LOFT PRIMARY SCIENCE GOALS
Neutron stars and black holes possess the strongest gravitational fields in the universe. They provide unique opportunities to reveal for the first time a variety of general relativistic effects and thereby investigate gravity in the strong-field regime, and to measure fundamental parameters of collapsed objects gaining important insights into the physics of matter at supranuclear densities and in supercritical magnetic fields.
LOFT is specifically designed to exploit the diagnostics of very rapid X-ray flux and spectral variability (already known to exist) that directly probe the motion of matter down to distances very close to black holes and neutron stars.
Its factor of ~20 larger effective area than RXTE’s PCA (the largest area X-ray instrument ever flown) is crucial in this respect. LOFT/LAD’s much improved energy resolution (better than 260 eV) compared to that of RXTE/PCA will also allow the simultaneous exploitation of spectral diagnostics, in particular the relativistically broadened 6-7 keV Fe-K lines. The timescales that LOFT will investigate range from submillisecond quasi-periodic oscillations (QPOs) to years long transient outbursts, and the relevant objects include many that flare up and change state unpredictably, so relatively long observations, flexible scheduling and continuous monitoring of the X-ray sky are essential elements for success.
LOFT’s Wide Field Monitor (WFM) will discover and localise X-ray transients and impulsive events and monitor spectral state changes with unprecedented sensitivity, triggering follow-up pointed observations and constituting an important resource in its own right.
THE NEUTRON STAR EQUATION OF STATE
Understanding the properties of ultradense matter and determining its equation of state (EOS) is one of the most challenging problems in contemporary physics. At densities exceeding that of atomic nuclei, exotic states of matter such as B ose condensates or hyperons may appear; a phase transition to strange quark matter may take place at higher still densities. Only neutron stars probe these densities in the ‘zero’ temperature regime relevant to these transitions. Very “soft” EOSs give a maximum neutron star mass in the 1.4-1.5 solar mass (Mo) range, whereas “stiff” EOSs can reach up to 2.4-2.5 Mo before collapse to a black hole becomes unavoidable. Apart from maximum mass, the relation between the neutron star mass and radius (M-R) is a powerful probe of the EOS. With the exception of redshifts of any narrow atmospheric lines (feasible for slowly rotating stars only), all tools devised to constrain mass-radius are based primarily on accurate time-resolved and high-throughput broadband spectral measurements. In ~25 neutron star s , s pi ns are now observed in burst oscillations and/or coherent pulsations at frequencies of up to 620 Hz, proving that millisecond spins and dynamically relevant magnetic fields are common among neutron stars in low-mass X-ray binaries. LOFT will measure the masses and radii of accreting millisecond pulsars to an accuracy of 5% by modelling their pulse profiles: their fast spin and strong gravity affect the radiation from the surface hot spots producing the pulsations through relativistic beaming, time dilation, red/blue-shifts, light bending and frame dragging (hence they provide an alternative probe of strong field gravity effects as well). LOFT will be able to cross-validate these results by flux and spectral modelling at photospheric touch-down of radius-expansion thermonuclear X-ray bursts. Since the maximum rotation a neutron star can sustain depends on its mass and structure, fastest spin periods also constrain the neutron star EOS.
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Mass-radius curves of representative EOSs for nucleonic (blue), nucleonic plus exotic (pink), and strange quark (green) matter. Horizontal bars represent mass determinations of binary radio pulsars. Regions to the upper left are disallowed by General Relativity and causality. The lower right region is excluded by the highest-frequency pulsar known (716 Hz). We simulated the pulse profile rising phase of a Type I X-ray burst in a way similar to Strohmayer (2004 AIPC 714, 245) The derived 90% confidence level limits on the neutron star M and R from observations with LOFT are shown by the green ellipse. The uncertainty is <5% on both M and R. The M-R constraint derived from fitting simulated pulse profiles of SAX J1808.4- 3658, the first millisecond X-ray pulsar discovered (401 Hz), are shown by a gray ellipse. The same technique applied to simulations of the signal of the first eclipsing millisecond X-ray pulsars, Swift J1749.4-2807 (518 Hz), gives in a much smaller uncertainty region (red); this is mainly due to the higher system inclination |
LOFT will search for periodic signals (and QPOs as well) with unprecedented sensitivity. Models indicate that the pulsation amplitude in fast spinning neutron stars in X-ray binaries should be as low as 0.1%, so the effective area of LOFT/LAD is needed to detect these pulsations in a typical 104 s observation of a 100 mCrab source. The recent discoveries of a few intermittent X-ray pulsars, with small pulse amplitudes over short periods of time, indicate that it should be possible to build up a much better spin period distribution for accreting neutron stars than has been possible with RXTE. These intermittent pulsations were also very hard, underscoring the need for a timing mission with a good hard X-ray response. LAD can search for them with an unprecedented sensitivity of 0.5-0.8% amplitude in 100 s (for 100-300 mCrab sources). A different approach has recently emerged from the discove ry of global seismic oscillations (GSOs) in the tens of Hz to kHz range from magnetars during the rare and extremely luminous giant flares emitted by these sources. The lower frequency GSOs likely arise from torsional shear oscillations of the crust and their frequency, in combination with the magnetic field inferred from the magnetar spin-down, tightly constrains the EOS. The identification of an n = 1 radial overtone allows to estimate crust thickness. LOFT will detect and study GSOs for the first time in ‘intermediate’ flares, which are tens of times more frequent than giant ones, down to amplitudes of 0.7%, an order of magnitude lower than seen up to now. This will open a new window in the study of neutron star structure through asteroseismology.
STRONG GRAVITATIONAL FIELDS
About 40 compact objects accreting matter in binaries are now known to display variability arising in, and occurring at the (millisecond) dynamical timescale of their inner accretion flows: black holes and neutron stars, respectively, show QPOs of up to 450 and 1250 Hz. These QPOs require an explanation that involves the fundamental frequencies of the motion of matter in the inner, strong-field gravity-dominated disk regions.
In the absence of sufficient guidance from observations, modelling has so far been to a large extent phenomenological, and different interpretations are still viable. For example, competing models variously identify observed QPOs with therelativistic radial and vertical epicyclic frequencies or relativistic nodal and periastron precession.
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Corresponding evolution for the relativistic precession model. The curves correspond to: 1 Crab, fractional rms 2.8% and 6.2% (blue), 400 mCrab, fractional rms 1.4% and 3.1% (green), 300 mCrab, fractional rms 0.7% and 1.5%. The exposure is 16 ks in all cases. |
LOFT simulations for black hole high-frequency QPOs. Left panel: prediction of the evolution within the epicyclic resonance model. |
Very high-signal-to-noise LOFT/LAD measurements of the QPOs will unambiguously discriminate between such interpretations and in the process tease out yet untested general relativistic effects such as frame dragging, strong-field periastron precession, and the presence of an innermost stable orbit. Crucially, LOFT will provide access for the first time to types of information in these signals that are qualitatively new due to the capability to measure dynamical timescale phenomena within their coherence time, where so far only statistical averages of signals were accessible. This will allow studies that directly witness QPO formation and propagation and tie in with what state-of-the-art numerical work is just beginning to address.
LOFT will allow direct measurements of the black hole mass and spin through timing measurements, to compare with other estimates such as mass from optical studies or spin from the thermal X-ray continuum or the Fe K-line profile. The spectral capabilities of LOFT will allow use of the energy dependence of amplitudes and phase delays in the QPOs together with the Fe-K line profiles to measure the compact object’s mass and spin, the disk inclination and to study massive black holes in the brightest active galactic nuclei (AGNs) by measuring with unprecedented accuracy the profiles and variability of their Fe K-lines. Additionally, LOFT's good response to higher-energy X-rays is crucial for most of this work; the discoveries of the highest-frequency quasi-periodic oscillations from black holes – the ones which can be used to probe the mass and spins of the black holes were made only above 13 keV, despite the much higher count rates at lower energies. Similarly, reliable measurements of Fe lines can only be made when both the Fe spectral edges around 8-9 keV and thecontinuum at energies significantly higher than these edge energies can be well measured.
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Simulated LOFT/LAD spectra of a 3 mCrab (2-10 keV) AGN, such as e.g. MGC 6-30 |
ADDITIONAL SCIENCE TOPICS
LOFT will additionally be a powerful tool for studying the X-ray variability and spectra of a wide range of objects, from accreting pulsars and bursters, to magnetar candidates (Anomalous X-ray Pulsars and Soft Gamma Repeaters), cataclysmic variables, bright AGNs, X-ray transients and the early afterglows of Gamma Ray Bursts.
Through these studies it will be possible to address a variety of problems in the physics of these objects. Coordinated optical/NIR and radio campaigns on specific themes, as well as spin measurements which can aid the Advanced Virgo/LIGO searches for gravitational wave signals from fast rotating neutron stars will add great value to the LOFT program.