In Orbit Operation of LAXPC Payload: Concerns and Care. R. K. Manchanda Department of Physics Mumbai University

Transcription

1 In Orbit Operation of LAXPC Payload: Concerns and Care R. K. Manchanda Department of Physics Mumbai University

2 LAXPC realization In orbit operation of LAXPC While a large number of things contributed to the success of LAXPC hardware the key reasons I believe are : People--good, dedicated, hardworking Cooperation among all the groups--team effort. Adequate funding when needed On the management side it requires Good planning and maintaining emphasis on schedule Being open, honest and frank in discussions with supporting groups Giving appropriate credit to the doers of good work Defining instrument descope options at the earliest Doing what is needed to be done and not standing on ceremony Sticking to design where good is good enough and not allow hardware changes just for the sake of improvement

3 In orbit operation of LAXPC LAXPC Chronology Written proposal was submitted April 2000 Seed Development Money 2002 Organization of Astrosat Project 2003 Management Structure 2004 Formal project approved 2005 Payload Delivery 2014 Launch 2015 Operation 10 Years nominal

4 RXTE Chronology In orbit operation of LAXPC Proposal Phase (Oct 17, 1980) Phase II Phase III Launch Dec 30, 1995 Switched Off January 5, 2012 Active life 16 years

5 In orbit operation of LAXPC About the talk. In the last 2 days you have heard several talks which describe the Science we can expect from the LAXPC payload and from the more qualified scientists than me, I therefore, chose a completely different topic but an extremely relevant one. In the title of my talk, I did not used the word Challenges, as I do not think it to be something, which we can not workaround The talk should not be taken as critique on the performance of LAXPC payload. From my 34 years experience of working with high pressure xenon MWPC counters, I have tried to foresee what operational issues may arise and what care we can take while making science out of the data. (First paper being in 1978 NIM 156, 57 )

6 In orbit operation of LAXPC LAXPC Capabilities LAXPC can be called as Super RXTE or say RXTE plus RXTE 6500 cm 2 LAXPC 8000 cm 2 Effective Area (cm 2 ) ASTROSAT-LAXPC RXTE-PCA SAX-PDS Energy (kev) RXTE Publication 1900 so far LAXPC??

7 Open problems In orbit operation of LAXPC Origin and Evolution of Black Holes Active Galactic Nuclei Accretion history of universe (obscured AGNs) Origin of the Hard X-ray Background (Spectra of all AGN -Seyferts to QSOs) Astrophysics of Compact Objects Black Hole Systems: emission mechanisms in disks and jets Low Magnetic Field Neutron Stars: accretion disk coronae QPOs in Black Hole and Neutron Star LMXBs - disk-corona connection Highly Magnetized Neutron Stars: cyclotron lines measures of B fields Neutron Stars or Black Holes in Giant Molecular Clouds

8 Open problems In orbit operation of LAXPC Supernovae & Nucleosynthesis Ti-44 survey of galactic plane: obscured SNe and SN rate in Galaxy Origin and Nature of Gamma-ray Bursts Faintest sources (LogN-LogS): GRBs as cosmological probes Luminosity functions, high resolution spectra (temporal and spectral), and precise positions Relation to supernovae Nature of ``Fast" GRBs and Soft Gamma-ray Repeaters Study of SGR throughout the Local Group Diffuse Inverse Compton Emission from Galaxy Clusters Measurement of magnetic fields in galaxy clusters

9 In orbit operation of LAXPC Proportional counters Advantage: Versatile instruments (wide variety of applications) Variety of shapes and sizes Highly sensitive (counter can respond to the formation of one ion pair) Size of pulse proportional to initial number of ion pairs Energy discrimination (spectroscopy) Ability to count at much higher rates Disadvantages: Highly Stable high voltage required due to nature of gas amplification External amplification (preamplifiers, amplifiers) required to produce pulse Proper operation requires more attention on the part of the user Instruments tend to be "finicky" (i.e., more attention to maintenance is required) Susceptible to environmental conditions (heat, humidity) Self-absorption/leaks possible in counters using entrance windows

10 In orbit operation of LAXPC There is no such thing as a low risk heritage detector. Each detector is one-of-a-kind and should be treated as a high risk item with an appropriate amount of contingency and a good risk mitigation plan XTE Mission (NASA news 1996)

11 In orbit operation of LAXPC RXTE operation history 70 days of problem free operation. Three of the PCUs became subject to periodic discharge after launch. (33% gain reduction in all detectors) Periodic Discharge seen in counters # 3 and # 4 (n March 1996 and # 1 in March Change in operation strategy Loss of Propane layer in 2 counters # 0 in May 2000 and #1 in December 2006 Even in the HEXTE payload - Pulse height analyzer failed for one detector on March 6, The system of measuring dead time failed for large energy loss ( wait time 2.5 msec)

12 In orbit operation of LAXPC RXTE operation history

13 In orbit operation of LAXPC Absolute efficiency, Gain stability Gas behaviour, Aging effect Other variables Pulse rise time Resolution vs Energy ( Recombination; function of gas quality) Effects due to flexure of window

14 LAXPC counters In orbit operation of LAXPC In order to have sufficient pulse height at low energies, one uses high gas gain which takes the detector in to region of limited proportionality and therefore the behaviour of the counter is more susceptible to small perturbations. e.g. In LAXPC, in order to have a reasonable pulse at 3 kev above the noise, we operate Detectors. HV ~ 2800 V CSPA gain ~ 1 x Volts per Coulomb Which gives ~ 240 mv pulse for 3 kev which is our threshold Gas gain ~ For a 3 kev photon, the average no. of ion pairs produced in the xenon gas are; 3000 ev/ 30 ev ~ 100 primary pairs with gas gain ~ 1.5 x 10 5 pairs Critical charge for local ionization above which non-linearity sets in Q~5X10 6 Which is about 90 kev in LAXPC and our upper threshold is 80 kev and hence small non linearity may come in if we increase the HV voltage during operation.

15 Absolute efficiency In orbit operation of LAXPC R e = 10-5 x E 1.63 kev g. cm -2 = 1.71 x 10-4 E 1.63 kev cm ; 2 atm

16 In orbit operation of LAXPC About the talk. 14 edges. Edge energies (kev): K E+01 L I E+00 L II E+00 L III E+00 M I E+00 M II E-01 M III E-01 M IV E-01 M V E-01 N I E-01 N II E-01 N III E-01 N IV E-02 N V E-02

17 In orbit operation of LAXPC Anode # Anode # counts Anode # 8 Anode # 7 Anode # 6 Anode # 5 Anode # 4 Anode # 3 Anode # MCA CH No. Anode # wire number wire number

18 Energy calibration In orbit operation of LAXPC MCA calibration or data taken through the electronics chain

19 In orbit operation of LAXPC Absolute efficiency Recombination ( Energy, Gas quality, Gas pressure) Change in the rise time Pulse height and noise ratio Gradual shift in energy scale, combined with difficulties in low energy incomplete charge collection are a challenge - RXTE

20 Energy resolution In orbit operation of LAXPC Measured energy resolution hides several known and unknown terms Variance in the observed pulse height at a given energy E may be written as where N is the mean number of primary ion pairs produced by a photon of energy E and mean track length L is the mean amplification factor and dp; dn ; dm and dl are the standard deviation of the relevant quantities. Third, statistically independent factor, which limits the resolution of each spectrometric system, normally the spread in the rise-time of current pulses generated in the counter due to finite track length of electron. A non-zero spread in the rise-time spectrum is determined among other things by the diffusion processes which terms depends on the relative contribution of weak fields occurring in the active volume of the counter

21 Energy resolution In orbit operation of LAXPC But what it hides is that ionization efficiency itself is not uniform and shows an oscillatory behaviour and the avalanche process which is taken of the form of a Polya distribution is itself an empirical fit.

22 In orbit operation of LAXPC Energy resolution

23 In orbit operation of LAXPC Gas gain

24 Pulse shape In orbit operation of LAXPC

25 In orbit operation of LAXPC Aging of the detector The formation of insulating layers on the anode of proportional counter is a major effect of its long-term exposition to the radiation. The presence of the insulating deposit modifies the surface charging the space distribution of the electric field in the active volume of the counter, thus degrading the energy resolution For example, generation of the layer, 0.5 μm in the thickness on anode wire, 25 μm in diameter can reduce gas gain by (20 40)%. In case of an insulator layer, the anode is shielded and the layer is negatively charged up. It conducts to further reduction in gas gain, by decreasing in V.

26 In orbit operation of LAXPC Influence of aging on counting rate The increase of radiation intensity measured by a proportional counter leads to undesirable changes of its parameters, such as pulse height and energy resolution. It has been found that both the pulse height and the energy resolution significantly decrease with increasing radiation intensity. These changes are called count rate effect -- Kowalski and Mindur Energy resolution as function of count rate, for for different value of collected charge. Relative change in the pulse height as a function of the count rate for different value of collected charge. The Y-axis is the relative decrease in pulse height from the value corresponding to the count rate below 400 cps.

27 Pre-amplifier In orbit operation of LAXPC Thermal noise of the first-stage FET Shot noise caused by gate current of first-stage FET and dark current of detector

28 HV Stability In orbit operation of LAXPC At high gain operation as used by LAXPC, the stability of HV is highly important. Observations show that even small change in the monitoring component (100K/100 M) and feed back chain, can lead to substantial effect in the gain. HV is design is a push pull design with voltage multiplier stages,,error amplifier and PWM comparator to obtain the Line Feedforward. With a voltage mode control the voltage ramp to the PWM comparator is fixed..

29 HV Stability In orbit operation of LAXPC The line rejection is not good under suddenly changing conditions. The reason is that a sudden change in line is not felt by the PWM comparator directly and so it continues with the same duty cycle for a while. But any change in input voltage ultimately requires a change in the stead-state duty cycle (as per the steady DC transfer function equation of the converter: D=V O /V IN ). So not changing the duty cycle quickly enough leads to an output overshoot or undershoot. The system has to wait for the output error to be sensed by the error amplifier, and that information to be communicated to the PWM comparator as a change in the applied control voltage. That eventually corrects the duty cycle and the output too, but not before some swinging back and forth (ringing) around the settling value. At large counting rate which impacts the current, HV can get into oscillation.

30 Gain stability In orbit operation of LAXPC

31 In orbit operation of LAXPC Summary Upshot of this short discourse in LAXPC proportional counter and associated electronics was to emphasize that performance of the detector is a function of several operating parameters and is essential to monitor and to calibrate these almost on continuous basis and take necessary remedial action to optimize the working of the detectors (Detectors were sealed almost 2.5 years ago). X 3 That is big job for POC - In orbit calibration and consistency with Spectral data from CZTI and the SXT - Keep a watch on the MTO and MTT signals apart from veto and use these to corelate the response matrix (Repeated blank field observations during the entire mission)

32 In orbit operation of LAXPC Detector Background, Dead Time effects Threshold sensitivity In order to have spectral results with respect to errors of background determination.

33 For 600 km, < 10 o orbit, the expected background is In orbit operation of LAXPC LAXPC background Energy (kev) ct/sec 3 sensitivity for observation of 10 4 s LAXPC_SEN SENSITIVITY (ph.cm -2 s -1 kev -1 ) millicrab Crab nebula JEM_X LASE HEXTE ENERGY ( kev)

34 In orbit operation of LAXPC RXTE PCA Counting rate HEXTE counting rate ~100 c s -1 per detector 200 cm 2 area in kev - all shielded Config. - Lead Florescence photons seen clearly kev band - Larger than anticipated dead time fraction

35 Gain stability In orbit operation of LAXPC

36 In orbit operation of LAXPC Threshold sensitivity

37 In orbit operation of LAXPC Detector background in orbit Aperture flux + Leakage from sides Aperture flux can be estimated to some extents Photons: + Charged particles and how well we can discriminate the charge particle background Second part is not straight forward Electrons, low energy protons and high energy protons show different behaviour Leakage from the sides depends not only the cosmic background but the local production which includes several radiation nuclides which have relatively long half life and depends on the orbital environment of the space craft.

38 In orbit operation of LAXPC Detector background in orbit On a low-inclination (i < 5 ) low-earth orbit (LEO; typical altitude of 550 km), a detector is exposed to a variety of background components Galactic cosmic rays (mainly protons, α-particles and electrons), whose fluxes are strongly modulated by the geomagnetic field Secondary protons, neutrons, electrons and positrons produced in the Earth atmosphere; and atmospheric gamma rays. The latter have two origins Reflection of cosmic diffuse X- and gamma-ray photons, which is the dominant component below ~100 kev, and High-energy emission induced by cosmic-ray particles impinging the Earth atmosphere.

39 In orbit operation of LAXPC Radiation fluxes and LEO The results of observations of intense ion and electron fluxes at low and equatorial latitudes (at altitudes of ~ km

40 In orbit operation of LAXPC Stopping Power and Range Tables for Protons

41 In orbit operation of LAXPC Energy loss in a proportional counter Ramnamurthy and Demeester 1967

42 In orbit operation of LAXPC LAXPC background Key problem as I see is the modeling of detector background in orbit in terms of known parameters and its short term and long term stability/trends My personal experience is that in the lab environment simulated data fit the measurements extremely well.

43 In orbit operation of LAXPC Laboratory simulations Radio-active sources for Spectral Calibration and GEANT4 simulation for background estimation

44 In orbit operation of LAXPC Simulation constraints But when it comes to fitting the in orbit measured data we to have use an addition adhoc term Good fit to the Measured Background = Simulated background + HOG part General Background Background due to secondary photons Time drift of the counting rate SAA flux variation versus altitude if any precession period Faint source model + Bright source mode SAA flux variation and data gaps lead to large systematic errors

45 RXTE background In orbit operation of LAXPC Short term Left panel and long term variations (right) 0

46 In orbit operation of LAXPC RXTE data epoch Epoch Start Time Stop Time PCU Epoch 1 Launch 21 Mar :33 0,1,2 Epoch 2 21 Mar :34 15 Apr : Epoch 3A 15 Apr :06 09 Feb :00 0,1,2 Epoch 3B 09 Feb :00 22 Mar :38 0,1,2 Epoch 4 22 Mar :39 13 May :00 0,2 Epoch 5A 13 May :00 01 Jan :00 2 Epoch 5B 01 Jan :00 01 Jan :00 2 Epoch 5C 01 Jan :00 Present 2

47 RXTE background In orbit operation of LAXPC

48 RXTE background In orbit operation of LAXPC

49 RXTE background In orbit operation of LAXPC AGN total spectrum, showing old (red) and new models (black). The obvious changes and improvements are in the kev band.

50 RXTE background In orbit operation of LAXPC : Description of PCU 0 Background Issues After Loss of Propane : New Background Models for Epoch : Revised versions of PCABACKEST and PCARSP : New Mission-Long Background Models : New Background Models Correct for Long Term Linear Drift : New Background Models Remove Long Term Trends : Propane Loss in PCU : Problems and Corrections to RXTE PCA Background : New PCA Breakdown history files : Improvements to the PCA Response Matrix : Correction to PCABACKEST spike artifacts : Improvements to the PCA Effective Area (HEASoft 6.11)

51 RXTE background In orbit operation of LAXPC

52 RXTE background In orbit operation of LAXPC

53 RXTE background In orbit operation of LAXPC Wilson-Hodge et al. 2011, ApJ, 727, L40;

54 Crab nebula In orbit operation of LAXPC < 15 kev kev

55 In orbit operation of LAXPC Experience from RXTE Background models for the PCA detectors are created from observations of the "blank" sky (i.e. the recorded signals are the sum of the Cosmic X-ray Background and instrumental background). We observe that the background is time variable So we parameterize the background as a function of spacecraft and instrument conditions. In particular, all useful models to date parameterize the background as a function of one or more of the coincidence rates with an additional time dependent term to account for an activation component due to passage through the radiation belts in the South Atlantic anomaly.

56 In orbit operation of LAXPC Experience from RXTE The faint models use a rate called "L7", which is the sum of all 2-fold coincidences between adjacent xenon signal chains (L1+R1, L2+R2, L3+R3, L1+L2, L2+L3, R1+R2, and R2+R3). Even without the propane layer, this quantity is unambiguously defined, and our first attempt was to examine whether the L7 rate still tracks apparent X-ray rate for PCU 0. Summarized in a phrase, the answer is "not well enough". There are at least two contributing factors. Without the propane layer, there is additional noise in the apparent X-ray rate due to electrons which deposit energy in the first layer (and which previously would have stopped in the propane layer or the propane+first xenon layer). Additionally, without the propane layer, our electron screening criteria is undefined. The scatter is sufficiently large that we have abandoned the idea that we can model the PCU 0 background as a function of PCU 0 parameters.

57 In orbit operation of LAXPC Experience from RXTE Systematic offset Unmodeled variance Long term effect Co adding the data (energy smearing- Gain and offset correction)

58 SAA effects In orbit operation of LAXPC The effect is periodic because of a complex beat between the spacecraft orbital period, the earth rotation period, and the begin-of-day sampling window. Solar Flares

59 Dead time issues In orbit operation of LAXPC Total counting rate of the detector before rejection Accepted Events - These are events which meet selection criteria Coincident Events - These events are detected in more than one anode simultaneously. These include both the good xenon events i.e. with a fluorescent photon and those most likely due to particles. [At very high count rates, however, it becomes increasingly likely that two coincident anode firings are due to two real photon events rather than one particle event] Very Large Events - These are events above the upper discriminator. Like the coincident events, they are mostly due to particles.

60 Dead time issues In orbit operation of LAXPC Accepted events = Good events + Compton scattered events 10 6 Photon cross section Xenon 10 5 cross section (barns) Energy (MeV)

61 Dead time issues In orbit operation of LAXPC Electron flux observations from low-earth orbit and near the magnetic equator during the event. Drew L. Turner, Yuri Shprits, Michael Hartinge & Vassilis Angelopoulos Nature Physics 8, (2012)

62 In orbit operation of LAXPC Radiation fluxes and LEO

63 Dead time issues In orbit operation of LAXPC About the talk. AW = 150 cm 2 sr 2-10 kev

64 Dead time issues In orbit operation of LAXPC

65 In orbit operation of LAXPC Total counting rate It is quite difficult to get exact estimate of the total background in the space environment `Based on old data from several missions ~ 2000 cts s -1 per detector Compare this with the Crab nebula ~ 2500 cts s % cts s -1 Following observation was made in the ROSAT PSPC operation The following types of events are seen by the PSPC in the space environment. The majority of events are produced by minimum ionizing particles with a count rate between 100 cts s-1 and 500 cts s-1. Depending on the incidence angle, they deposit an energy of 10 to 100 kev in the sensitive volume of the detector. Heavily ionizing particles with a count rate of about 2x10-2 cts s-1 and deposit up to 100 MeV in the detector. ( 1200 times the ULD of laxpc)

66 In orbit operation of LAXPC LAXPC electronics We have used 50 microsec window for the amplifier in which a very large event which saturates the pre-amplifier and cause ringing. The time actually depends on the energy of the incoming particle will come to base line (can be several hundred microsec). This may have spectral consequences? RXTE used 4 commandable windows, set nominally to 20, 60, 150, and 500 μs. The pre-launch intention was to allow extra clean data for very faint sources and high throughput data for very bright sources. The size of the VLE window affects the shape of the power spectrum Early observations of Cyg X-1 with the VLE window set to the smallest value demonstrated a failure associated with this window on one of the analog chains; use of the shortest VLE window was discontinued. 150 μs was the default value.

67 Dead time issue In orbit operation of LAXPC Total dead time of LAXPC is 50 microsec and Trigger logic dead time of 5-6 microsec Energy distortion may be also seen due to pulse pile of effect

68 In orbit operation of LAXPC Energy distortion due pile up effect

69 dsvle deadtime per event (microseconds) The values vary from one detector to the next: the figures above are accurate to 10 percent. Dead time issues VLE settings: In orbit operation of LAXPC The VLE setting is "2" by default which corresponds to a dead time per event of 150 microseconds. It is only changed on the explicit instruction of the PI before or during the observation. ds value Dead time (microsec) The values vary from one detector to the next: the figures above are accurate to 10 percent

70 Summary In orbit operation of LAXPC Real time gain monitoring of the background to resolve hardware issues Careful analysis for the copper K_alpha fluorescence at 8 kev Background modeling on 6 monthly basis after stabilization During SAA the voltage should be zero Operating warmer detectors Once optimized, the Hardware parameters to be only changed under exceptional circumstances Dead time corrections and any energy distortion during the observations of X-ray flares or bright X-ray sources

71 In orbit operation of LAXPC

72 Dead time issue In orbit operation of LAXPC

73 Crab nebula In orbit operation of LAXPC 20 year light curve kev kev

74 Dead time issues In orbit operation of LAXPC

75 Suzaku HXD In orbit operation of LAXPC

76 In orbit operation of LAXPC About the talk.

77 In orbit operation of LAXPC