An absorbed view of a new class of INTEGRAL sources

References Winkler et al A&A astro-ph: / / / / Bodaghee et al A&A Walter et al A&A White N Nature

INTEGRAL (International Gamma-Ray Astrophysics Laboratory) launched on 17 October 2002 main instruments: SPI (20 keV–8 MeV) IBIS (15 keV–10 MeV) Monitors: JEM-X: in the (3−35) keV X-ray band OMC: in optical Johnson V-band complement the payload Scientific objectives: nucleosynthesis, nova and supernova explosions, the interstellar medium, cosmic-ray interactions and sources, neutron stars, black holes, gamma-ray bursts, active galactic nuclei and the cosmic gamma-ray background Winkler et al A&A

IGR Sources more than 50 of these IGR sources reported up to April 2005 one third of the IGR sources can be classified as persistent or transient LMXBs or HMXBs, CVs, accreting millisecond X-ray pulsar, AGN but two third of them are unclassified. they are all distributed along the galactic plane, with concentrations in the direction of the Galactic Center and Galactic arms a lot of the INTEGRAL observations are concentrated on regions around the Galactic plane and the detection of new (especially transient) sources may, therefore, be biased towards these regions.

Highly absorbed IGR sources very strong absorption one to two orders of magnitude higher than the Galactic value of around hard spectra (cut-off power-law model ) high-energy cut-off value four of the (well-studied) highly absorbed IGR sources have been seen to also vary on a regular time scale (between ≃ 4 to ≃ 100min, see Table 1)

IGR J Two emission lines at 6.46 ± 0.02(±0.06) keV and 7.05 keV  fluorescence of neutral or weakly ionized material  most of the absorption observed in the X-ray spectrum of IGR J takes place in the close vicinity of the source.

Near- IR spectra

Counterpart The likely infrared/optical counterpart indicates that IGRJ16318−4848 is probably a High Mass X-Ray Binary neutron star or black hole enshrouded in a Compton thick environment. No radio counterpart is observed by Australia Telescope Compact Array (ATCA).  68% X–ray matches with 2MASS counterparts.  several X–ray sources are positionally associated with more than one infrared counterpart (Sidoli et al. 2006)

Contribution to the X-ray background Strongly absorbed sources, not detected in previous surveys, could contribute significantly to the Galactic hard X-ray background between 10 and 200 keV.

Compared with GX strong photoabsorption flourescent iron lines The likely infrared/optical counterpart The exponential cutoff powerlaw model (tipical for accreting X-ray pulasrs)  similar to GX (with massive shells), which is an X-ray pulsar accreting via a powerful wind from the B supergiant Wray 977  the source is a wind- fed high-mass X-ray binary similar to GX But NO pulsations searched which would be a clear signature of a neutron star in the system. NO QPO either (Swank & Markwardt 2003)  IGRJ16318−4848 is probably a High Mass X-Ray Binary neutron star or black hole enshrouded in a Compton thick environment.

IGR J16393−4643 the persistence of IGRJ16393−4643 in the 20–60 keV energy range. we find that the source flux varies by a factor larger than 20.

Pulsations A pulse period of 912.0±0.1 s was discovered in the ISGRI and EPIC light curves.

Spectral analysis Its persistence, pulsation, and spectrum place IGR J16393−4643 among the class of heavily-absorbed HMXBs. The shape of the spectrum does not change with the pulse.

Other systems? Background AGN? Though similar X–ray spectrum shapes and the parameters of the iron line, Fe line is not red-shifted or lack of X–ray temporal variability and radio counterparts. X-ray binary in eclipse? Though Similar photoabsorption is observed during eclipses/dips in X-ray binaries, almost complete absence of flux at energies < 3 keV is unusual. Source in a molecular cloud? In the case of IGR J , the ASCA and XMM observations, separated by approximately 8.5 years, revealed very similar spectra and fluxes, similar as molecular cloud SGR B2  but ruled out given the maps in the molecular CO line (Dame et al. 2001)

Observation Summary Some of the highly absorbed IGR sources seem to be more or less persistent (such as IGRJ16318−4848; see, e.g., Matt et al. 2005) some of them are clearly transient (e.g., IGRJ16358−4726: Patel et al. 2004; IGRJ16465−4507: Lutovinov et al. 2005b) The highly absorbed IGR sources vary in brightness on time scales of minutes to hours, as well as from observation to observation, both at soft and hard X-ray energies (e.g., IGRJ16318−4848: Walter et al. 2003, Matt & Guainazzi 2003, Matt et al. 2005; IGRJ16320−4751: Rodriguez et al. 2003, Foschini et al. 2004) In IGRJ16318−4848 the line emission also varies on time scales of ∼ 15min and longer (Matt & Guainazzi 2003) Of the highly absorbed IGR sources, some (for example, IGRJ16138−4848) shows very strong emission lines, while others only show weak (or undetectable) line emission. The absorption column is also seen to vary, from observation to observation (e.g., IGRJ16318−4848: Revnivtsev 2003; IGR J16320−4751: Rodriguez et al. 2003;IGRJ : Rodriguez et al. 2005a)

X-ray property hard X-ray (cut-off; E cut >10 keV) power-law emission and some, or most of the time, strong and variable absorption. Four of them have been found to show (long-period; >4min) ‘pulse’ periods.  the highly absorbed IGR sources are HMXBs. If highly absorbed IGR sources indeed contain (slow) pulsars, their compact object is evidently a neutron star. If not ( for no identified pulsation sources), a black hole can not be excluded either. The location of the highly absorbed IGR sources also support the HMXB hypothesis. strong absorption in the X-ray domain  compact object embedded in a dense circumstellar envelope, originating from a dense stellar wind from the donor  This (relatively cold) envelope also serves as the source of the fluorescent emission, especially in IGRJ16318−4848 (e.g., Walter et al. 2003; Matt & Guainazzi 2003; Revnivtsev et al. 2003).

IR/optical property The near-IR excess in B[e] stars points to the presence of hot circumstellar dust. A few of the highly absorbed IGR sources have been identified in the optical and/or IR  supergiant B[e] donor present in a dense and absorbing circumstellar environment. The column density derived from the optical extinction is found to be one to two orders of magnitude less than that derived from the X-ray measurements (IGRJ16318−4848: Walter et al. 2003, Filliatre & Chaty 2004; IGRJ16320−4751:Rodriguez et al. 2003; IGRJ16465−4507: Smith 2004)  dense circumstellar envelope must be rather compact and concentrated towards the compact object (e.g., Revnivtsev et al. 2003).

Soft X-ray excess Soft X-ray excess emission between 0.3 and 5 keV has been reported for IGRJ16318−4848, as well in IGRJ16320−4751, IGR J16393−4643 ~ similar feature is seen in the soft X-ray spectrum of several other X-ray binaries (e.g., XTE J )  emission from a compact object which is strongly absorbed by a partionally ionised dense envelope.

Foreseeing The increasing sample size these objects represent should enable meaningful statistical studies to be performed. Understanding the nature of sources such as IGRJ16393−4643 could shed light on the structure of stellar winds Constraints on the masses of neutron stars help elucidate the evolution of binaries.

Conclusions The above described X-ray and optical/IR properties suggests that the highly absorbed IGR sources are HMXBs containing either a neutron star or black hole in orbit around a (super)giant donor. The stellar wind accreting onto the compact object could form a dense envelope in which absorption, fluoresence and ionization takes place. This circumstellar envelope does not seem to cover much of the (super)giant donor. These sources are concentrated along the galactic plane and in the spiral arms. Because of the wavelength window INTEGRAL is able to observe, we are now starting to find more of this previously poorly known class of sources.

Properties strong photoelectric absorption hard 2–10 keV spectra, non-thermal often display powerful fluorescent iron emission lines Most of them also show X–ray pulsations

Indeed, thanks to INTEGRAL “we can see clearly now...” (White 2004). THANK YOU