K., 1993, http://heasarc.gsfc.nasa.gov /docs/xanadu/xspec/



presence of the Fe emission line is still significant at ~6.5 keV, and the low-energy CRSF feature (fit with cyclabs) is found at 26.0±0.4 keV. For the flare in revolution 382, nine IBIS pointings are used, and the low-energy CRSF feature is found at 27.0±0.3 keV. The SPI spectrum obtained by combining the flares of Vela X-1 in revolutions 373 and 382 is presented in Fig. 11. The result of the fit using the previously presented model with two CRSF is reported in Tab. 3 (3rd data column). We compare those parameters with the ones found for the persistent average spectrum using either SPI alone or SPI+ISGRI data. We conclude that no important change in the structure of the CRSF features can be noticed for those Vela X-1 flares.

features (CRSF) at ~27 keV and ~54 keV using IBIS/ISGRI and SPI data separately and in a combined fit. Assuming canonical parameters for the neutron star (mass 1.4 M , radius 10 km), a 27 keV CRSF implies a neutron star magnetic field of 3â1012 Gauss. Using the JEM-X data we also found the presence of a Fe emission line at ~6.5 keV. During two strong Vela X-1 flares we found that the energy positions of the CRSF features remain unchanged with respect to their persistent average values. Furthermore we obtain a spin period of 283.6 s by folding the Vela X-1 light curve over one pointing of 1 h duration. In order to complete the Vela X-1 analysis, work is currently ongoing. We will derive a precise orbital ephemeris, which is important for the study of this system, since it allows to determine detailed time-resolved spectra during the eclipse ingress and egress phases in different energy bands, and to monitor details of the companion wind. Orbital and pulse-phase resolved spectroscopy will permit us to understand the underlying physics of the high-energy emission. Using a relativistic description of the polar-cap emission of the neutron star, the variation of the pulse morphology with energy and time will permit to model the pulsed flux in energy and constrain the system geometry.