Science Requirements for Estremo

1. Science Requirements for Estremo X-RAY BURST PHYSICS • Detection of redshifted lines in X-ray bursts may provide information on NS mass and radius • discrete spectral features in a superburst from 4U1820-30 (6.4 keV line + abs edge) observed by RXTE • other indication for discrete lines may come from observation of deviation from pure blackbody spectra • XMM detection of redshifted lines from EXO 0748-676 (Cottam et al. 2002) • Superbursts are obvious candidates for detection by ESTREMO/WFM: ~1000 times longer (~h) and more energetic than normal X-ray bursts. Detected at least from 6 objects Lorenzo.Natalucci@iasf-roma.inaf.it ISTITUTO NAZIONALE DI ASTROFISICA-IASF Roma

2. Cottam, Paerels & Mendez: absorption lines in the burst spectra of EXO 0748-676 (28 events average, XMM/RGS)

3. Lines -> redshift -> determination of M/R ratio -> guess on the NS EOS Fe XXV, XXVI (n=2-3) and O VIII (n=1-2) with redshift z=0.35 -> in agreement with normal nuclear matter (some exotic models excluded) M = 1.4-1.8 Msun, R = 9-12 km Better NS atmosphere models (taking into account also burst effect) are expected

4. <- ‘intermediate’ bursts <- ‘superbursts’ (carbon) He bursts -> mixed H/He bursts ->

5. Looking for Bursts lines: 1) normal bursts: look at the longer bursts (eg GS 1826-238, KS 1731-2609) GS1826-238 is the best candidate + easy trigger + most frequent bursts (1/3h) + pretty long events (~120 s) + GS 1826 shows quasi-periodic bursts (other pointings possible between consecutive events) - need for fast slew (<20 s not to miss the first phase) - need for low energy sensitivity in WFGC (~2-3 keV)

6. Looking for Bursts lines: 2) superbursts: + very long bursts (hours, excellent statistics) + no real need for quick repointing (a few minutes is ok) + WFGC low energy threshold can be higher (4-5 keV) as more statistics available and integrating on larger timebins - tricky trigger, possibly manual (high persistent intensity sources) - rare events (1-2 per year for 5-10% Edd sources) - LEO gaps likely interfering with the burst light curve and overall statistics

7. Superburst profiles in ‘t Zand, Cornelisse & Cumming 2004 Strohmayer & Bildsten 2006

8. Science Requirements for Estremo FAST X-RAY TRANSIENTS (I) IGR J17544-2619 • New class of fast transients associated to HMXBs with early type supergiants: • short outbursts (~hour), fast rise (~minutes). Mainly absorbed sources (not easily seen in X-rays by monitor instruments), low quiescent luminosity (~ few 1032 erg/s?) • ~ 7 objects detected so far (INTEGRAL, RXTE). A given source may show outbursts after several months. There is clear indication for a large population • They add to the “classical” HMXB class associated to Be/X-ray binaries (mostly, transients with bright, long outbursts) (Negueruela et al. 2005) XTE J1739-302 = IGR J17391-3021 (Sunyaev et al. 2003) Lorenzo.Natalucci@iasf-roma.inaf.it ISTITUTO NAZIONALE DI ASTROFISICA-IASF Roma

9. Science Requirements for Estremo FAST X-RAY TRANSIENTS (II) • New class of fast transients associated to HMXBs with early type supergiants: • outburst most probably triggered by event during wind accretion • is this similar mechanism for giant outburst of e.g Cyg X-1, V4641Sgr? Cyg X-1 V4641 Sgr Unresolved issue is the mechanism for instability: is this intrinsic to the wind accretion or is that related to the capture of “clumpy” wind structures? Lorenzo.Natalucci@iasf-roma.inaf.it ISTITUTO NAZIONALE DI ASTROFISICA-IASF Roma

10. Science Requirements for Estremo FAST X-RAY TRANSIENTS (III) • Recent X-ray Observations by Chandra • IGR J17544-2619 (in’t Zand 2005) • “Lucky” observation: covers both quiescent phase and outburst. The author claims for NS type accretor, by spectral analysis of persistent emission • XTE J1739-302 (Smith et al. 2005) • foreground object (2-3 kpc) • Hard X-ray spectrum, well described by PL. Detected strong variability in absorption column IGR J17544-2619 Chandra light curve Observations of fast transients below 10 keV are useful to measure absorption and also for possible diagnostic of compact object nature Lorenzo.Natalucci@iasf-roma.inaf.it ISTITUTO NAZIONALE DI ASTROFISICA-IASF Roma

11. Rev#58 Rev#56 Rev#55 H1743-322 spectral variability Typical X-ray nova evolution Hard state during rise Intermediate (steep power-law) state Go to soft state (disk prominent) Return to hard state at the end, through intermediate state INTEGRAL pattern of hardness behaviour (Joinet et al., ApJ 2005; see also Capitanio et al., ApJ 2005)

12. Science Requirements for Estremo SPECTRAL STATE TRANSITIONS IN SOFT X-RAY TRANSIENTS • Study of the 2003 outburst from 4U1743-47 (Kalemci et al. 2004) • Evidence of state transition during decay phase (soft/intermediate/hard state) • Sharp changes associated to the soft to IS transition: (a) increase of PL flux on the time scale of hours, (b) sudden appearance of rms variability and QPO • The appearance of a radio jet seems to occur after LH state has been reached 4U 1743-47 ASM light curve Still unclear is the mechanism related to these state changes and the forming of the PL components (comptonization, synchrotron, feeding of electrons by jets, etc) and how do they interplay during transient outburst evolution? Lorenzo.Natalucci@iasf-roma.inaf.it ISTITUTO NAZIONALE DI ASTROFISICA-IASF Roma

13. Science Requirements for Estremo CONCLUSIONS • We identify three mostly interesting topics for ESTREMO: • Spectroscopy of X-ray bursts, in particular search for redshifted lines in superbursts and selected bursters (needs relatively fast repointing – 1min) • Study of fast X-ray transients population associated to HMXB Supergiant class (wind-accretion) and other selected objects. Needs relatively fast repointing (~few minutes) • Follow up of state transitions in Soft X-ray Transients (disk-accretion) , e.g. triggered by detection of sharp increase in the hard X-ray flux. Re-pointing should allow full broadband coverage and X-ray spectroscopic follow-up (relativistic iron lines, presence of absorbers/outflows) Lorenzo.Natalucci@iasf-roma.inaf.it ISTITUTO NAZIONALE DI ASTROFISICA-IASF Roma