7.2 IGR J00291+5934
7.2.4 New analysis of the INTEGRAL data with OSA 7.0
In the framework of a “Travaux Pratiques 3” work at the University of Geneva with an under-graduate student (C. Tchernin), we performed a new analysis of the existing INTE-GRALdata with the latest Offline Scientific Analysis (OSA) software, version 7.0. Indeed, Falanga et al. (2005) used OSA v4.2 to analyze the INTEGRAL data, and several very important improvements in the calibration of the instruments have been implemented in further OSA versions, so a new analysis could lead to refined results. For our analysis, we used all available ScWs from revolutions 0261-0268 (Dec. 02-25, 2004), for a total observation time of 760 ksec (ISGRI) and 165 ksec (JEM-X1).
Figure 7.3 shows the significance images of the region obtained by ISGRI (left) in the 20-40 keV band and by JEM-X1 (right) in the 3-6 keV band, extracted with the standard OSA 7.0 software. The source was clearly detected by both instruments at the level of 84σ (ISGRI) and 35σ (JEM-X1). The images clearly demonstrate the need for instruments with sufficient angular resolution in the hard X-ray domain. Indeed, the position of the source is found only 17 arcmin away from the Cataclysmic Variable V709 Cas and∼1 de-gree away from the blazar 1ES 0033+59.5, which are clearly detected by both instruments.
The discovery of IGR J00291+5934 therefore demonstrates the ability ofINTEGRAL to detect new hard X-ray sources and locate them with sufficient accuracy.
Analyzing the ISGRI and JEM-X mosaics, we extracted the list of sources which must be taken into account by the spectral extraction tool, and extracted the spectrum of IGR J00291+5934. The quality of the totalINTEGRAL spectrum is excellent, and leads to a broad-band coverage (3-200 keV) of the source. To model the high-energy emission of the source, we used the XSPEC spectral fitting package version 11.3.2. We started from the simplest possible model, a single power law, and then tried more complex models in order to improve the quality of the fit. Fitting the data with a single power law, we find an unacceptable fit (χ2red= 3.05 for 65 d.o.f). Positive residuals are found below 10 keV and around 50 keV. In the standard model which describes the emission from such objects, a soft emission (thermal emission from the surface of the neutron star or from an accretion disc) is comptonized by a hot corona surrounding the system. Therefore, the excess at low energies could be explained by black-body-like emission from the surface of the star or from the accretion disc. Adding a second component to the fit under the form of a soft black-body, we find a significantly better fit (χ2red= 1.87 for 63 d.o.f.) with a black-body temperature kTbb ∼ 2 keV. However, the quality of the fit is still not optimal, and sig-nificant negative residuals are found above 80 keV. This could indicate the presence of a high-energy cut-off in the power-law component. Including a high-energy cut-off in our
7.2. IGR J00291+5934 123
Figure 7.3: ISGRI (left) and JEM-X1 (right) significance images of IGR J00291+5934 during the 2004 outburst. Two other known sources (the Cataclysmic Variable V709 Cas and the blazar 1ES 0033+59.5) are found less than one degree away from the source.
kTbb [keV] 2.08±0.06 kTcor [keV] 31±3
τ 3.3±0.2
Table7.1: Results of spectral fitting when modeling the spectrum of IGR J00291+5934 as soft black-body emission comptonized by a hot corona. The parameters of the fit are the black-body temperaturekTbb, the coronal temperaturekTcor and the optical depth of the coronaτ.
modeling further improves the fit (χ2red = 1.61 for 62 d.o.f.), although the cut-off energy cannot be well constrained. This indicates that the radiation could indeed be described as comptonized emission from a hot corona at a temperature kT ∼ 50 keV (the cut-off energy).
In order to model the emission as black-body emission from the surface of the neutron star comptonized in a hot corona, we used the CompST model (Sunyaev & Titarchuk 1980), which models the comptonized emission from the corona using approximate solutions of the radiative transfer equation. The parameters of the model are the coronal temperature kTcor and the optical depth τ of the medium. This model provides the best fit to the data (χ2red = 1.42 for 62 d.o.f.). The spectrum of IGR J00291+5934 with the best-fit model is displayed in Fig. 7.4. The left panel shows the folded spectrum and the residuals compared to the model, while the right panel shows the unfolded spectrum in an EFE representation. Table 7.1 gives the best-fit parameters for this model.
Fitting the data using more complex models for the comptonized emission (CompTT, CompPS) did not provide significant improvements to the fit, so we decided to keep the
10−30.010.1
Figure 7.4: Left: Folded spectrum of IGR J00291+5931 extracted with JEM-X (black) and ISGRI (red). The bottom panel shows the deviations of the data compared to the best fit with a black-body+CompST model (solid line). Right: Unfolded spectrum of IGR J00291+5931 in EFE representation. The solid green line shows the total model made of black-body emission from the neutron star (dashed red) plus comptonized emission from the corona, approximated by a CompST model (dashed green).
simplest acceptable model to describe the source. Instead of a black-body emission, it is also possible to model the soft emission as an optically-thick accretion disc using the DiskBB model (Shakura & Sunyaev 1973). The quality of the fit is similar to our best fit, so based only on INTEGRAL data it is not possible to determine if the soft emis-sion comes from the surface of the neutron star (black-body) or from the accretion disc (DiskBB). Using a Chandra observation of the source, Paizis et al. (2005) detected an additional very soft component which they model as an accretion disc at kT ∼ 0.3 keV.
Since the bandpass of JEM-X starts only at 3 keV, it is not possible for us to detect this component. Therefore, the 2 keV soft component is probably due to black-body emission from the surface of the neutron star, while the cooler disc component cannot be detected by INTEGRAL.
Overall, the results are consistent with the results of Falanga et al. (2005) extracted with OSA 4.2. The calibration improvements introduced in OSA 7.0 allowed us to refine some physical parameters such as the black-body temperature of the soft emission and the temperature of the corona.
7.2.5 Conclusion
The transient X-ray system IGR J00291+5934, discovered by INTEGRAL in Dec. 2004 by INTEGRALduring a bright outburst, is the fastest known accreting milli-second pul-sar to the present day (598 Hz). It is a close binary system consisting of a fast-spinning neutron star and a low-mass star which fills its Roche lobe and transfers material onto the compact object. The angular momentum of the accreted material is used to spin up the neutron star, which explains the extreme rotation velocity (>0.1c) of the pulsar. Spectral analysis of the source reveals that the emission can be well-described by soft black-body
7.2. IGR J00291+5934 125 emission from the surface of the neutron star at a temperature kT = 2 keV plus comp-tonized emission from a hot corona atkT ∼30 keV surrounding the system. Optical and soft X-ray emission also reveal the presence of an accretion disc around the compact object.
While the existence of milli-second pulsars has been known for a long time, it was not understood how these objects can reach such high rotation velocities. The discovery of IGR J00291+5934 and the detection of a spin-up by Falanga et al. (2005) provides a natural explanation to this mystery: milli-second pulsars are normal pulsars in close binary systems which have been spun up by accretion of material from the companion star.