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Super Giant X-Ray Binaries (SGXB) & Super-giant Fast X-Ray Transient (SFXT) different type of HMXBs. HMXB are X-Ray sources composed by young massive stars accreting on a compact companion (BH, NS or WD)
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HMXB are X-Ray sources composed by young massive stars accreting on a compact companion (BH, NS or WD)
A subclass contains an early type supergiant accreting via wind (SGXB): they are bright and persistent (12 known before Integral).
Integral detect new SGXBs, but highly absorbed and mixture of persistent and variable, with slow, weakly magnetised NS as companion
A new subclass of SGXB is emerging characterised by FAST TRANSIENT behaviour, namedSupergiant Fast X-Ray Transient (SFXT):
Sharp rise, complex time profile (multi peack) with absorbed spectra (lines?); fit compatible with pwl+cut-off or thermal breemsstrahlung, Lx~10exp36 at peack.
What are they?: possibly wind clamps on accretion disk in a wide orbit NS system to explain low emission during quiescence ~ 10 exp 32
The luminosity is very low when off and they are difficult to see also with XMM and Chandra
The spectrum is complex in X-ray and difficult to observe if not triggered by ASM or Integral becouse of thye short on-time
One shows recurrent outburst with perdiod of 330 days >>> orbit period?
Why ESTREMO: to catch and study the spectral and time evolution when on; periods of ON are from 1/2 hour to 31 hour:
Important is total absorption vs time >> accretion process physics
Line or absorption feature >> material of the wind, velocity and line width
Spectral shape and evolution with time is a signature of the accretion process: power law >> Jet, magnetic reconnection in disk?- plasma ejection?; thermal/non thermal Comptonisation >> electron population;
High energy tail >> disentangle between steep power law, thermal, comptonosation
IBIS 2nd catalogue lists 43 transient sources on 209 detections→ 25%
Contrary to BeppoSAX result:
The sample is based on 2 years monitoring, 10 Msec and 50% of sky coverage
Sources have average flux in between 1-200 mCrab in the 20-40 keV band
BeppoSAX/WFC monitored 36 active LMXB transients with
peak in between 10-12000mCrab,
duration 0.1-1700 d
on 6 years monitoring of the sky within 20 degree from G.C. for a total of 6 Msec
Differences with BeppoSAX are due either to energy band & source nature
(highly absorbed) and to sky coverage
Discovery of transient source with Be companion was expected since they represent the
larger class of HMXB in the Galaxy.
INTEGRAL with the GPS regular scans and instrument characteristic has discovered this 2
other kinds of HMXB with supergiant companion missed by previous Mission because of Energy range/sensitivity
Association with High Mass X-Ray Binary is due to either
identification at optical/infrared wawelenghts with blu supergiant or Be star
based on their X-ray properties typical of HMXB e.g. periodic pulsation or hard x-γ spectra
Supergiant HMXB (SGXBs) are believed to be rare objects because of very short lifetime of stars in supergiant phase
only ~ 12 have been discovered so far associated to persistent and bright X-ray sources INTEGRAL is now detecting a new subclass of SGXBs labelled as
Supergiant Fast X-ray transient (SFXT) characterized by outbursts lasting less that a day, typically few hours
Outbursts from 10sources detected so far, 4 recently optically identified as SGXRB
6still unclassified but sharing similarities
IGR J 11215-5952, identified as supergiant, archival data show recurrent outburst spaced by intervals of 330 days !!!one of the few covered also by X-ray data
5-100 keV spectrum well described by a cut-off power law (α=0.5), cut-off energy
~ 15-20 KeV, luminosity ~3x 10 36 erg/s. Observed with RXTE as outburst exepected 15-18 march giving p=329 day!! PCA data spectrum in the 2.5-15 keV consistent with either a bhc in hard state or accreting puòlsar , NH =(11±3) x 10 22 cm-2
IGR J 16195-4945, 3outbursts with duration0.5-1.5 h and peak 35 mCrab
IGR J 16418-4532 2 outburst duration 1h and peak at 80 mCrab
IGR J 16479-4514 8 outbursts, durtaion 0.5-3.5 h , peak 60-850 mCrab
IGR J 17391-3021=XTE 1739-302, 6 outburst with duration 0.5-14h
peak flux 60-450 mCrab
IGR J 17407-28081 outburst alsting 3 minutes and peak 800 mCrab
IGR J 1749.1-27331 outburst with duration 31 h and peak 40 mCrab
IGR J 17544-2619 5 outburst with duration0.5-10h
peak flux 70-400 mCrab
IGR J 18410-0535=AX1814.1-0536, 3 outburst with duration 1.1-7 h 120 mCrab
X-ray simultaenous observation are missed most of the time
Most X-ray transients, wheter in HM or LM X-ray binaries display outbursts lasting from a few weeks to months. Such durations are compatible with viscous timescaless in a typical accretion disc.
Fast outbursts must be due to a completly different mechanism.
Now we showed that at least a fraction of the recurrent fast transient are associated with luminous OB stars, suggesting the mass transfer mechanism feeding the accretion compact object is a radiative wind.
We remind that not every sources showing short X-ray outburst may be assigned to SFXT:
Flare stars, RS CVn binaries and superbursts in LMXB have similar duration , thogh rather different luminosity and lightcurve.