Proposal Details - 0820033
Variability of the cyclotron line energy E_cyc in Hercules X-1 (Staubert)

Proposal Summary (based on proposal abstract)

It is proposed to observe Her X-1 during INTEGRAL's AO-8 for a total observing time of 600 ksec: covering one "Main-On'' with two INTEGRAL orbits,the first near the maximum of the "Main-On'', the second during its decay, plus one INTEGRAL orbit during the following "Short-On". The prime objectives are to systematically measure the centroid energy of the fundamental cyclotron line E_cyc as a function of time, X-ray flux, 35 day phase and 1.24 s phase and to study the variation of the shape of the pulse profile with 35 day phase. These observations will contribute to answering the following questions: Is there a slow secular decrease in the value of E_cyc with time? Does E_cyc depend on 35 day phase? How stable is the positive correlation of the value of E_cyc with the X-ray luminosity? Does the neutron star itself really show precession? The results will yield information about fundamental physics in this disk-accreting binary pulsar with respect to: the structure of the magnetic field at the polar caps, the physics of sub-Eddington accretion, the generation of spectra and pulse profiles.

Observation Strategy (Recommended by TAC)

Total of 600 ks split into three observations of 200 ks each in the standard 5x5 dithering mode centered on RA, Dec = 254.458, 35.342 (l, b = 58.149, 37.523). The observations will take place at specific times to cover distinct pulse phases with three INTEGRAL orbits: the first near the early maximum of the "Main-On" (phases 0.05 to 0.12 of the 35-day cycle), the next one during the decay of the same "Main-On" period (phases 0.18 to 0.25 of the 35-day cycle) and, finally, one INTEGRAL orbit during the following "Short-On" (phases 0.60 to 0.70 of the 35-day cycle). The exact observing times are not defined yet since "Main-Ons" appear with a somewhat irregular clock. The timing of the observations will be predicted on the basis of RXTE/ASM, SWIFT/BAT and MAXI observations (with a confidence of ±1 day two months before), and more reliable one month before the actual INTEGRAL observations (between April-June and October-December 2011).

Proposal grade: A

Data Rights (Recommended by TAC)

The TAC has allocated the following specific Data Rights to this proposal: Her X-1.

Exposure Map

Exposure maps are in units of s and shown using Galactic coordinates. The intensity scale is given by the colourbar, and there are contour levels at 100, 300 and 500 ks.