Dear Colleague,
Most of us have learned about the recommendation by the ESA Senior Survey Committee for the two scientific themes for the next L-class missions. You may question how this effects the LOFT proposal since it is the intention of ESA to set the L-class themes first and then organize the M-class missions around this. The Senior Survey Committee has recommended the selection of two fundamental and exciting themes for the L2 and L3 mission slot: the Hot and Energetic Universe and the Gravitational Universe. These themes call for a large X-ray observatory for L2 and a gravitational wave observatory for L3. As the science goals and optimization of LOFT are, to a large extent, complementary to the selected L missions, we believe that there are compelling reasons to select LOFT for a launch in 2022 and capitalize on the expected breakthroughs of this mission prior to the L2 and L3 missions.
LOFT is proposed as a mission devoted to very clearly defined science topics, which GRANT a TRASFORMATIONAL ADVANCE in the field. Below we summarize some of the key differences and the relevance of LOFT as M3 mission.
Effective area for different missions, showing the transformational step by LOFT.

The two fundamental science topics at the core of the LOFT mission’s science case are the understanding of ultra dense matter and of strong field gravity. These two topics are relevant both to the physics and astrophysics communities, and are prioritized in the Cosmic Vision program. A prime window on the properties of ultra dense matter and strong field gravity is provided by the high-energy X-ray photons (2 – 30 keV) emitted by accreting neutron stars and black holes. For a full study of these two topics we need a very large area and a wide bandwidth. It is shown in the figure on the left how LOFT differs from existing or planned missions.

Mass/radius relation for a number of observations and models. Only with a reasonable sized sample it is feasible to recover the true nucleon-nucleon interaction model

Due to its very large collecting area (factor 15 higher than any other X-ray satellite) LOFT will reconstruct the Equation of State (EoS) of dense matter by measuring the mass/radius relation for 1) a large number (>10) neutron stars and 2) using different techniques to overcome any systematics. Other missions, not optimized for this goal, are expected to provide some constraints on 1-2 sources but not to reconstruct the EoS. Only the reconstruction of the EoS with different methods for a sizeable set of sources will answer the question how the strong force behaves at higher density than that of atomic nuclei. This is illustrated in the figure on the left where one can immediately see the difference between one or several M/R measurements and the necessity of a sufficient number of neutron stars to reconstruct the EoS.

Signal/noise in terms of 1Rg/c showing that LOFT can observe the space-time curvature down to 1 rg with good significance in 100 ks observation for galactic X-ray binaries and bright AGN. The shaded area show the differences as a function of area, the solid and dashed lines represent the effect of the background.
Strong field gravity plays a crucial role in astrophysics: phenomena such as supernovae, gamma ray bursts and the very existence of neutron stars and black holes are inherently related to extremely strong gravitational fields. Supermassive black hole feedback influences the evolution of galaxies and their nuclei. Strong field gravity is also of great importance to fundamental physics. To measure the effects of strong gravity in the stationary fields of neutron stars and black holes at time scales relevant to the orbital motion of matter in the closest vicinity of neutron stars and black holes, it is necessary to study the brightest sources (X-ray Binaries) with a 10 m2 high-throughput detector with CCD-class resolution. For this combination LOFT offers a factor of >200 improvement over previous instruments. Only satisfying these requirements we can reach the sensitivity necessary to study the effects of strong field gravity on accreting black holes with a sufficient resolution to map it down to a few Schwarzschild radii. The difference with existing and planned missions is shown in the figure on the left. Timing/spectral studies of the effects of strong field gravity near XRB will make a quantum leap ahead when LOFT is selected. Of course, the mission addressing the gravitational Universe will complement these studies in stationary fields by the study of dynamic space-times for merging compact objects.
LOFT research is timely and it is indeed possible to reach these unique breakthroughs by the first half of the next decade. LOFT is also complementary in time to the proposed themes for L2 and L3 with an anticipated launch date for LOFT of 2022.
Finally the argument that “the X-ray community has been served” with the selection of the hot and energetic Universe as theme for L2 is not a real issue. For the other M-class proposals, which all will provide exciting science, there are complementary missions under development by ESA or with ESA contributions, which have a more or less overlap with the M3 mission proposals in both science and communities (and closer in time). In this sense LOFT’s position after the selection of the themes for L2 and L3 has not significantly changed and it is certainly not unique. The exciting science addressed in the L2 and L3 themes does not make the LOFT mission less relevant and each mission will be evaluated on its own scientific merits and LOFT promises unprecedented returns for its core science goals.
The M3 mission selection context. Although the science of each of the M3 missions is unique, they all are part landscape where other missions will address some of the key science themes of the M3 proposals as well.
M3 Candidate
Science Theme
Approved Missions
CHEOPS (2017)
JWST (2018)(*)
TESS (2017)
JWST (2018)
SPICA (> 2022)
BepiColombo (2016)
Solar Orbiter (2017)
EXOMARS (2018)
JUICE (2022)
Hayabusa2 (2014)
Athena+ (2028)
ASTRO-H (2016)
Fundamental Physics
LISA PF (2015)
MICROSCOPE (2016)(**)
ACES (2017)(**)
eLISA (2034)
(*) ~M-class contribution by ESA
(**) Small ESA contribution
We like to conclude with the very positive and encouraging preliminary feedback coming from the ESA review of the LOFT Phase A study (PRR – Preliminary Requirements Review), indicating LOFT as a well defined and low risk mission.
Looking forward to the selection of this great mission!
The yellow book is now finished and will be made available shortly