Документ взят из кэша поисковой машины. Адрес оригинального документа : http://xmm.vilspa.esa.es/sched_tour/too_paper.pdf Дата изменения: Wed Nov 7 12:57:51 2001 Дата индексирования: Mon Oct 1 19:47:04 2012 Кодировка: Поисковые слова: vela satellite

The life cycle of "Targets of Opportunity" with XMMNewton
M. Santos-LleС, N. Schartel, M. Guainazzi, P. M. Rodriguez-Pascual, M. Ehle, M. Breitfellner, L. Tomas, F.A. Jansen
Introduction The term "Target of Opportunity" (ToO) is used in astronomy to identify unpredictable events whose study is of the highest scientific interest. For XMM-Newton a ToO is an astronomical event observable by its instruments, which cannot be predicted and scheduled on the time scale of one year, yet is scientifically significant to justify the interruption of the ongoing XMM-Newton programme. In this paper , we present what kind of objects are suitable to be observed as ToO with XMM-Newton and which is their scientific interest. Then we explain how they are implemented in the schedule. This is done in two parts: first the nominal target selection and mission planning procedures are detailed, then the ToO selection and scheduling are described. The last section shows what has been done so far and the preliminary results of a few examples. The XMM-Newton (X-ray Multi-mirror Mission) observatory is the second cornerstone of ESA's Horizon 2000 Science Programme. It offers the astronomers simultaneous: - high throughput non-dispersive spectroscopic imaging, - medium resolution dispersive spectroscopy, - optical-ultraviolet (UV) imaging and timing from a co-aligned telescope (Optical Monitor, OM) The three European Photon Imaging Cameras (EPIC) provide a large effective area over the energy range from 0.1 keV to 15 keV. Each of the two modules of the Reflection Grating Spectrometer (RGS) covers the energy range from 0.35 keV to 2.5 keV. Thus, XMM-Newton provides a unique opportunity for a wide variety of sensitive X-ray observations accompanied by simultaneous optical/UV measurements. For XMM-Newton, ToO types of interest are -ray bursts (GRB), supernovae, classical novae, X-ray transient binary stellar systems and X-ray transient active galactic nuclei (AGN). "Hot" topics in astrophysics, e.g. newly detected objects that are of high scientific interest can be awarded discretionary time by the project scientist. Some of the scientific issues relating to potential ToO targets are : GRBs are extremely energetic events whose nature is still far from being understood. Current models claim either the coalescence of a binary system of compact objects (such as black holes, neutron stars and white dwarfs) or the collapse of a massive star (hypernova). Observational evidence discriminating against the various models should come with measurements of X-ray emission lines. X-ray lines would also provide an estimate of their distance and a diagnostic tool of both the nature of the central engine and its environment, probing the early universe. The X-ray emission from supernova explosions comes from the interaction of the shock wave with the interstellar matter. XMM-Newton observations of supernovae shortly after the expansion should provide spectra to test the models for the different types and light curves to monitor the interaction between the shock wave and the interstellar matter. Classical novae are stellar systems formed by a white dwarf and a giant star where the mass transfer from the giant to the dwarf generates nuclear explosions in the hydrogen shell-burning white dwarf. They emit Xrays during outburst via three different mechanisms: a) luminous super-soft X-rays by the shell, b) thermal X rays (0.5 to 20keV) from shocks in the wind or the interaction between the ejecta and the circumstellar material and c) hard X-ray emission due to Compton degradation of radiactive decay, which was predicted but never detected. XMM-Newton observations can be used to understand the properties of the shocked nova shell and the composition of the hard X-ray emission, to detect the super-soft component and to test the theoretical model predictions. AGN is a term that refers to very energetic phenomena that occur in the cores of certain galaxies. They are suspected of containing super-massive black holes surrounded by accreting material moving so fast that it becomes sufficiently hot to produce X-rays. XMM-Newton observations of AGN in either outburst or extremely low state would allow constraining the physical characteristics of the central engine. Moreover they will provide tools to separate the emission from the different components (like the inner relativistic accreting gas or the outer cold material that reprocesses the primary radiation) and to understand the accretion events that trigger an outburst (like tidal disruption of nearby objects). They could also provide tools to discriminate between the outburst of the AGN itself and the circumnuclear starbursts observed in many AGN.


The routine XMM-Newton mission planning XMM-Newton as an Observatory XMM-Newton activities are prepared long in advance. The process starts more than one year and a half before the observations are performed. At that time, the ESA Director of Science makes a public call for observing time proposals. Astronomers from all over the world can respond submitting proposals for observations of carefully selected objects by the specified deadline. All the proposals are technically evaluated and then sent to the Observing Time Allocation Committee (OTAC) for independent peer review of their scientific merits. The OTAC selects the proposals, and the observations within a proposal, that should be performed and assigns them a priority. In addition to this "Open Time" programme, routine calibration observations are necessary to understand the response of the instruments. Based on the OTAC recommendations, it is the task and the responsibility of the Science Operations Centre (SOC), located at Vilspa, to ensure that all are "visited" for the approved time in the safest and most efficient way. Being efficient means minimize idle periods and the time spent in manoeuvres. There are some additional XMM-Newton specific items: 1) whenever possible, the six on-board instruments should operate in parallel (even if this was not requested). 2) The exposures for all the instruments should start and end at the same time. 3) It may be that for safety reasons some instruments have to remain closed. Scheduling constraints The orbit of the XMM-Newton observatory (Figure 1) is the basis of every planning. The spacecraft (S/C hereafter) orbit is highly elongated, from ca. 7000 km altitude at perigee to 114000 km at apogee. It takes almost 48 hours to complete one revolution around the Earth. In the lowest part of the orbit, the platform passes across the Earth radiation belts, high-energy atomic particles that extend for about 40000 km from the Earth. The radiation environment is so high that all the instruments need to remain inactive and, if possible, closed. Otherwise they would be seriously damaged. The S/C, however, can start manoeuvring to the next target shortly after its perigee passage and it is usually ready to start an observation as soon as the conditions required for some of the instruments are fulfilled. At an altitude of 46000 km (about 4 hours after the perigee passage) the RGS and OM instruments are ready for scientific exposures. Currently, the EPIC instruments are only exposed to the sky after the S/C has reached an altitude of about 60000 km (7 hours after perigee). Similarly, at the end of the revolution all the instruments are put into a safe configuration at 46000 km altitude. This means that every 48-hour orbit, about 37 hours are available for uninterrupted observations with the EPIC cameras and about 3 more hours with the RGS and OM detectors. The are more severe restrictions on the observations. The most important are related with the pointing constraints and driven by either the instrument avoidance angles of bright objects or the S/C requirements on the alignment with respect to the Sun, to ensure sufficient energy supply. The S/C solar aspect angle is limited within the range 70 ­ 110 degrees. The angle with the Earth limb and Moon has to be larger than 47 and 22 degrees, respectively. Major planets and solar system objects have to be avoided for OM observations. In total only 34 per cent of the sky is visible by XMM-Newton at a given revolution and even less at a given time within the revolution. The position of the Sun relative to the stars as seen by XMM-Newton is changing during the year (in the same way as it changes for us on Earth!) and most of the sky can be visited for a few weeks only. Figure 2 shows, as an example, the sky that was visible during the revolution number 221 (February 21-22, 20001). In particular, the Earth position in the XMM-Newton sky changes during the revolution following the satellite path along its orbit. Therefore some visible objects are far enough from the Earth, while some others are only visible either at the start or at the end of the revolution. Additional constraints are driven by scientific reasons. Examples of this are systems of two stars that should be observed at a given phase of their orbits; also multi-wavelength studies of variable objects that need coordination with other satellites or ground-based observatories; or variability monitoring projects that require an specific time lag between consecutive observations within the programme. Approved targets that because of the different constraints have short visibility periods are called "critical targets". Special care is taken to get all critical targets in the planning within their visibility windows. Long and short term planning The planning of XMM-Newton observations starts with the identification of all "critical" objects and the time slots when they can be observed. The optimal date (usually the first possible one) for each critical target is then "reserved" in a long-term plan. The rest of the available time is filled with objects that are close to the critical ones


considering the assigned priorities. An observing plan is made public 3 months in advance, with half of the time left free. The detailed plan is prepared 3 to 4 weeks in advance. The SOC mission-planning group takes care of the optimisation of the XMM-Newton scientific activities and creates a timeline, which is passed to the Flight Dynamics group at ESOC. Flight Dynamics checks that all the constraints are considered and introduces additional information for the manoeuvres. The final timeline (which is the list of the actual telecommands and their execution dates and times) is generated at the Mission Operations Centre (MOC) which is located at ESOC. The operations' team at the SOC takes care of any additional manual commanding that is needed, though these are minimized to nearly no significant manual routine intervention. It is therefore a team task that involves most of the groups at the SOC and MOC. The teams work in a close collaboration, which is one of the keys for the success of the entire project. As soon as one revolution is scheduled, the scientists that requested the scheduled observations are informed via electronic mail and the details of the schedule are made public in the XMM-Newton web site at Vilspa. The whole process can take 1-2 days for each revolution. At this point the nominal planning is finished. No changes would be introduced unless an eventuality arrives. A ToO alert is, by definition, one of such eventualities. Reception and scheduling a ToO alert The scientific value of many ToO observations will significantly increase with decreasing reaction time. For example, a delay of 2h in a GRB observation implies a 10 % decrease in the expected flux. Therefore the procedure must ensure the shortest possible reaction time for such observations after reception. ToO alerts reach the SOC via the GCN (GRB Coordinates Network) circulars or through the XMM-Newton SOC web site at Vilspa (which allows every astronomer to propose a ToO observation). Every incoming ToO alert is immediately checked by the SOC instrument operator on shift. There are three pre-defined selection criteria: 1. Does the object require immediate reaction, like e.g. GRB's? 2. Is the target visible for XMM-Newton in the current orbit? 3. Are the target coordinates sufficiently accurate to allow an observation, i.e. would the target be in the field of view? If these three questions have a positive answer, then the SOC scientist on call is informed. After arrival at the SOC, the scientist evaluates in detail the feasibility of the proposed observations (e.g. the earliest time the new target could be reached), the impact on the ongoing revolution (e.g. which observation(s) has(have) to be substituted) and the scientific expectations (e.g. expected flux and spectra). Based on this evaluation, the Project Scientist, who is by default on call, decides whether "to go" or "not to go". In case he decides to go, the instrument and spacecraft operators are informed and one expert from the Flight Dynamics group is called at the MOC. The goal is to start the slew to the ToO within four hours after the alert was received. The whole scheduling process described in the previous section, which in normal conditions requires from one to two days and a half, has to be done in 2 to 3 hours. This can only be achieved because all involved persons work in a team. Many tasks that are performed one after the other during routine scheduling are now done in paralell or are pre-planned in advance. At the MOC, the flight dynamics is evaluated based on the new coordinates and the approximate starting time of the additional slews. The instrument operators carefully check the possibilities to interrupt ongoing observations and the time to swap timelines. The SOC scientist acts as mission planner. The observation of the ToO, i.e. instruments and modes, is prepared. Wherever possible observation templates, which were prepared in advance, are used. The schedule for the current revolution is opened (without interruption of the ongoing satellite activities). One or more targets are removed from it and the ToO observation is fitted into the available slot. However, in all this excitement the security of the S/C and its instruments has absolute priority and cannot be forgotten. A schedule generated in 2-3 h is not expected to be as optimised as a nominally generated one. In this respect, ToO are expensive. However, the expected scientific results well deserve to pay the price. After reception of the schedule generated at the SOC, the MOC generates the new version of the timeline. Half an hour before the time chosen for the swap of the old timeline with the new one, the current one is interrupted. The spacecraft and instrument operators manually finish the ongoing observation. Immediately after the timeline swap the spacecraft starts the slew to the position of the ToO. As a backup solution, the Flight Dynamics group and the spacecraft operators are ready to manually command the slew, which allows some flexibility in the procedure. When the slew is finished and the ToO observation starts more and more scientists are coming to the operations room. The excitement there increases with every further minute of accumulated observing time. Do we see the target? Is it well centred in the field of view? Do we see emission lines? Is the target bright enough to get a spectrum in the RGS detectors? These are the questions rising in the room and leading to the scientific analysis described in the next section.


Example: scheduling GRB010220 An example of a revolution when one of the two GRB's observed with XMM-Newton so far is illustrated in Figure 2. The figure shows a projection of the sky for XMM-Newton revolution 221, that started on February 21, 2001, at 6:38 (UT). The original plan for this revolution was to spend first 56ks on 47 Cassiopeae, then 33ks on 3C 58 and finally 47ks on HR 1099. 47 Cassiopeae is an active cool star (a "solar analog"), but about 1000 times more luminous in Xrays than the Sun because of the presence of a close companion. 3C 58 is a point-like source at the centre of a supernova remnant, almost certainly the youngest neutron star ever observed. HR 1099 is another cool star, much brighter in X-rays than 47 Cassiopeae, used as calibration for the RGS wavelength scale. On February 21, at 4:30 (UT) an alert was sent by the team of the X-ray BeppoSAX satellite. A GRB (GRB010220) had been detected on February 20, 22:51 (UT), it was visible with XMM-Newton, but the coordinates were only preliminar. A following communication with the refined position reached the Vilspa SOC on the same date at 6:08 (UT). The error box was sufficiently small to full fill the pre-selection criteria. Therefore, the ToO alert was triggered, i.e. the SOC scientist was called and the Project Scientist approved the GRB observation. At this time the revolution 221 was just starting, so the EPIC cameras could observe about 7 h later (13:38 UT). The goal was therefore to be on target by 13:38 (UT). The requested exposure time was of the order of 40ks. This was about the EPIC exposure time in the first observation scheduled in the original timeline, which was, just by chance, close to the GRB position. The obvious way to re-plan the revolution was to replace 47 Cassiopeae by GRB010220, leaving the two remaining observations unchanged. Following the described ToO implementation procedure, a new timeline was generated. In this case, the manoeuvre to the GRB was manually performed before the timeline swap. The observation was successful and X-ray emission from the GRB, though already very weak, was detected. The results are outlined below in this paper. Results of some ToO observations The first XMM-Newton ToO observation was done as early as March 2000, in revolution 44. However, it was just after the end of the "Commissioning" phase, and there was still some lack of knowledge of the instruments' performance. In addition, the target resulted weaker than expected. The next object in the XMM-Newton ToO list was a newly discovered high redshift quasar which was awarded discretionary time by the Project Scientist. The results are outlined below. It was followed by observations of a nova detected in the Large Magellanic Cloud (Nova LMC 2000). This nova was discovered on July 12, 2000 and first observed with XMM-Newton on July 25. This is one example of a slow reaction time ToO. Follow up observations of the nova were requested to monitor its different phases. The third observation was performed on March 29, 2001. The analysis on the data received is ongoing. The first GRB observed with XMM-Newton was GRB001025, on October 27, 2000, at 0h (UT), though weak, X-rays were detected and an International Astronomical Union circular was published with the precise position as measured in the EPIC images. Later on, another GRB and two binary systems have been observed. More details are given below. X-ray binary systems Quite recently two binary systems have been observed as "slow reaction time" ToO targets. Many X-ray binaries are transient. They are bright only occasionally (once every few months to tens of years) and briefly (between days and months) in comparison to the quiescent state. Such ToO observations enable the study of a wide dynamic range of luminosity states. In the X-ray faint state, the optical emission is less affected by X-ray irradiation and the companion can be most easily studied, particularly if it is lighter than a few solar masses and intrinsically faint. The cause for the quiescent X-ray emission is not completely understood. Is there a different kind of less energetic accretion present, or does the emission relate more directly to the compact object? In the X-ray bright state, the high-energy radiation enables the detailed study of the compact object and its immediate neighbourhood. Because of the sensitivity of the instruments on XMM-Newton, the platform is well suited for studies at both ends of the flux scale if the X-ray binary is in our Galaxy. At low fluxes, the quiescent emission is easily detected. At high fluxes, X-ray may potentially reveal details about the chemical composition, density and morphology of the accreting matter, and relativistic effects close to the compact object may be detected. Transients in outburst necessarily have to be observed as ToO, because the outburst times cannot be predicted. SAX J1711.6-3808, is a new X-ray transient discovered on February 8, 2001 by BeppoSAX and observed by XMMNewton on March 2. GRS 1758-258, is a well-known, very bright black hole candidate micro quasar in the centre of the Galaxy. Although it is classified as a non-transient system, it suddenly moved into an extremely dim state between February 21 and 27, 2001 as detected by the RossiXTE satellite, providing a unique opportunity to study


the low state X-ray emission. It was observed on March 22, 2001. Figures 3 and 4 show the EPIC and RGS images of this bright source. The analysis of the observations is currently ongoing. GRS 1758-258 was observed before with XMM-Newton, on 19 September 2000. At this time, its spectrum displayed the typical signature of an accretion disc around a black hole and the first indications of a transition state (more details are given in the ESA Science web pages). The comparison of the source spectrum in two different states promises even more valuable results. A high redshift quasar The most distant known celestial object, the quasar SDSJ1024-0125 was observed by XMM-Newton on May 28 2000. Discovered in the Sloan Digital Survey (Fan et al. 2000), SDSJ1044-0125 has a redshift of z = 5.80 which corresponds to an age of only one million years after the Big Bang. The XMM-Newton observation yielded a statistically significant detection in the EPIC p-n camera in about 32 ks exposure time (Brandt et al. 2001). Albeit exciting the discovery of X-ray sparkles from the farthest borders of the Universe was for the scientist team, it was also a source of puzzlement. If SDSSJ10.24-0125 would have an optical to X-ray luminosity ratio similar to objects of the same class, the X-ray flux should have been about two orders of magnitude higher. A possible explanation for this unexpected X-ray faintness is significant X-ray absorption. Intriguingly enough, that would match the idea that ancient quasars are preferably embedded in dusty environments, possibly due to regions of intense nuclear star formation, which are cleared up during subsequent phases of the quasar evolution. Alternatively, SDSSJ1024-0125 could represent an early stage of the quasar evolution, when the potential well required to produce their immense output energy was still in the process of being formed. The XMM-Newton observation was unable to discriminate between these competing scenarios. Nonetheless it underlines the capability of the scientific payload onboard XMM-Newton to investigate these remote phenomena, at the origin of the baryonic age in our Universe. ray bursts Research in the field of GRB's has undergone an impressive acceleration in recent years. In parallel with the advances in our understanding of these huge explosions in the far Universe, new areas of investigations are opened by the incoming data. The unprecedented capability of XMM-Newton will prove crucial in addressing some of the most important issues of the field. After the distance-scale determination, the study of afterglows of GRB's has allowed to achieve a fairly good understanding of the radiation mechanisms in terms of a fireball model. There is evidence that at least a fraction of GRB's are the result of the explosion of a massive progenitor, going off in a starforming region. The latter scenario is supported by the discovery of iron lines and edges in the X-ray spectrum of 5 GRB's. The data are still sparse, and we still have to understand whether those lines are a common feature of GRB, and we are missing them in other events because of lack of sensitivity or because they fade away shortly after the burst. This is an area in which the combination of high throughput, spectral resolution and reaction time of XMMNewton, can hardly be surpassed by other experiments, provided that a moderately bright afterglow is observed. So far, XMM-Newton has observed two GRB's (GRB001025, GRB010220), finding in both cases a candidate afterglow, too dim, however, for significant search of iron features. Nonetheless, the case of GRB010220 is of particular interest for another feature: the time decay connecting the early X-ray emission, observed by BeppoSAX, with the Newton-XMM data point, taken 15 hours after the GRB, is very steep, actually one of the steepest ever observed. Such behaviour is usually connected with either a collimated fireball or with an expansion in a very dense medium, typical e.g. of star-forming regions. Figure 5 shows a picture of the EPIC p-n image of this GRB. New and old mysteries wait to be unveiled, and the key observation leading to their solution can just be around the corner. What is the origin of GHOST's (GRB Hiding Optical Source Transient), a.k.a. dark GRB, i.e. events without optical afterglows (but with X-ray afterglows)? If GRB's are indeed associated with massive progenitors and therefore lie in regions of star formation, it is likely that in a large fraction of events, the optical emission is heavily absorbed by dust. However, we cannot exclude that these events are GRB at z>5, such that optical photons are absorbed by the Ly forest. BeppoSAX has also revealed the existence of another new class of events, the socalled X-ray flashes, or X-ray rich GRB's. In these events the bulk of the energy is not produced in -rays, but in Xrays. An extremely intriguing possibility is that these phenomena occur in very distant galaxies (z>10). Finally, very little is known on short GRB's, i.e. events lasting less than 1 s, since no counterpart has been so far identified. It is speculated that they may be produced by mergers of two neutron stars, that should result very short bursts. No counterpart/afterglow has been so far identified at any wavelength for the last two classes of objects. Here, the high sensitivity and reaction time of Newton-XMM can be decisive in solving these mysteries. We acknowledge Drs. Luigi Piro and Jean in `t Zand for their contribution to this article and B. Montesinos for useful comments. Figure Captions


Figure 1. XMM-Newton orbit. XMM-Newton travels around the Earth describing a highly eccentric ellipse. The plane of the ellipse is tilted 40 degrees with respect to the Earth's equatorial plane. The fraction of the orbit that is above the Earth's equatorial plane (i.e. pointing to the North) is shadowed. At its maximum distance, or apogee ­marked with an "A" in the figure-, the spacecraft reaches 114000 km of altitude and then it returns to the closest distance of 7000 km at perigee, "P" in the figure. The duration of one orbit (its period) is very close to 48 h. The figure shows in red the fraction of the orbit which lies within the Earth's radiation belts. The green line shows the part of the orbit that is available for science. The time at which the EPIC observations can start (7 h after the perigee passage) is also shown. With the spacecraft moving much faster at perigee than at apogee, this orbit offers the astronomers the possibility of long uninterrupted observations of close to 140 ks. Figure 2. The XMM-Newton sky for revolution 221 (February 21-22, 2001). This figure shows a projection of the sky as seen by XMM-Newton, the equatorial coordinates are shown in the middle (right ascension in hours goes horizontally, declination in degrees vertically) The Sun, Earth and Moon avoidance regions are shown in red, blue and orange respectively. The Sun position is marked with an "S", the anti-Sun with an "A". Different Earth positions are shown and appear as deformed ellipses, the numbers at nearly the centres of these ellipses indicate the time elapsed since the start of the revolution. Though these numbers cannot be read in part of the figure, they are used as an indication of the path followed by the Earth. The concentration of numbers at the top-right of the figure shows where the Earth is at XMM-Newton apogee, when the satellite is moving more slowly. It is also when the Earth size projected on the sky is smaller, though this is not clearly seen in the figure because of the projection effects. Only three different Moon positions (at the start, middle and end of the revolution) are shown. For this particular revolution the Moon does not impose additional constraints on the visibility because it is itself located in the Sun avoidance region. However, a few days later, when it moved a bit higher and to the left (i.e. further to the North / West) it lies just inside the visibility window band. It should be noted that the Moon avoidance angle is larger for the optical monitor (OM). The positions of some major planets and asteroids are also indicated in orange. They should be avoided for observations with the OM. This is a revolution that was quickly re-planned to allow a ToO observation. The positions of the scheduled targets and the one that was removed from the schedule are shown. Note that it is just by chance that GRB010220 is very close to the second target in the schedule: 3C 58. Figure 3. EPIC p-n image of GRS 1758-258. The sharp arcs that appear in the upper part of the image are caused by single mirror reflections of photons possibly from GX 5-1 which is about 40 arcmin northward and outside the EPIC field of view Figure 4. RGS data of GRS 1758-258. The dispersion axis runs horizontally, with shorter wavelengths (higher energies) to the left. The top panel shows the image of the dispersed light on the detector (the spatial, or cross dispersion direction is along the vertical axis). The bottom panel shows the CCD's intrinsic energy on the ordinates, used to separate photons reflected in first and second order off the gratings. The lack of photons at long wavelengths is the result of the high absorption the GRS 1758-258 photons suffer on its way to the Earth from the centre of the Galaxy. Figure 5. EPIC p-n image of the field of GRB010220. The GRB position is shown, well within the BeppoSAX error region.