NASA's Chandra X-Ray Observatory, designed for three years of operation with a goal of five years is now entering its 15-th year of operation. Thanks to its superb angular resolution, the Observatory continues to yield new and exciting results, many of which were totally unanticipated prior to launch. We discuss the current technical status, review recent scientific highlights, indicate a few future directions, and present what we feel is the most important lesson learned from our experience of building and operating this great observatory

Baggett, Randy

Brissenden, Roger

Edmonds, Peter

Mattison, Edward

Tananbaum, Harvey

Tucker, Wallace

Weisskopf, Martin C.

Wilkes, Belinda

NASA Technical Reports Server

Fifteen years of Chandra operation: scientific highlights and lessons 
learned  
Martin C. Weisskopf
1
, Harvey Tananbaum
2
, Wallace Tucker
2
, Belinda Wilkes
2
,  
Randy Baggett
1
, Roger Brissenden
2
, Peter Edmonds
2
 and Edward Mattison
2
 
1
NASA/Marshall Space Flight Center, ZP12, 320 Sparkman Dr. Huntsville, AL, USA 35801, 
2
Chandra X-Ray Center, Smithsonian Astrophysical Observatory, Cambridge MA 02138 
ABSTRACT  
NASA's Chandra X-Ray Observatory, designed for three years of operation with a goal of five years is now 
entering its 15-th year of operation. Thanks to its superb angular resolution, the Observatory continues to 
yield new and exciting results, many of which were totally unanticipated prior to launch. We discuss the 
current technical status, review recent scientific highlights, indicate a few future directions, and present what 
we feel is the most important lesson learned from our experience of building and operating this great 
observatory. 
 
Keywords: X-ray Astronomy, Chandra X-ray Observatory, High-energy astrophysics 
1. INTRODUCTION  
Almost fifteen years after launch, Chandra continues to be one of the world’s most powerful tools for 
exploring the Universe and its extreme and interesting physics as part of the desire to understand our place in 
the Universe and the fundamental laws that govern it. From the edge of space-time around black holes (BHs), 
to the outer reaches of the known Universe, the Chandra X-ray Observatory is deepening our understanding 
of the behavior of matter and energy under conditions that cannot be probed on Earth. Chandra is also part of 
a larger picture wherein high-resolution imaging across a wide band of wavelengths is enabling advances in 
understanding the cosmos, on scales from the first supermassive black holes (SMBHs) to the characteristics 
of exoplanets and their atmospheres.  
The Observatory and its instrumentation have been described in detail in a number of publications. A 
complete description may be found in the Chandra Proposer Observatory Guide
2
 and references therein. 
Briefly, Chandra consists of three main elements: (1) the telescope containing the X-ray focusing optics 
which provides sub-arc-second angular resolution, two X-ray transmission gratings (the low and high-energy 
gratings) that can be inserted into the X-ray path, and a 10-meter-long optical bench that separates the optics 
from the focal plane imaging instruments; (2) a spacecraft module that provides electrical power, 
communications, and attitude control; and (3) the instrument module that holds two focal-plane cameras – the 
Advanced CCD Imaging Spectrometer (ACIS) and the High Resolution Camera (HRC) – and mechanisms to 
adjust their position and focus. Chandra is 13.8 m long, has a 19 m wingspan, and a mass of 4800 kg. 
ACIS contains two arrays of charge coupled devices that provide position and energy information for each 
detected X-ray photon. The imaging array is optimized for spectrally resolved high-resolution imaging over 
17-arcminute wide field-of-view; the spectroscopy array, when used in conjunction with the high energy 
transmission grating, provides high-resolution spectroscopy with a resolving power (E/ΔE) up to 1000 over 
the 0.4-8 keV band. 
                                                 
1
 martin.c.weisskopf@nasa.gov; phone 1 256 961-7798  
2
 http://cxc.harvard.edu/proposer/POG/ 
https://ntrs.nasa.gov/search.jsp?R=20140011796 2019-08-31T19:20:25+00:00Z
  
 
 
The HRC is comprises two microchannel plate detectors, one for wide-field imaging and the other serving as 
a readout for the low energy transmission grating (LETG). The HRC detectors have the highest spatial 
resolution on Chandra and, in certain operating modes, the fastest time resolution (16 μs). When operated 
with the HRC’s spectral array, the LETG provides spectral resolution >1000 at low (0.08 – 0.2 keV) energies 
while covering the full Chandra energy band.  
Finally, a system of gyroscopes, reaction wheels, reference lights, and a CCD-based star camera enables 
Chandra to maneuver between targets and point stably while also providing data for accurately determining 
the sky positions of observed objects. The blurring of images due to pointing uncertainty is <0.10", negligibly 
affecting the resolution. Absolute positions can be determined to ≤0.6" for 90% of the observed sources, 
providing an unrivaled capability for X-ray source localization. 
2. TECHNICAL STATUS 
Chandra’s spacecraft and instruments are in excellent health and performing superbly, a result of sound 
design and construction, as well as careful and skilled operations over the past 14+ years. Figure 1 
summarizes the current state of the Observatory subsystems. Based upon thorough analysis, we have every 
expectation that the Observatory will continue to be capable of delivering ground-breaking science for a 20-
year mission (and likely substantially longer). 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 1. The state of health of the main components within each of Chandra’s three major systems, the 
spacecraft, the telescope and the Science Instrument Module (SIM). Key: blue = no known problems; green = 
minor problem but meeting all requirements; yellow = moderate problem with manageable performance 
effects; red = major problem affecting performance. “Radiation Detection” comprises IEPHIN, HRC and 
ACIS as they apply to this function. 
The Observatory has experienced very few anomalies in its first 14+ years of operation; spacecraft 
performance has been exceptional, and fully single-fault tolerant. That is not to say that the Observatory has 
not experienced any problems. The spacecraft’s multi-layer insulation and other thermal protective surfaces 
have degraded during the mission due to particle and UV radiation, resulting in elevated temperatures. This 
has reduced the ability to provide uninterrupted extended observing time at particular spacecraft orientations 
with respect to the sun. Mitigating the effect has increased the complexity of mission planning. In addition, 
the insulation degradation has raised the temperature of the Integrated Electron Proton Helium Instrument 
(IEPHIN, used to safe the science instruments to prevent damage by cosmic rays) to unacceptable levels. As a 
  
 
 
consequence new, autonomous on-board procedures were developed that use the focal plane x-ray cameras to 
initiate instrument safing. Another, potentially serious, consequence of the thermal insulation degradation is 
that the spacecraft’s temperature control system no longer adequately maintains the temperature of some of 
the thruster propellant supply tubes above the freezing point of the hydrazine fuel, introducing a risk of 
rupturing the tubes. These temperatures are now also managed using mission planning constraints that limit 
observations in an unfavorable (cold) direction. 
Another problem which has had an impact on a subset of scientific observations concerns the ACIS 
instrument’s low-energy detection efficiency which has gradually decreased due to the buildup of 
contamination on the ACIS optical blocking filter. Of course the contamination has its largest effect at lower 
energies (<0.5 keV) and the filter transmission currently ~35% of its launch value at 0.7 keV, and ~82% at 
1.5 keV. Nevertheless, these values are acceptable when compared to requirements and, based on current 
extrapolation, requirements will not be exceeded for many years to come 
3. RECENT RESULTS 
Chandra’s ability to locate sources with sub-arc second accuracy enables, e.g., the stacking of X-ray ob-
servations at the positions of galaxies found in Hubble Space Telescope images to constrain X-ray emission 
from accreting SMBHs at redshifts z>6, and thereby constrain models for the formation and growth of these 
objects (Figure 2). Some SMBHs in the early Universe likely grew at a prodigious rate, and Chandra is 
proving indispensable at studying this process through observations of active galactic nuclei (AGN) emitting 
strongly in X-rays. But not all SMBHs grow rapidly. For example, the SMBH, Sgr A*, at the center of the 
Galaxy is growing very slowly. Chandra data has been used to make the first detailed study of the flow of hot 
gas in the vicinity of Sgr A* and for the SMBH in the nearby galaxy NGC 3115. The results suggest that the 
flow is a complex mixture of inflowing and outflowing zones, likely driven by the need to transfer angular 
momentum away from the inflowing material. 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 2: The inner 8'×8' of the 4-Ms CDFS, the deepest X-ray image currently available [1], with smoothed 
Chandra images in the 0.5–2.0 keV (red), 2–4 keV (green) and 4–8 keV (blue) bands. The polygon shows 
coverage by the Hubble Ultra Deep Field. A current Chandra project will extend the total exposure to 7-Ms. 
Figure adapted from [1]  
  
 
 
A recent long observation of the galaxy cluster Abell 133 (Figure 3) has revealed, for the first time, a picture 
of the conditions in the rarified hot gas, some 1.5 Mpc from the center of the cluster at the virial radius, where 
gas falling into the cluster from the intracluster medium is predicted to be shocked and thermalized. Extensive 
surveys of galaxy clusters have shown that Chandra measurements may be combined with other observations 
to place ever tighter constraints on the mix of baryonic matter, dark matter, and dark energy in the Universe. 
The results show consistency with the picture of the standard cosmological model described by General 
Relativity (GR), with the energy density dominated by a cosmological constant, and the mass density by dark 
matter. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 3: A 2.4 Ms mosaic of Chandra images of the galaxy cluster Abell 133 in the 0.7–2 keV band. All 
detectable point sources and small-scale clumps were removed and the data smoothed with a σ=30'' (32 kpc) 
Gaussian. The dynamic range is ~106 and the color map saturates to the red. The circle shows the virial 
radius. Note filaments in the outer region of the cluster. [2] 
On a Galactic scale, observations of a young star cluster have been used in conjunction with infra-red (IR) 
data to determine the characteristic survival time (~ 6 Myr) of protoplanetary disks. Chandra has also 
detected the transit of an exoplanet in X-rays for the first time, providing evidence for an extended, X-ray 
absorbing atmosphere. Chandra’s unrivaled ability to probe supernova remnants (SNRs) continues with a 
striking image of the remnant W49B (Figure 4), which provides evidence for an explosion in which a rapidly 
rotating BH formed and ejected material in a high velocity jet. Another Chandra discovery was made when 
the X-ray Binary (XRB) Cir X-1 transitioned to a low state, and enterprising X-ray astronomers used 
Chandra to detect the natal SNR, which had been hidden from our view by the glare from Cir X-1 since its 
discovery more than 4 decades ago. At an age < 4600 years determined from the SNR data, Cir X-1 is the 
youngest known XRB, and affords a golden opportunity to explore how these systems evolve. 
 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 4: Composite image of W49B, with Chandra X-ray data shown in blue and green, radio data (VLA) in 
pink and near-IR data (Palomar) in yellow. The distorted shape provides evidence that material near the poles 
of the SN progenitor, aligned roughly from NW to SE, was ejected at a much higher speed than material near 
its equator. [3] 
4.  FUTURE SCIENTIFIC PROSPECTS  
It should be noted that Chandra observations are determined through the peer review process.  
 
The increased exposure of the Chandra Deep Field South will enable advances in determining the space 
density of AGN which will be used to further constrain models for the origin and growth of SMBHs, provide 
insight into the co-evolution (or not) of SMBHs and galaxies in the redshift range z~1–4, extend our 
knowledge of star formation versus red shift, and provide a unique set of faint X-ray sources to be used to 
guide studies in other wavelengths for years to come.  
 
A novel statistical approach applied to substructures merging into clusters may provide much tighter 
constraints on the self-interaction cross section of dark matter. Sunyaev-Zel'dovich surveys and the upcoming 
eROSITA (extended Roentgen Survey with an Imaging Telescope Array) X-ray survey mission are (or will 
be) discovering large numbers of galaxy clusters. When these observations are combined with follow-up 
Chandra mass determinations, the resulting mass functions will be used to measure the growth of structure in 
  
 
 
a dark energy dominated Universe and thereby constrain changes in the dark energy equation of state with 
time.  
 
A much longer Chandra observation of the background blazar 1ES 1553+113 could detect many additional 
absorption lines due to foreground intergalactic filamentary systems, and thus provide evidence for the 
whereabouts of the baryons (~ 50% of the total) seemingly missing from the low-redshift Universe.  
 
A very deep observation of 30 Doradus in the Large Magellanic Cloud will use Chandra’s superb spatial 
resolution to study the tableau of stellar evolution presented by one of the most active star-forming regions in 
the Local Group. Closer to home, Chandra will continue to study the interaction of host stars and their 
exoplanets. Chandra, along with the Jansky Very Large Array and other telescopes, continues to monitor the 
passage of the gas cloud G2 around the SMBH Sgr A*. Over the coming months/years, G2 could serve as a 
source of test particles to probe the flow of matter into a BH, as well as provide unique insights into a range 
of phenomena driven by previous outbursts in the vicinity of Sgr A*. 
5. LESSONS LEARNED 
There are two other papers in this conference [4,5] that discuss in some detail the lessons learned in the 
process of building and operating this Great Observatory. The entire process has taken thirty-eight years (and 
counting) from the initial unsolicited proposal of Giacconi and Tananbaum in 1976 to the upcoming 
celebration of the 15-th anniversary of the launch in July 2014. Clearly there were numerous factors that 
contributed to the outstanding success of the Mission that includes Riccardo Giacconi’s Nobel in 2002. 
Perhaps the foremost amongst these has been the active and dedicated involvement of teams of scientists that 
have been involved in every aspect of the program including the optics, the spacecraft, the science 
instruments, the ground and space calibration, and the science and spacecraft operations. These teams both 
held the line on requirements and also saw to it that there was essentially no “requirements creep” throughout 
the lengthy development of the mission. They recommended removal of one of the original instruments and 
signed up, albeit reluctantly, to the bold strategy of changing the mission to one that was unserviceable. This 
change, which fortunately turned out well, was at the price of one of the most novel instruments that had been 
originally selected --- an X-ray-sensitive calorimeter, soon to be launched on the ASTRO-H mission 
discussed elsewhere in these proceedings. We urge those interested in more details of the Chandra-lessons 
learned by reading the two papers referenced. 
REFERENCES 
[1] Xue, Y. et al. 2011, ApJS, 195, 10 
[2] Vikhlinin, A. et al. 2014, in preparation. 
[3] Lopez, L. et al. 2013, ApJ, 764, 50 
[4] Schwartz, D. A.  “Development lessons from the AXAF/Chandra X-ray observatory”, Paper 9144-27 this 
conference. 
[5] Arenberg, J. et al. “Lessons We Learned Designing and Building the Chandra Telescope”, Paper 9144-25  This 
conference 
 
 


Fifteen Years of Chandra Operation: Scientific Highlights and Lessons Learned

Fifteen Years of Chandra Operation: Scientific Highlights and Lessons Learned

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