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Intermittent Accreting Millisecond Pulsars

Intermittent Accreting Millisecond Pulsars. Introduction. An exciting new type of sources Pulsations appear and disappear either on timescales of hundreds of seconds or on timescales of days

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Intermittent Accreting Millisecond Pulsars

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  1. Intermittent Accreting Millisecond Pulsars

  2. Introduction • An exciting new type of sources • Pulsations appear and disappear either on timescales of hundreds of seconds or on timescales of days • In the early 1980s it was predicted that the progenitors of millisecond radio pulsars should be accreion-powered millisecond X-ray pulsars in neutron star low-mass X-ray binaries • The first one was discovered in 1998 • A total of seven AMSPs have been found • Recently three intermittent AMXPs were found

  3. HETE J1900.1-2455 • A transient source • Remain active for ~3 years but pulsations were detected during only the first ~ 70 days • On three occasions during this period, it was observed an abrupt increase in the pulse amplitude, approximately coincident with the time of a thermonuclear burst, followed by a steady decrease on a timescale of 10 days • Pulsations at 377.3 Hz and in an 83.3 minute circular orbit with a companion of mass greater than 0.016 M_sun and likely less than 0.07 M_sun • No burst oscillations have been detected ApJ638,963-967, 2006

  4. Aql X-1 • A ~550 Hz burst oscillation transient source from which pulsations were detected only for ~150 sec out of the ~1.5 Msec • The pulsations were discovered at the end of a one-orbit ~1500 seconds onservation • Given the data structrue, we cannot exclude the occurrence of X-ray burst ~1400 seconds before the pulsations or immediately after the pulsations disappeared ApJ674, L41-L44, 2008

  5. The Scenarios Most Often Considered • The magnetic field in nonpulsating LMXBs is too weak to allow channeling of the matter onto the magnetic poles • The magnetic field has comparable strength inside most LMXB neutron star, but in the large majority of them, it has been “buried” by accretion • Pulsations are produced in all LMXBs, but in most of them they are attenuated by a surrounding scattering medium that washes out the coherent beamed pulsation • The pulsations are attenuated by gravitational lensing from the neutron star

  6. SAX J 1748-2021 • A transient source with a frequency of 442.36 Hz from which intermittent pulsations were detected during both 2001 and 2005 outbursts • The pulsations appeared and disappeared on timescales of hundreds of seconds • A suggestive relation between the occurrence of type-I X-ray burst and the appearance of the pulsations was found, but the relation is not strict • The reason for the intermittence of the pulsations remain unclear • It is now evident that a strict division between pulsating and nonpulsating neutron star systems does not exist • An orbit with a period of 8.7 hr • The companion star might be a main-sequence or a slightly evolved star with a mss of ~1 M_sun • No burst oscillations have been detected ApJ674, L45-L48, 2008

  7. The Neutron Star Transient SAX J1748.9-2021 In NGC 6440 • NGC 6440 is a GC at 8.5±0.4 kpc. • Bright X-ray outbursts from a LMXB were reported in 1971, 1998, 2001, and 2005 • In’t Zand et al. (2001)), from X-ray and optical observations, concluded that the 1998 and 2001 outbursts were from the same object, which they designated SAX J1748.9-2021

  8. Colors, Light Curves, and States • The PCA observations sample three different outbursts • The color-color diagrams show a pattern typical for atoll sources • No kHz QPOs have been found • No thermonuclear bursts were detected in the first outburst, 16 during the second , and four during the third • No burst oscillations have been found

  9. Pulsations

  10. Best Parameters Estimates

  11. The Pulse Frequency As A Function Of Orbital Phase

  12. Discussion • From the Doppler shifts on the pulsations the authors determine that the system is in a near-orbit with period of 8.7 hr and projected radius of 0.39 lt-s • The stability of the pulsations strongly suggests that the pulsation frequency reflects the neutron star spin frequency and that SAX J 1748.9-2021 is an accreting millisecond X-ray pulsar • In SAX J1748.9-2021 and Aql X-1 the timescales on which the pulse amplitude can fluctuate are as short as ~100 s, too short for the properties of the neutron star core to change. So these changes must orginate in the disk or the outer layers of the neutron star envelope • Pulsations only around a mass accretion rate of ~2*10^(16) M_sun/s, indicating that instantaneous mass accretion rate rather than total accreted mass is the important quantity

  13. Discussion

  14. SOURCE STATES AND PULSATIONS • Two sub-types of NS LMXBs: the Z sources and the atoll sources • The Z source are the most luminous while the atoll sources cover a much wider range in luminosities • In the case of atoll sources, the main three states: EIS, IS, BB • All AMSPs with persistent pulsations have been observed only during their island or extreme island state • The 3 intermittent AMSPs are the only pulsating sources that have been observed not only in there island states, but also in their banana states

  15. POWER SPECTRA IN THE INTERMITTENT AMSPS • The shape of the power spectra between AMSPs and non-AMSPs are very similar • The frequency correlations for AMSPs and non-AMSPs and non-AMSPs are found to be shifted in frequency and only one non-AMSP might show frequency shifts as those seen in AMSPs • Confirming whether the pulse mechanism in AMSPs is related or not with these frequency shifts might give important clues that help us understand the differences between AMSPs and non-AMSPs

  16. 2005, ApJ 634, 1250-1260

  17. POWER SPECTRA IN THE INTERMITTENT AMSPS • For SAX J1748-2021 most of the power spectra are typically of the (upper) banana state, during both pulsating and non-pulsating periods; the low-frequency noise seems to be weaker when pulsations are present • The average power spectrum of the observation of Aql X-1 in which pulsations were detected is typical of the upper banana state • Most of the power spectra of HETE J 1900.1-2455 are like those typically seen during the EIS and IS

  18. They found identical (within errors) power spectra during the pulsating and the non-pulsating periods. Their results suggest that frequency correlations within a given intermittent source will not differe between pulsating and non-pulsating periods

  19. Questions • Can a unique scenario explain all the intermittency observed? • Is there a relation between the pulse mechanism and the thermonuclear burning on the neutron star surface? • What is the relation between the pulse mechanism and the burst oscillation mechanism? • Interestingly, bursts and burst oscillations have been detected in the AMSPs SAX J1808.4-3658 and XTE J1814-338 and bursts without burst oscillations in the two intermittent AMSPs SAX J1748-2021 and HETE J1900.1-2455 • The only connecting system might be Aql X-1, since no X-ray bursts have been detected in the other AMSPs • Does the pulse mechanism affect in any way the observed aperiodic variability?

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