Manickam, Sivakumar G

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Indian J. Phys. 73B (6), 925-930 (1999)IJP Ban in ternational journalIntroduction to quasi periodic oscillations (QPO) in the X-ray emission from neutron stars and black hole candidatesS iv a k u m a r G . M a n ic k a mS.N.Bose National Centre for Basic Sciences JD Block, Sector III, Salt Lake, Calcutta-700091, IndiaAbstract* We review the present status of the nature of Quasi Periodic Oscillations (QPO) observed in X-rays from black holes and neutron star candidates. We also present model existing in the literature for these QPOs.K eyw ords : Black holes, neutron stars, X-ray sources PACS N os, : 97.60.Lf, 97.60.Jd, 98.70.Qy1. IntroductionThe sources w hich produce X-ray radiation are Active Galactic: Nuclei, Super­nova rem n an ts , B inary  system s, C luster, Stellar corona, and s o m e  unidentified ob jec ts. A m ong these , sources which accrete m atter, namely Active Galactic Nuclei and  b inary  system s, show Q PO  behaviour in their X-ray emission. The detection  o f th ese  X -ray rad iation  is made mostly using satellites, as e a rth ’s a tm osphere  is n o t tra n sp a re n t to  X-rays. The study of Q PO  acts as probe for th e  dynam ics o f m a tte r  in the  accretion disk as direct imaging is not pos­sible w ith  th e  c u rre n t in strum en ts. T he energy dependence of Q PO  tim ing behaviour also  gives ind irect inform ation of the accretion disk structure.2* Physical mechanisms for X-ray generationT here a re  basically  th re e  physical m echanism s for producing X-rays, namely therm al em ission from  a  h o t gas, synchrotron radiation from relativistic elec­trons in th e  presence o f a  m agnetic field, and blackbody radiation [1].© 1999 LACS926 Sivakumar G Manickam2.1. Thermal emission from a hot gasT he electrons which a re  in therm al equilibrium  have a  M axwellian d istribu tion  of velocities. T he average energy of all electrons in therm al equilibrium  is the sam e and  is d ic ta ted  by th e  tem p era tu re  T . T he  electrons passing close to  a  positive ion experiences au acceleration and hence produces rad iation  via therm al b rem sstrah lung . T he expression for th e  energy spectrum  is, I ( E , T )  =  A G (E , T ) Z 2nen i(k T )~ 1/2e~E /kT, where G ( E , T )  is th e  G au n t fac to r, Z  is the  atom ic num ber of th e  ion, n e, m are the  num ber density  of electrons and ions respectively, and k is th e  B oltzm ann con stan t. In addition to  this continuum  spectrum  there  can also be line emission.2.2. Synchrotron radiatton from the relativistic electronsT he electrons gy ra ting  in a  m agnetic field a t relativistic velocities produce syn­chro tron  rad ia tion  via m agnetic brem sstrah lung . T he radiation-energy spec­tru m  is given as, 1 (E ) =  A E ~ a for an assum ed power-law of energy d istri­bution. T he  m agnetic  field is som ew hat aligned bu t th e  particle velocities are expected to  be isotropic, and  hence the  observed spectrum  depends only on the  m agnetic  field B  and  the energy spectrum  of electrons.2.3. Blackbody radiationT he expression for blackbody rad ia tion  is given as, I ( E ,  T ) =  ^ ^ i/ k r ^ yM ore energetic photons are produced when th e  tem p era tu re  is higher. The spectrum retains the overall shape, although it is strongly modified by the s te llar a tm osphere . T he  surface tem p era tu re  of a  newly form ed neutron  s ta r  is a round one million degrees and will em it in th e  X-ray range.3. Quasi Periodic Oscillations(QPO)T here  a re  tw o types o f ob jec ts  which show quasi periodic oscillations, namely, Low M ass X -ray B inaries (LM XB) and  A ctive G alactic  Nuclei (AGN) [2]. In LM XB, th e  com pact accreting  o b jec t could be e ither a  neu tron  s ta r  or a  black hole cand ida te . In LM XB th e  m ass of th e  com panion is less th a n  1M@. They are  p a rt  o f o lder galac tic  popu lation  of age g rea te r th a n  109 years. T hey  have X -ray b u rs ts  and  soft X -ray sp e c tra  b u t lack X -ray pulsations and  eclipses. T h e  op tical light is dom inated  by th e  disk. T he  m agnetic  field s tre n g th  is much less th an  1012- 1013 gauss. In c o n tra s t, the  m assive X -ray binaries (M X B) with com panion m ass g rea te r th an  10M© have age less th a n  107 years. X -ray bursts a re  ab sen t b u t X -ray  pu lsations and eclipses are  seen often in M X B, and the m agnetic  field s tre n g th  is a round  1012-1013 gauss. B righ test LM XB are in galactic  bulge and  a re  less likely to  show X -ray b u rsts  th an  fa in ter ones.927Introduction to quasi periodic oscillations (QPO) etc4. QPOs from neutron starsThe tuning analyse and spectral analysis could give information about the decay of the magnetic field, disk-magnetosphere interaction, inner radiation pressure dominated accretion disk near Eddington limit, and the evolutionary connection between LMXB and millisecond radio pulsars. The X-ray radiation is detected in different energy channels as function of time. When the QPOs of a source are plotted in Hardness-Intensity Diagram (HID) or in Colour-Colour Diagram (CCD) i.e. hard colour vs. soft colour they trace out distinctive pat­terns and hence creating a possibility for their classification.4.1. Z sourcesThey trace out a Z  pattern in HID and CCD. The parameter which is varying along Z  is most likely the accretion rate. There are no ‘jumps’ when the source traces out the Z  pattern. The three branches of the Z are named as Horizontal Branch (HB), Normal Branch (NB), and Flaring Branch (FB). The HB has Very Low Frequency Noise (VLFN), Low Frequency Noise (LFN), and High Frequency Noise (HFN) components. The frequency of QPO (v q p o ) is high, and it is strongly correlated with position (hence the intensity lx)- The QPO frequency is in the range 15-5.') Hz and the sharpness of the QPO, defined as i'qpo' ’s 'n ran£e 0.12-0.4. The X-ray spectrum of LFN in HB QPO is harder than time averaged flux. The HB QPO has hard lags and there are soft lags in LFN. In the NB, the QPO frequency is anti-correlated to the intensity, and the change in vqpo is small. The range QPO frequency is 5*7 Hz. In the FB, the QPO frequency increases as the intensity increases. The width of the QPO, Jk/y increases and finally becomes HFN.There is a discontinuous jump in the QPO frequency at the HB-NB transi­tion. The HB QPO may persist along with NB QPO in the NB. These indicate that there are different physical phenomena for HB QPO and NB QPO. FB QPO is probably the same as NB QPO* A possible understanding of the Z  sources should explain the process which drives the source along Z  and what happens at the branch transitions. The HB QPO and LhN can possibly be explained by beat frequency model [3].4.2. Atoll sourcesThe Atoll sources are fainter than Z sources, but some of them are reliable X-ray bursters. They are further classified as Island and Banana sources as the pattern traced in the CCD is like an island and banana. The island sources are less active in CCD. Their X-ray intensity is low and HFN dominates. Bananas have s tro n g e s t VLFN which is approximately the I f f  noise. Bananas recur at roughly the same place. One transition from island to left end of banana928 Sivakumar G Manickams ta te  is observed, b u t th ere  is no clear fixed position of island w ith respect to  banana.If th e  accretion ra te  is th e  only difference between Z  and Atoll sources, then  th e  tran sitio n  from  Z  to  A toll o r vice versa is possible in LM XB with large range o f accretion ra te . It is passible th a t  the  Atoll sources have low m agnetic field s tre n g th , so th a t  th e  b ea t frequency model does not work.5. QPOs from Black hole candidatesT he s trong  (>  20% rm s) and rapid (>  1 Hz) variations are no longer good argum en t for Black Hole C and ida tes  (BH C s). M any pulsars are now known to  be strong ly  and rapidly variable. T he following are the  exam ples of some black hole cand ida tes and  their m ain characteristics.1. L M C  X -3 : T h is a  s trong  BH C based on the  com panion s ta r ’s orb ita l velocity curve, b u t does no t show strong , rapid variability.2 . Cyg X - l :  T h is is a  good BH C which shows rapid variability bu t has noise power spectrum  sim ilar to  th e  pulsars.3. L M C  X -3 : T h is a  s tro n g  B H C  based on the  com panion s ta r ’s orbital velocity curve, b u t does no t show strong , rapid variability.4. L M C  X - l :  A ten ta tiv e  BHC based on th e  low quality  optical radial curve and th e  u ltrarso ft X -ray spectrum . It has a  slow Q P O  of frequency 0.075 Hz.5. G X  339-4: T h is is a  BH C based on th e  u ltra  soft spectrum  and the  bi- m odal spectra l behaviour.6 . G R S 1915+105: T his is a  BH C w ith X-ray transien ts . Very chaotic hard X -ray profile.T he  BH C have th ree  source s ta te s , low, high, and very high. T he accretion ra te  is th e  dom inan t p a ram ete r which decides th e  s ta te . T he  Power Density S pectrum (P D S ) of very hard  s ta te  has 3-10 Hz Q PO s.6. Cyg X-lT he d a ta  o f 1100 days ob tained  using BA TSE (B urst And T ransien t Source E xperim ent) on C G R O  (C om pton  G am m a Ray O bservatory) is reported  in C ra ry  e t al. [4]. T he  presence of hard  com ponent indicates th a t  the  source is in low s ta te . T here  is a  s tro n g  correlation between power law index or and fractional rm s am plitude  f, and  also betw een a  and th e  flux F, determ ined by a  basic system  p aram ete r, th e  accretion ra te (A f). T he fas t tim e variability can be explained in te rm s of shot-noise m odels [5-7]. w here th e  decay tim e of sh o ts  im plies th e  b reak  frequency in the  PD S. M ineshige e t al. [8-9] propose th a t  the  disk is in self-organized critical s ta te . Q ualita tive  a rgum en ts lead to th e  conclusion th a t  f  increases as a  decreases.929Chakrabarti & Titarchuk [10] predicts a shock at several tens of Schwarzschilc radius. In the low state the post shock region is quite hot and the emergent spectrum is very hard (energy spectral index n ~  0.5). The photon index a  increases with accretion rate in the low state. Molteni et al. [11] finds oscilla­tion of shock location when a “resonance” condition is satisfied. The centroid frequency of oscillations increases as M increases. The cv-f correlation can be accounted for by [10-11]. At super Eddington accretion rate, [10] predicts that the flow inside the shock is cooled by Comptonization of low energy photons from the disk, and post-shock flow is predominantly radial and converging. Comptonization in this region can still occur due to bulk motions producing hard tail, a  «  1.5. The photon index increases weakly with the accretion rate.7. GES 1915+105The source was observed over a long period (see, [12] for recent results). The regular bursts can be due to periodic infall of matter in black hole from an oscillating shock front [10-11]. The peculiarities of this source are regularity over a time-scale of several days, presence of secondary peaks, and spectrum is hardest near the end of the burst. As the burst progresses, the temperature of the infalling matter increases which implies spectral hardening. Irregular and long bursts can be produced when accreting matter has large angular momentum, so that momentary disk is formed before advection. Details of some of the RXTE observations are in Manickam and Chakrabarti [13].8. GX 339-4This source was discovered in 1973 [14]. The companion of BHC is a low mass, spectral class F2 star. The maximum magnitude in blue region of 21 during the OFF  state implies that luminosity of the star less than 1-2 Lq . It has an orbital period of 14.8 hrs and its distance is 4 kpc. The ON and OFF states are distinct, each last for a time of the Order of months. It has three X-ray states, namely O FF, low(hard), and high (soft). There is an anti-correlation between X-ray and visible region luminosity in low and high states. The time duration for state transition is of the order of less than a day. Intensity changes by a factor greater than 60 in time-scale of months. The photon index is 2.4- 2.7 in the high state and 1.6 in the low state. The period of observed QPOs are 1.2s, 10s, 20s in low state. In very bright state it shows 6 Hz QPO along with LFN and VLFN. Rapid variability is seen only in hard and OFF state.Introduction to quasi periodic oscillations (QPO) etc930 Sivakumar G  ManickamA cknow ledgm entsThis work is partly supported by a RESPOND project of Indian Space Research Organizations with Prof. S.K. Chakrabarti.R eferences1. F K Lamb, N Shibazaki M Alpar, J Shaham Nature 317 681 (1985)2. T Belloni and G Hasinger Astron. Astrophys. 227 L33 (1990)3. S K Chakrabarti and L Titarchuk Astrophys. J. 455 623 (1995)4. P A Charles and F D Seward Exploring the X-ray universe (Cambridge Univ. press) (1995)5. D J Crary et al. 1996, Astrophys. J. 462 L716. T H Marker! et al. Astrophys. J. 184 L67 (1973)7. S Mineshige, M Kusunose and R Matsumoto Astrophys. J. 445 L43 (1995)8. S Mineshige, M Takeuchi and H Nishimori Astrophys. J. 435 L125 (1994)9. S Miyamoto and S Kitamoto Nature 342 774 (1989)10. D Molteni, H Sponholz, S K Chakrabarti Astrophys. J. 457 805 (1996)11. B Paul, et al. Astrophys. J. 492 L63 (1998)12. P G Sutherland, et al. Astrophys. J. 219 1029 (1978)13. M van der Klis Ann. Rev. Astron. Astrophys. 27 517 (1989)14. S G Manickam and S K Chakrabarti (this volume)

Introduction to Quasi Periodic Oscillations (QPO) in the X-Ray Emission from Neutron Stars and Black Hole Candidates

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Introduction to Quasi Periodic Oscillations (QPO) in the X-Ray Emission from Neutron Stars and Black Hole Candidates

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