Updates: 04.05.2012 (Tolga)
The future of X-ray astronomy does not look very bright for almost a decade, as there is no new observatory class X-ray satellite in the foreseeable future. However, this does not mean there won't be any X-ray/gamma-ray missions, in fact there will be many of them but in smaller scales. These smaller scale missions will, of course, actually need more cooperation and follow-up observations. Furthermore most of them (including some of the currently operating ones) will always have open data access.
A list and brief descriptions of X-ray/gamma-ray missions, which the author thinks will create follow-up observation opportunities for a ground based optical/NIR 4m class telescope are given below. The list is based on planned or scheduled approximate launch dates:
Swift has been operating since November 2004. Swift has a complement of three co-aligned instruments: the Burst Alert Telescope (BAT), the Xray Telescope (XRT), and the Ultraviolet/Optical Telescope (UVOT). The whole purpose of the largest instrument, BAT, on SWIFT is to monitor almost a sixth of the entire sky to detect transient events, often GRBs it is sensitive to 15-150 keV. After the detection SWIFT will autonomously repoint itself to aim the XRT and UVOT at the transient event to obtain high-precision X-ray and optical positions and X-ray spectra to be determined. This satellite has been acting as an all sky monitor since its launch and creating many opportunities for ground based telescopes. All of the data this satellite obtains is open and it almost always needs further follow-up observations by ground based telescopes (see notes on GRBs or notes on the near-infrared observations of Galactic black hole candidates). Finally, it is seen as one of the most successful missions by NASA (by means of its effectiveness) therefore it will be operating for quite a long time, depending on the health of the satellite. It is also the only "all-sky" monitor at these energies at the moment.
Fermi is a gamma-ray mission launched in 2008. It has two main detectors. LAT (Large Area Telescope) is a gamma-ray detector with a wide field of view (one-fifth of the whole sky) and is approximately sensitive to the energy range 30 MeV - 300 GeV. Again all of the data this satellite obtains is public immediately and this satellite has also been creating many opportunities for ground based telescopes some examples are of course monitoring of active galaxies, follow-up observations of GRBs etc.
nuSTAR will be the first imaging telescope at hard X-rays. Its launch date is planned as this summer. The telescope will be sensitive to 5-80 keV range and will have a focal length of 10 m yielding images with FWHM 10 arc seconds. The field of view will be about 13x13 arc minutes so this will not be an "all-sky" monitor. However with its imaging capability and long exposures it will be able observe a number of obscured AGN, which are hard to detect in the soft X-rays. Again all of the data this satellite will obtain will be public immediately.
SRG is a German-Russian mission with an ambitious gaol of surveying the whole sky in the soft (with eROSITA) and hard (ART-XC) X-rays for about 4 years. The aim is to detect the hot intergalactic medium of 50-100 thousand galaxy clusters, to detect systematically all obscured accreting Black Holes in nearby galaxies and many (up to 3 Million) new, distant active galactic nuclei and to study the Galactic X-ray source populations. The word detect is important because often this is what this satellite will only be able to do. So it will create many great opportunities for ground based telescopes. In fact they are planning to use RTT150 as one of their main telescopes to obtain low resolution optical spectra of some of the objects SRG detects. However the data policy is not very clear and we may need to wait several years to reach their catalog of sources.
LOFT is a proposed mission to ESA with a planned launch date somewhere in 2020s. The plan is to have a detector that has about 12m2 photon collecting area @ 8 keV, which is about a factor of 20 larger than RXTE PCA. Basically it is being planned as the almost ultimate X-ray timing mission for observations of Galactic neutron stars, black-holes, X-ray binaries and some extragalactic sources. Clearly it will detect a number of new sources and will create many opportunities for ground based observations.
All of the Galactic or extra-galactic high energy sources also emit in optical and near-infrared wavelengths and it will only be possible to understand the nature of these sources by performing observations in coordination with X-ray / gamma-ray satellites and cover the wides possible wavelength range. Some example projects have already been given in this document (see the discussion about the gamma-ray bursts or the Galactic black hole transients), more examples can be given. However, in order for a 4m class telescope to be successful in these observations it should have a number important capabilities.