The Mikhail Pavlinsky Astronomical Roentgen Telescope – X-ray Concentrator (ART-XC) is one of two X-ray telescopes of the Spektr-Roentgen-Gamma (SRG) observatory – the flagship astrophysical project of the Russian Federal Space Program. ART-XC is designed to produce true images of the X-ray sky using the grazing incidence imaging X-ray optics technique. ART-XC can also be used in a mode with a much larger effective field of view but without good angular resolution (”Concentrator mode”). The ART-XC and eROSITA (the other instrument of the SRG observatory) telescopes complement each other, being sensitive in the 4–30 keV and 0.2–8 keV energy bands, respectively.
The only all-sky survey carried out previously with a grazing incidence X-ray telescope is the ROSAT all-sky survey (RASS) in the 0.1–2.4 keV energy band. At higher energies, all-sky X-ray surveys have been performed either with collimator instruments or with coded-mask aperture telescopes (e.g. INTEGRAL/IBIS and Swift/BAT). The sensitivity of these surveys was strongly limited by their poor angular resolution. The ART-XC telescope will provide the first ever true imaging all-sky survey performed with grazing incidence optics in the 4–30 keV energy band, i.e. at significantly higher energies compared to RASS. ART-XC is optimized for conducting surveys in the 4–12 keV band with maximum sensitivity at around 8–10 keV.
ART-XC was developed by the Space Research Institute (IKI, Moscow) and the Russian Federal Nuclear Center – All-Russian Scientific Research Institute for Experimental Physics (RFNC-VNIIEF, Sarov). The development of the X-ray optics for ART-XC proceeded independently at VNIIEF and NASA’s Marshall Space Flight Center (MSFC). The VNIIEF mirror systems were installed in the qualification model of ART-XC, which was subjected to vibration and endurance tests. This led to a significant reduction of the ART-XC projects execution time. The flight models of ART-XC X-ray mirror systems were simultaneously developed, fabricated and calibrated by MSFC. It is important to note that the great experience, gained by IKI in the development and operating of the ART-P telescope on board the GRANAT observatory as well as in development and working with instruments of the Roentgen Observatory on board the MIR space station, played a significant role in the creation of ART-XC and formulation of its scientific tasks.
ART-XC is the first Wolter grazing incidence X-ray telescope developed and launched into space by Russia.
Structure of ART–XC
ART-XC consists of the telescope itself and four separate electronics units that are mounted on a thermo-stabilized platform located 0.5 meters under the telescope (Fig. 1, Pavlinsky et al. 2012).
The telescope has a weight of about 350 kg, an approximate dimensions of 3.5 m of height and 0.9 m in diameter and power consumption of 150 W. The main elements of the telescope are seven X-ray mirror systems (MSs) and corresponding X-ray detectors. Each pair “mirror system – detector” forms a “telescope module”, these modules are co-aligned and being referred to below as T1…T7. The MSs are mounted on a platform situated in the upper part of the telescope structure. The seven X-ray detectors compose a focal plane assembly. The detectors and mirrors are enclosed in a carbon fiber conic housing. The upper part of the conic housing is covered with a copper shield. It blocks the side aperture stray light, which would otherwise significantly increase the overall detector X-ray background.
The telescope has an on-board calibration system, which is used to determine detector gain and energy resolution, and a thermal control system. The calibration system contains a drive control unit and seven calibration X-ray source blocks – one for each detector. The calibration X-ray source consists of Am241 and Fe55 and is pulled out of the lead box using a stepper motor.
The telescope is a rather complex object from the point of view of ensuring its thermal stability. The telescope’s thermal control system consists of 36 active elements – heaters mounted in different places on the telescope’s structure. The instrument has two strictly thermally stabilized zones: X-ray mirrors and detectors. A stable temperature of mirror shells in the range of 20 ± 2°C is provided by the heated outer mirror shell and the thermal mirror baffle. The lower part of the outer mirror shell has a temperature of 27°C, while the upper part has 28°C. The temperature of the thermal mirror baffle is 22°C with the time stability of 0.01°C.
The separate electronics units are the information collection and control unit, two blocks of electronics for detectors and the thermal control unit.
A BOKZ-MF star tracker is mounted on the MS platform next to the MSs.
The design of the ART-XC X-ray optics was developed independently at VNIIEF and MSFC based on specifications by IKI. MSFC used the classical parabolic/hyperbolic shape of Wolter type I mirrors instead of a conical approximation to Wolter I geometry used by VNIIEF. The major characteristics, such as mass and effective area, of the MSs were similar, but the modules produced by MSFC had a significantly better angular resolution. VNIIEF fabricated seven MSs. All of them were installed in the qualification model of ART-XC which was subjected to vibration and endurance tests. MSFC fabricated, tested and calibrated eight identical X-ray MSs for ART-XC (see Fig. 2). Seven of them were installed into the flight model of the telescope and the eighth became a spare.
Each MS (see Fig. 3) contains 28 Wolter-I nested mirror shells. The shells were fabricated using an electroformed-nickel-replication technique. The shells have diameters ranging from 49 to 145 mm. Their thickness varies with radius from 250 to 350 µm. The outer shells are made thicker, which increases their rigidity and, hence, improves the angular resolution of the MS. The inner surface of the ART-XC nickel-cobalt mirrors is coated with a ∼10 nm layer of 90% bulk density iridium (Ir). This metal has an X-ray reflective index higher than that of gold at energies above 10 keV. The upper ends of the shells are glued into a supporting “spider”.
The weight of each ART-XC MS is about 17 kg. The nominal focal length of the MSs is 2700 mm. During installation into the telescope (Fig. 4), the MSs were defocused by 7 mm to provide a more uniform angular resolution across the field of view (see Fig. 5). Thus, the distance from the MS to the detector plane is 2693 mm.
X-ray detectors system
The ART-XC focal plane X-ray detectors were developed in IKI specifically for the SRG mission.
The ART-XC detector system consists of seven units of roentgen detectors (URDs with numbers from 01 to 07 according to the number of telescope modules), two blocks of electronics (BE) and one serial interface connection block (CB). The power consumption of the detector assembly is 42 W, and the total mass budget is 39.6 kg. The two BEs consist of seven identical modules (four in BE01 and three in BE02). One BE module services one URD. Each module is connected to the supply distribution network and receives two pulse commands from the spacecraft control system. The BE module includes a primary power supply switcher, an electromagnetic interference filter, a set of low voltage DC-to-DC converters, a high voltage regulated DC-to-DC converter, and a current measure and protection circuit. The BE module provides all necessary low and high voltages to the URD and protect the latter from high current consumption. The CB is used for distribution of command/housekeeping and time synchronization interfaces. Both interfaces are based on the RS-485 standard with galvanic isolation.
The URDs and BEs are separated in order to lower the power dissipation inside the URD. One detection channel (one URD and one BE module) consumes 6 W from the on-board supply network. A power of 3.5 W is dissipated inside the URD and another 2.5 W is dissipated in the BE.
The two BEs are placed on the thermally stabilized platform of the spacecraft. The seven URDs and CB are situated inside the ART-XC telescope tube (Fig. 6).
The URD performs the following tasks: receiving and processing commands from the subsystem storing on-board information (SSOI), processing time synchronization signals, providing all necessary biases to the detector, amplification and analog-to-digital conversion of the detector output signals, processing the digitized data, finding the areas with registered events, packing data to housekeeping frames, and transmitting housekeeping frames to the SSOI.
The X-ray detector, situated inside URD, is a big hybrid integrated circuit (Fig. 7). The sensitive elements of the detectors are high quality CdTe dies, manufactured by Acrorad (Japan). The size of the die is 29.95 × 29.95 × 1.00 mm. The electrodes structure of the die is: (top) Au/Pt/CdTe/Al/Ti/Au (bottom). The structure CdTe/Al on the bottom electrode forms a Schottky barrier. A high-purity CdTe with the Schottky barrier provides a very low leakage current. To operate in a double side strip detector configuration, the top and bottom electrodes of the die are patterned by photolithography. On the top side, 48 parallel strips are formed, which are surrounded by a guard ring. The same pattern is formed on the bottom side, but it is rotated by 90° relative to the top side, which enables a reconstruction of two coordinates of an incoming event on the detector plane. The strips’ width is 520 µm, and the gaps between strips are 75 µm. The sensitive area of the detector is 28.48 × 28.48 mm. Given the focal length of the MS (∼ 2700 mm), the angular size of the strip is about 4500.
The ART-XC electronics includes several functional parts:
– X-ray detectors system;
– Data processing unit (DPU, Russian abbreviation SSOI);
– Star tracker (Russian abbreviation BOKZ-MF);
– Thermal-control unit;
– Calibration sources electronics.
On-board data handling structure
The data processing unit (SSOI) of ART-XC performs:
– data collection and storage from the seven detectors;
– receiving and storage of orientation data and quaternions from the star trackers and gyros;
– survey of the instrument’s sensors (analog and digital) by the S/C telemetry system;
– synchronization of subsystems and data binding to the timeline;
– upload of telecommands for the instrument;
– download of scientific and housekeeping data.
ART-XC Observing Modes
There are three modes of observations with ART-XC aboard the spacecraft: (1) pointed observations mode, (2) survey mode, and (3) scan mode.
In the pointed observations mode, the optical axis of the telescope is fixed in a given direction. This mode is usually used for calibration observations.
The survey mode is used during the all-sky survey. In this mode, the telescope’s optical axis is rotated with a period of 4 hours around the spacecraft axis, pointed towards the Sun. This enables the full sky coverage in about 6 months. We can control movement parameters of the rotation axis by setting the plane of rotation, rotation speed and initial direction. Typically this is done once per week. During the first two surveys the rotation plane coincided with the Ecliptic plane.
The scan mode is the third observing mode realized in the SRG mission, which enables observations of fairly large regions of the sky (12.5° × 12.5° maximum) with uniform exposure. In this mode, the optical axis of ART-XC is performing a “snake scan”. The spacecraft control system is automatically conducting a series of repeated consecutive rotations around two spacecraft axes with a set of predefined parameters, this set is being called a ”template”. Before being used this ”template” has to be checked by and agreed with the spacecraft control team.
The scan mode was widely used during the Calibration and Performance Verification phase as well as during additional calibration sessions, and provided excellent results (see Fig. 8).
Mission planning and timeline
The SRG mission is operated by Lavochkin Association (also referred to as NPO Lavochkin or NPOL), the developer and creator of the Navigator platform. Mission planning is mainly done on a monthly basis. The plan of observations for the next month is prepared by IKI and checked by NPOL for possible observational constraints. The approved month observational program is normally divided into schedule blocks, which can be sent to the spacecraft on a daily basis during the ground contacts. This naturally sets the constraints for the SRG response time for possible transient events from one day to several days. The actual SRG observing schedule is available on a dedicated website http://srg.cosmos.ru.
Scientific goals and expectations
The main goal of ART-XC is to survey the whole sky in the medium X-ray energy range of 4–12 keV with the record-high sensitivity of ∼ 10¯12 erg s¯1cm¯2 (∼ 10¯13 erg s¯1 cm¯2 near the ecliptic poles) and sub-arcminute angular resolution. The relatively hard energy band of ART-XC is particularly well suited for studying populations of astrophysical objects that are significantly affected by intrinsic X-ray photoabsorption, such as active galactic nuclei (AGN), high-mass X-ray binaries (HMXBs) and cataclysmic variables (CVs).
Over the last two decades, a number of all-sky (serendipitous) surveys have been performed in energy bands close to that of ART-XC. The RXTE Slew Survey (XSS) achieved a sensitivity ∼ 10¯11 erg s¯1cm¯2 and angular resolution ∼ 1° in the 3–20 keV energy band in the extragalactic sky (|b| > 10°). A somewhat better sensitivity (∼ 5 × 10¯12 erg s¯1cm¯2) has been reached in the 4–10 keV energy band in the MAXI /GSC all-sky survey, which similarly to XSS is conducted by a collimator instrument. A similar average depth in the 2–12 keV energy band, but with excellent angular resolution (thanks to the use of mirror X-ray optics), characterizes the XMM-Newton Slew Survey (XMMSL); however, XMMSL has covered the sky very non-uniformly and not in its entirety (84%). In addition, there are hard X-ray (above 15 keV) all-sky (also serendipitous) surveys carried out by the codedmask INTEGRAL/IBIS and Swift/BAT instruments, which have reached a depth ∼ 10¯11 erg s¯1cm¯2 at angular resolution ∼ 5–10 arcmin.
The ART-XC all-sky survey will significantly improve on all of these surveys in terms of the combination of angular resolution, sensitivity and uniformity, and will provide a rich astrophysical database for explorations of Galactic objects X-ray binaries and cataclysmic variables, Supernova remnants, Galactic X-ray background, Transients, dark matter etc. and extragalactic objects, like active galactic nuclei, Clusters of galaxies.
Upon completion of the CalPV phase, on 12 December 2019 the SRG observatory started its 4-year all-sky X-ray survey. By 10 June 2020 the entire sky had been covered by ARTXC for the first time and by 15 Dec. 2020 for the second time.
Pavlinsky and colleagues in their recent paper showed results of preliminary analysis of all available data which revealed a lack of bright flares on short timescales. They also showed that, though there were no major Solar events during the first year of the SRG mission. With the onset of Solar cycle 25 in the next years the situation might change dramatically.
“Our preliminary analysis of the all-sky maps constructed from the ART-XC data of the first year of the survey has revealed ∼ 700 sources detected in the 4–12 keV energy band and ∼ 400 sources detected in the 7–12 keV energy band (of which ∼ 100 sources are not detected in the softer 4–7 keV band). Note that at energies above 6–7 keV ART-XC is more sensitive (in survey mode) than eROSITA (in preparation). Based on our current knowledge of the instrument and in-flight background conditions, we can predict that after completion of the 4-year all-sky survey ART-XC will detect ∼ 5000 sources in the 4–12 keV band.”— told Pavlinsky, principal investigator and lead author of the study
Figure 9 shows a ∼ 1000 sq. deg fragment of the allsky map along the Galactic Plane obtained by ART-XC in the 4–12 keV energy band during its first scan of the sky.
It nicely demonstrates the unique characteristics (hard X-ray band, all-sky coverage, good angular resolution, high dynamic range, and uniformity) of the on-going ART-XC survey.
Reference: M.Pavlinsky, A.Tkachenko, V. Levin, N. Alexandrovich, V. Arefiev et al., “The ART-XC telescope on board the SRG observatory”, ArXiv, 2021. Pp. 1-19, 2021. https://arxiv.org/abs/2103.12479
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