About NHXM

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About NHXM

The New Hard X-ray Imaging and Polarimetric Satellite Mission has a high spatial and energy resolution over a very broad energy band, from 0.2 to 80 keV, together with a sensitive X-ray imaging polarimetric capability.

It is based on four identical mirror modules that, for the first time, will extend from 0.2 keV up to 80 keV the fine imaging capability today available only at E<10 keV. At the focus of three telescopes there will be three identical spectro-imaging cameras, at the focus of the fourth the X-ray polarimeter. The addition of a Wide Field X-Ray Monitor, sensitive in the band 2-50 keV, will permit also to detect highly variable sources (e.g. GRB, soft-Gamma Ray repeaters, transient sources like CV, novae, binary sources, or relativistically boosted blazars, etc.).

For the first time, with this instrument complement it will be possible to obtain simultaneously images, spectral energy distributions and polarization properties (degree and angle) of sources in the X-ray sky, including when they are in an active state.

NHXM will provide a real breakthrough on a number of hot astrophysical topics, by exploiting the most advanced technology in grazing incidence mirrors, detectors and in polarimeters. These topics do broadly fall under two main headings: (i) Censing the black holes in the Universe and probing the physics of accretion in the most diverse conditions. (ii) Investigating the particle acceleration mechanisms at work in different contexts, and the effects of radiative transfer in highly magnetized plasmas and strong gravitational fields.

The scientific case

Science with NHXM

Taking advantage of new technology in mirror and detector manufacturing, NHXM will push grazing incidence imaging up to ~80 keV and will open a brand new window for accreting sources and non-thermal sources and for X-ray polarimetry. NHXM will benefit on one side of high-resolution mirrors with multilayer coating, providing an improvement of roughly three orders of magnitude in sensitivity and angular resolution compared to all instruments that have operated so far above 10 keV (see Figure 1). On the other side, the new development of imaging photoelectric polarimetric detectors provides an increase from 2 to 3 orders of magnitude in X-ray polarimetry sensitivity, with respect to conventional instrumentation. These technological breakthroughs will allow to access a huge discovery space in two large areas of astrophysics and cosmology:

  • Probing black hole census and accretion physics;
  • Acceleration mechanisms and radiative transfer in strong electromagnetic and gravitational fields. These two broad topics define the core scientific objectives of NHXM.
  • File:NHXM_sensitivity.png

    Fig. 1 NHXM sensitivity as function of energy compared with current and previous missions. At low energies the two lines are for two different operating modes of the low energy detector. Clearly, NHXM is orders of magnitude more sensitive than non-focusing instruments.

    Black hole census and accretion physics

    Because of the tight links between galaxy bulges and their central Supermassive Black Hole (SMBH), it is crucial, for our understanding of the formation and evolution of galaxies and their nuclei, to obtain a complete and unbiased census of SMBH. This has to be performed by means of direct observations at the energies where the Cosmic X-Ray Background (CXB) energy density peaks. CXB synthesis models predict a large number of heavily obscured AGN: the Compton thick (CT) AGN, objects with an obscuring screen of gas and dust suppressing the nuclear radiation by photoelectric absorption and Compton recoil (see cartoon in Figure 2).

    File:AGN_cartoon_fig2.png

    Fig. 2 A cartoon of the inner region of Active Galactic Nuclei (adapted from Urry & Padovani 1995).

    The problem is that the population of CT AGN is so far loosely constrained. A blind search for highly obscured AGN among serendipitous X-ray sources at energies <10keV is inefficient, even by using ultra-deep exposures, meaning that all present X-ray surveys miss most of the highly obscured, but still strongly accreting objects. The few tens CT objects discovered so far by BeppoSAX, INTEGRAL and Swift at low redshift, and by Chandra and XMM-Newton at moderate redshift, probably represent just the tip of the iceberg of the CT AGN population. Only high sensitivity, hard X-ray observations like those that NHXM will be able to perform up to 70-80 keV, will allow to pinpoint and study the sources that make the bulk of the CXB. The measurement of their obscuration will allow assessing the real power of the active nucleus. NHXM will complete the AGN census as a function of both luminosity and redshift and provide a comprehensive picture of the SMBH growth in the universe.

    NHXM broadband X-ray spectroscopy capability will then allow, for these objects, a tight constraint on the Compton reflection component (see Figure 3). NHXM will break the degeneracy between AGN spectral shape and its dependence from luminosity and epoch, also determining the CT AGN contribution to the CXB.

    File:simulation_small_fig3.png

    Fig. 3 Simulated 100 ks NHXM observation of the highly obscured AGN, IRAS19254-7245 (an Ultra- Luminous Infrared Galaxy) using the spectrum detected up to 30 keV by Suzaku. Note the complexity of the spectrum below ~7 keV, which includes contributions from thermal gas, reflection of cold matter, iron fluorescence lines and absorption edges, plus scattered AGN continuum. To properly identify and disentangle all these components it is mandatory to have good sensitivity below and above this energy value, as in the case for NHXM.

    At the same time polarimetric observations will provide constraints on the geometry and orientation of the circumnuclear matter responsible for scattering and absorption in all AGN (the torus), and that of the region responsible for the scattering and polarization of the optical radiation (see Figure 2).

    Finally, while past and present generation of X-ray observatories allowed spectacular advancements in our understanding of the accretion physics and the development of a standard model for both extragalactic and galactic accreting compact objects, many basic questions are still today unsolved. The broadband spectroscopy and polarimetry capabilities of NHXM will give us the clue to attack the most important of these topics such as the nature of primary X-ray emission, General Relativity (GR) and relativistic accretion disks and the presence of massive and high velocity outflows in several radio quiet AGN, sometime with an ejection rate comparable to the accretion rate.

    Acceleration mechanisms

    A huge amount of observations confirm that electron acceleration is a ubiquitous process in an incredible number of different astrophysical sites, from the size of a neutron star (~10km) to the scales of jets and clusters of galaxies (Mpc). Nevertheless, the answers to fundamental questions are still lacking about the formation of jets in galactic and extragalactic BHs, the energy transport in these jets and the details of the jet emission mechanisms, the acceleration of cosmic rays, the acceleration mechanisms and radiative transfer in the strong electromagnetic and gravitational fields of pulsars and magnetars, the acceleration of particles in the intracluster medium. Sensitive broadband imaging, spectroscopy and polarimetry in the X-ray band can provide breakthroughs on all these problems.

    The variable X-ray sky

    It is well known that the hard X-ray sky is highly variable (GRB, soft-Gamma Ray repeaters, transient sources like CV, novae, binary sources, or relativistically boosted blazars,tc.). Observing these sources at different state of activity allow us to test the parameters of the various models in a much larger range, testing them in extreme conditions. Often this is the only way to understand the underlying physics.

    NHXM in the context

    The next decade will see an exquisite suit of multiwavelength facilities, including JWST, ALMA, LOFAR and maybe the first ELTs. They will provide unprecedented capabilities to explore the high-redshift Universe, from the time of formation of galaxies and QSOs to the golden age of galaxy and QSO activity at z=1÷3, where a large fraction of the CXB is likely produced. At the same time, there will probably be two missions carrying high-energy X-ray imagers, the NASA mission NuSTAR and the JAXA mission Astro-H. NuStar, a NASA SMEX mission, is aiming to a launch date within 2012; while the JAXA Astro-H mission, originally NeXT, is currently for a launch by 2014. Moreover, NASA has selected the SMEX mission GEMS, a high sensitivity photoelectric polarimeter with no imaging capability (see below). NHXM will arrive after these missions, or with some overlap, therefore a comparison is mandatory.

    The case of NuSTAR and Astro-H: Table 1 compares the main characteristics of NHXM against those of NuSTAR and ASTRO-H. Note that for NuSTAR the nominal value of HPD<60 arcsec as declared by the mission team is given, but probably the real value will be worse. In any case for our calculations below we used the declared value.

    The collecting areas of the three missions in the 10-60 keV range are comparable. Unlike NuStar, NHXM and Astro-H maintain a large collecting area below 10 keV, down to 0.5 keV, allowing broadband spectroscopy. The field of view of Astro-H is a factor of two smaller than that of NHXM and NuStar. But, above all, NHXM has by far better imaging capabilities with respect to NuStar and Astro-H. NHXM will provide images 10-20 times sharper than the competitors, therefore reaching much deeper sensitivity limits (a factor of 5 and 10 lower, respectively, see Table 1 and Figure 4).

    Table 1: Key performance parameters comparison between NHXM and the two competitors in the coming decade: NuSTAR and Astro-H. See text for details.

    NHXM NuSTAR Astro-H
    # of Telescopes 3+1 2 2
    Energy Band (keV) 0.5 - 80 7 - 80 0.5 - 80
    Effective area (cm2) at 30 keV 350 300 320
    Effective area (cm2) at 5 keV 1000 0 500
    Orbit inclination Low equatorial Low equatorial Low equatorial
    Inclination (deg) < 5 6 30
    Focal Length (meters) 10 10.14 12
    Field of View Diameter (arcmin) 12 12 9
    Hal Power Diameter (arcsec at 30 keV) 20 60 100
    10-40 keV flux sensitivity at confusion limit (erg cm-2 s-1) 3x10(-15) 2x10(-14) 4x10(-14)
    Sources per field 40 6 1

    Assuming 5 Ms of total exposure time dedicated to this scientific topic for the three missions, and by using the predicted 10-40 keV number counts, about 100 sources are predicted in the NHXM and NuStar surveys and between 30 and 40 sources in the Astro-H survey. However, while NHXM will resolve a CXB fraction as high as 70%, before reaching its confusion limit, NuSTAR and Astro-H will resolve only ~20% of the CXB. Therefore, while these two missions will begin to tackle the issue of directly identifying the sources that produce the CXB at its peak energy, only NHXM has the power to solve this crucial and long-standing issue. File:3sigma_sensitivity.jpg

    Fig. 4 the 10-40 keV 3sigma sensitivity of NHXM (purple curve), NuStar (blue curve) and Astro-H (red curve) as a function of the exposure time. The horizontal dashed lines represent the confusion limit for the three missions, computed assuming 30 beams per source.

    NHXM will be able to obtain broadband spectroscopy of the faint sources making the bulk of the CXB, thanks to its good sensitivity between 0.5 and 80 keV. Note that NuStar will not be able to probe the photoelectric cut-off and the iron line of highly obscured AGN, because its low energy threshold that is above 6 keV will thwart the characterization of both the 2-10 keV continuum and the iron complex. Astro-H will not have the latter limitation, but due to the limited image quality of its mirrors it will be confusion limited at low energy even for relatively bright hard x-ray sources.

    Figure 3 clearly shows the power of NHXM in studying the broadband X-ray spectrum of these sources.

    In conclusion, NuStar and Astro-H will contribute in exploring the high energy Universe, in particular for what regards spectroscopy of relatively bright sources. NuStar will presumably be the first of these mission to fly and will certainly open up a wide discovery space. However, the full exploitation of this crucial energy band must await the launch of NHXM, thanks to the quality of its mirror, high sensitivity over a wider energy band and sensitive imaging-polarimetric capability. This will help in keeping the high-energy astrophysics topics at the forefront of astrophysics in the JWST and ELTs era.

    The case of GEMS: the recent approvals by NASA of the SMEX mission GEMS (Gravity and Extreme Magnetism SMEX mission) is the witness of the high interest of this topic in the international community. GEMS is a pathfinder mission completely devoted to polarimetry but is not an imaging instrument and everything that falls inside the 14 arcmin FOV is measured together. The lack of imaging capability is a strong limitation for GEMS. For instance, the X-ray emission of the Crab nebula is highly structured, as shown by the wonderful Chandra images, with a torusplus- jet geometry (see Fig. 5). Detailed and space-resolved polarization measurements by NHXM will allow to study the dynamics of the plasma in this and other nebulae and the acceleration mechanism of the pulsar, which is able to convert the large part of its rotational energy in accelerating particles which eventually shine in the nebula by synchrotron emission.

    File:crab_nebula.png

    Figure 5: X-ray emission of the Crab nebula is highly structured, with a torusplus-jet geometry. Detailed space resolved polarization measurements by NHXM (the white spot shows the spatial resolution element of NHXM) will allow the study of the plasma dynamics in the nebula and the acceleration mechanism of the pulsar. For comparison the NASA mission GEMS will mediate everything falling within a bin of 14 arcminutes, much larger than the all figure.

    The better control of systematic, together with the imaging capabilities, allow for deep NHXM polarimetric observations both on single sources, as well as on crowded fields and extended objects. All this will not be possible with GEMS. Moreover, the extension of polarimetry to higher energies is foreseen for NHXM. With an Ar based mixture a high sensitivity in the 6 – 35 keV range is expected. In this range NHXM can cover a wide choice of topics of physics and astrophysics not covered by GEMS (which is sensitive only below 10 keV). For instance cyclotron lines, reflection bumps, hard tails, with a large overlap with major targets of the hard X-ray telescopes. In this range NHXM will be in any sense unique (even compared to IXO). Figure 6 shows the sensitivity of NHXM for the softer and harder X-ray bands. In conclusion, while the selection of the SMEX mission GEMS by NASA shows the relevance of the X-ray polarization science, NHXM will do much better in exploiting this topic so far precluded to astronomy.

    File:MDP_2-10.png


    File:MDP_8-35.png

    Figura 6: NHXM Minimum Detectable Polarisation (MDP) as a function of flux for the softer (2-10 keV) and harder (6-35 keV) energy bands. The MDP for some well-known sources are also plotted.

    Scientific payload Configuration

    Mirror Modules and Auxiliary Items

    The feasibility study performed in Phase A

    The NHXM mirrors will be electroformed NiCo shells with Wolter I profile. This technology has been developed and consolidated in the past two decades in Italy by the INAF Brera Astronomical Observatory in collaboration with the Media Lario Technology Company. For the NHXM mirrors, a few important modifications are foreseen:

    • 1) the use of multilayer reflecting coatings, allowing us to obtain a larger FOV and an operative range up to 80 keV and beyond;
    • 2) the NiCo walls will be a factor of two thinner than the XMM Nimirror shells, to maintain the weight as low as possible.

    Monolithic pseudo cylindrical shells have been developed at Media Lario, where a multilayer coating facility has been developed and installed. Engineering models with two integrated shell have already been developed and tested at the Panter-MPE X-ray calibration facility. Figure 7 shows two images taken at 0.93 keV and in the band 30-50 keV. Note how the image quality is extremely good also at the higher energies. The structure of the spider arms can be seen. The HEW at 0.93 keV is about 18 arcsec, while that one at 30 keV is ~25 arcsec, not very far away from the requirements.

    In summary the Simbol-X phase A study has shown that mirrors that fully satisfy the Scientific requirements of the NHXM can be developed.

    We are now evaluating also the possibility to use a different multilayer for the inner shells, a multilayer based on Ni/C instead of Pt/C, in order to improve the response around 80 keV, extending it up to 120 keV.

    File:Image_quality.png

    Figure 7: X-ray point source image quality already achieved with a prototype with two mirror shells at the Panter-MPE facility: (left) HEW=18 at ~1 keV; (right) HEW=25 in the band 20-50 keV.

    The Payload

    NHXM will operate in a circular, low inclination orbit at 600-km mean altitude, that provides a very low background, as proven by the BeppoSAX and Swift missions. It will host 4 X-ray telescopes with at their focus either a focal plane camera (for three of them) hosting a combination of two detectors sensitive from 0.5 up to 80 keV or two interchangeable polarimeter cameras covering the range 2-35 keV at the fourth telescope. The four MM will achieve a net area comparable or higher to that one of the Simbol-X single mirror module (MM), with the following advantages: i) each MM is smaller and lighter; ii) segmentation in four telescopes allows to implement a redundancy policy; iii) it is possible to implement polarimetric cameras in one of the telescopes.

    The mirror optical design

    Longer focal length ensures that shells with larger diameter will efficiently reflect high-energy photons. If we decrease the focal length then only smaller diameter shell will efficiently focus high-energy photons, implying a smaller effective area. Therefore, to reach the same values we need to add more MM. There are advantages and disadvantages in this solution of course. From the scientific side a big advantage is that we can add polarimetric capability to one of the telescopes. From the mirrors point of view the advantage is that we can replicate more shells from the same mandrels, minimising the number of mandrels and making them smaller. Being the fabrication of the mandrels one of the most critical items in the mirror production, these are both big advantages. From the detector point of view, the advantage is redundancy, if one detector fails we can recover this by simply extending the exposure time. The disadvantage being that we need to develop more than one detector of course. The current baseline of the NHXM optical design foresee 70 shells for each MM with a diameter ranging from 15.5 to 39 cm. The effective area of a single MM is reported in Fig. 8.

    We are now also studying the possibility to include smaller shells with a different multilayer recipe, based on Ni/C bilayers instead of Pt/C, in order to increase the effective area around 80 keV and to extend it up to 120 keV.

    File:effective_area.png

    Fig. 8 Effective area of one single NHXM mirror module.

    The focal plane spectro-imaging camera

    Three spectro-imaging cameras have to match the high-energy X-ray multilayer optics performance over the broad 0.5 - 80 keV energy range, providing a very low background environment. A hybrid detector systems will be used with two detection layers, plus an effective anticoincidence system:

    • the Low Energy Detector (LED), with the role to detect with a high quantum efficiency and energy resolution the soft X-ray photons between 0.5 and 1o keV, placed on the top of the focal plane assembly. The baseline configuration foresees a CCD from e2V with 120 micron depletion well;
    • the High Energy Detector (HED), placed in series beyond the LED, with the role to detect with a high quantum efficiency and energy resolution the hard X-ray photons in the 7-120 keV band. It will be formed by four pixellated, 1mm thick and 2x2 cm size each, Schottky barrier CdTe crystals. Each pixel is connected to its own read-out ASIC electronics by a proper bonding. A first prototype using the same ASIC developed for the polarimeter (XPOL) is currently under development;
    • the Anti-Coincidence system (AC), completely surrounding the LED and HED systems, in order to efficiently screen the particle and gamma ray background. The AC will be realized using well-known inorganic scintillators like NaI or CsI crystals, already successfully used in a number of focal plane detectors aboard X- and gamma-ray telescopes. As an alternative, the use of new scintillators as e.g. LaBr3, currently investigated by ESA, is very attractive due to the fast rise time and high light output.

    X-ray polarimeter camera

    Also for the polarimeter Camera the goal is to cover the largest energy range as possible. The polarimeter already developed by the INFN of Pisa and INAF-IASF_Roma currently covers the band from ~1 to 10 keV. By changing the gas pressure and mixture inside the detector cell it is possible to get polarization measurements up to 30 keV. However, to cover this larger band, it is necessary to use two detectors that can be put at the focus of the telescope by means of a rotating wheel. The detector already developed is based on a Gas Pixel Detector (GPD), a position-sensitive counter with proportional multiplication and a finely subdivision of the charge collecting electrode in such a way that photoelectron tracks can be accurately reconstructed and their emission direction derived. The linear polarization is determined from the angular distribution of the photoelectron tracks (see Fig. 9).

    File:photoelectron_track.png

    Fig. 9 The photoelectron track and Auger track drift (and diffuse) towards the GEM. They are multiplied by the GEM and collected by a pixellated readout plane (the ASIC CMOS top layer).

    From a constructive point of view the GPD is a conventional proportional counter coupled to VLSI chip. From the existing data on aging tests made with mixtures based on noble gases and various quenching, included DME, the GPD gas mixtures can withstand the radiation levels foreseen for NHXM. A sealed detector is working since more than 15 months in our laboratories without showing any degradation of its performances. In the energy range 2–10 keV, the mixture with the best performance is He-DME 20-80. For the polarimeter to be use in the higher energy band a mixture of gas with higher atomic number is foreseen. Candidates are a Neon based mixture and an Argon based mixture. The Argon based mixture would permit to extend the energy band up to 30-35 keV (see Fig 11 and Fig 12).

    File:GPD_small.png

    Fig. 11 The sealed Gas Pixel Detector with the 50 μm Be window.

    File:focal_plane.png

    Fig. 12 The accommodation of the polarimeter in the focal plane. On the left side is the Control Electronics, on the right the box containing the filter wheel.


    Wide Field X-ray Monitor

    The WFXRM design is based on a coded-mask coupled to Silicon Drift Detectors (SDD). SDD allow for good imaging performance over sr-wide fields of view, with a moderate spectroscopic resolution (300–570 eV FWHM) in the 2–50 keV band at room temperature. The baseline configuration foresees 4 units, co-aligned with the 4-telescopes and this will minimize the length of the slew when NHXM will re-poiting a newly detect (or discovered) bright object. The WFXRM will have a sensitivity limit of 2 mcrab in 50ks at 5 σ in the band 2-50 keV and the capability of triggering on a 0.7 crab source in 1s, with a FOV = 2.9 sr partially coded and 0.5 sr fully coded.

    Satellite properties

    NHXM is a single satellite mission that will operate in low Earth orbit. In particular, to get very low background, as proven by the BeppoSAX and Swift missions, a circular, equatorial or low inclination orbit at 600-km mean altitude is requested. The satellite needs to be stabilised on three axes and with good pointing capability and has to be designed for the accommodation of two sets of payload on different modules, separated by 10 m, and this distance has to be maintained with a certain alignment and stability.

    In the frame of the ASI NHXM Phase A study the satellite system has been studied by Thales- Alenia-Space-Italy (TASI) with the available information and preliminary assumption on the Mirror Module and Detectors.


    The Service Platform

    The NHXM phase A study has identified a satellite platform (named Common Platform) common with the Simbol-X Service Module. Consequently all the achieved progress on Simbol-X Service Module design can be implemented in the context of the NHXM mission.

    An important achievement of the NHXM Phase A study is the compatibility of the Common Platform with VEGA, the smallest and cheapest European launcher. Further advantage of the Common Platform is the extensive reuse of the subsystems of PRIMA (Piattaforma Riconfigurabile Italiana Multi-Applicazione), whose performances and reliability has been widely proven by the Cosmo-SkyMed Mission.

    In the NHXM satellite (Fig. 13):

    • the central cylinder (main platform structure) hosting the extendible bench canister on one side and the WFXRM on the other one;
    • the extendible bench connecting the extendable platform, on which the Detector Modules are accommodate;
    • the four Mirror Modules arranged on the external wall of central cylinder.

    The use of Common Platform for the NHXM mission is considered feasible, because the NHXM estimated mass of 2000 (including 20% margin) kg is compatible with a Common Platform design sized for the Simbol-X maximum launchable mass of 2300 kg.

    File:common_platform_small.png

    Fig. 13 NHXM Phase A Common platform configuration overview.

    The expandable X-ray telescope main features

    The NHXM satellite is made of the platform, the four mirrors on the external wall of platform cylinder, the WFXRM inside the front-end of the platform cylinder, the deployable truss boom hosted inside the back-end of the platform cylinder and of the focal plane assembly (FPA) arranged in the Detector Platform (DP) located at the focal length of 10m by the deployable truss. In Fig. 14 it is shown the HEXITSat satellite configuration with the Detector Module fully deployed.

    File:configuration.png

    Fig. 14 The NXHM configuration from the Phase A study

    The overall system requirements on the Detector Platform and deployable truss can be summarized as follow:

    • High Bending stiffness;
    • Low dimensional distortion;
    • Low interference with satellite control system (high natural frequencies);
    • Low mass and envelope (to enclose the deployable truss in the platform cylinder);
    • Possibility to carry harness;
    • No electromagnetic interference with instrumentation.

    During the phase A study a trade-off considering several extensible boom concepts (inflatable structure, telescopic boom, coilable truss, circulated truss) has been performed. This trade-off allowed the identification of the articulated truss boom for the NHXM implementation. This extendible truss is an off-the-shelf item of ATK-Able Engineering Company, Inc.

    The Detector Platform

    After injection into orbit and commissioning, the expandable truss will position the Focal Plane Assembly (FPA), accommodated onto the Detector Platform, at the operating mirror focal lengths of 10 m. In the figure below it is shown the conceptual design of the detectors platform as it has been foreseen by the NHXM phase A study.

    File:detector_platform.png

    Fig. 15 The HEXITSat Detector Platform with the 4 focal plane cameras (upper and bottom view).

    The Detector Platform provides also the thermal control of FPA (by means of Sun shield, heaters, Mli…), the mechanical structure, the harness connecting the Detector equipment and the metrology devices. The driving performance requirements of this architecture concept is the focal plane stability over the timescale of an observation, and under large temperature gradients (both axial and circumferential). The focal length must not change by more than 1 cm and the alignment between the optics reference axis and the focal plane reference axis must be stable within 1.5 arcmin. This condition is mainly guarantee by the selection of the material and by the thermal control of the Detector Platform and expandable truss. The compensation of the residual variations of the focal plane position (impacting on HEW) can be performed by the Service Metrology and post facto image reconstruction.

    Mission profile

    The mission profile requirements arise from a combination of the scientific objectives, the provision of the observing programme, and the operational characteristics of the scientific payload itself. The key mission profile elements are summarized in the following table, and are briefly described below.

    NHXM Mission Profile
    Orbit 600 km, circular
    Orbit period 95 min
    Orbit inclination Equatorial (<5°)
    Visibility (Malindi) Vega (compatible with Soyuz, Long-March, ...)
    Launch date 2016
    Mass budget (payload + spacecraft) < 2000 kg
    Power budget(payload + spacecraft) 1000 W
    Mission lifetime 3 (+2 goal) Years
    Mission science X-ray Observatory


    Launch scenario

    During NHXM Phase-A study a detailed trade-off has been performed between the scientific requirements, the mass, power budgets and the launcher choice. The important result of this study is the consolidation of the Common Platform compatible with the VEGA launcher. The compatibility with the VEGA launcher assures also the compatibility with bigger launcher system, like the Soyuz, the Chinese Long March or the indian PSLV.

    In the NHXM case the estimated launch mass is of about 2000 kg (including 20% margin), above the VEGA launcher capability, e.g. 2300 kg at the chosen orbit.

    File:vega_small.png

    Fig. 16 The folded satellite inside the VEGA fairing.

    Lifetime & operations

    The mission is composed by the standard operational phases: Launch and Early Orbit (LEOP), Commissioning and Calibration (CCP), Nominal Operational (NOP), Extended Operational (EOP), end of life. The total expected lifetime is of a minimum of 3 years with the provision of another 2 yrs (TBC).

    Ground Station(s)

    The nominal Ground Station during LEOP, CCP, NOP and EOP will be Malindi (-2.92°, 40.21°). During LEOP, the satellite will be supported by at least a further Ground Stations: Kourou Ground Station is the best candidate for the equatorial position (+5.07°, 307.4°). The coverage pattern for the target altitude of 600 km is a regular sequence of contacts, once per orbit, each one followed by a 85 min gap. At the beginning of the mission lifetime, and assuming

    a 3.5° inclination, the visibility for Malindi is 650 s/orbit (or 150 min/day). The duration of the visibility passes will decrease with time, due to altitude decrease and consequent orbit velocity increase.
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