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The Peculiar Evolutionary History Of The Terzan 5 Transient IGR J17480-2446

22/05/2012

During its Galactic bulge monitoring program, performed between 08:45UTC and 12:27UTC on October 10th 2010 (Bordas et al. 2010) and again on October 11th (Chenevez et al. 2010), the IBIS/ISGRI instrument onboard INTEGRAL discovered an outburst from a hard X-ray source originating from within the globular cluster Terzan 5. This source, now known as IGR J17480-2446, is an analogue to the clock-like burster GS 1826−238 (Chakraborty et al. 2011) and was swiftly identified as a neutron star (NS) low-mass X-ray binary (LMXB) with a relative slow rotation frequency of 11Hz (Strohmayer & Markwardt 2010). The LMXB system containing IGR J17480-2446 has an orbital period of 21.3 hr around a companion star with a mass of 0.4 (Papitto et al. 2011).

N.B. Terzan 5, discovered in 1968, is a globular cluster lying at approximately 18000 light years or 5.5±0.9 kpc (Ortolani et al. 2007) from Earth rich in a class of pulsar known as the millisecond pulsar (MSP). This cluster contains at least 33 of this pulsar type (Freire 2007). It is worth mentioning that the MSP population inside Terzan 5 includes the millisecond pulsar “PSR J1748–2446ad”, the fastest known millisecond pulsar spinning at 716 Hz (Hessels et al. 2006). Although IGR J17480-2446 is not an MSP, recent work by Papitto et al. (2010) coupled the high rate of MSPs in Terzan 5, has indicated that through accretion from its 0.4 companion IGR J17480-2446 is in the process of spinning-up. Being in this stage in the pulsar evolutionary track will reveal key information about the development of millisecond pulsars from slower spinning neutron stars via the recycling mechanism.

Millisecond radio pulsars are believed to be a result of the recycling scenario: a transfer of angular momentum via accretion of matter from a main sequence companion star to a compact neutron star (Alpar et al. 1982, Radhakrishnan & Srinivasan 1982). Such scenarios cause the spin-up of millsecond pulsars in binary star systems known as low mass X-ray binaries (LMXBs). The relative slow rotation period for IGR J17480-2446 is quite the peculiarity. Such a slowly spinning neutron star is unusual for LMXBs, because we expect accretion-induced spin-up of such stars (Bhattacharya & van den Heuvel 1991). IGR J17480-2446 shows thermonuclear bursts (Chenevez et al. 2010, Linares et al. 2011b) with burst oscillations phase locked with the accretion powered pulations (Cavecchi et al. 2011), and the magnetic field is within the range  (Cavecchi et al. 2011, Papitto et al. 2011). Such a field measurement is indicative of a pulsar which has started the recycling mechanism phase. However, the 11Hz rotation frequency is curiously slow, where the majority of rotation periods for accretion powered pulsars in globular clusters being of the order of milliseconds (Patruno et al. 2012). So what is this cause of such a low rotation frequency?

As mentioned, understanding this anomaly is valuable because IGR J17480–2446 can be the only accreting pulsar discovered so far which is in the process of becoming an accreting millisecond pulsar. Recent work by Patruno et al. (2012) concluded that IGR J17480-2446 is in an exceptionally early Roche-lobe overflow stage (RLOF) in its evolution, where the 0.4 companion star to IGR J17480-2446 is spilling its outer layers over the L1 point onto the compact neutron star. They predict that the total spin-up timescale to transform IGR J17480-2446 from it’s current state as a slow pulsar (~1 Hz) into a millisecond pulsar (>100 Hz) is approximately a billion years. Since the current RLOF is predicted to last for approximately one billion years they expect to observe perhaps 1 to 10 accreting millisecond pulsars that have followed a similar evolutionary history as IGR J17480-2446.

The prior spin-down history gives results which are not compatible with a binary having an age of several billion years as expected if IGR J17480–2446 is primordial or if it has not suffered exchange interactions in the last few billion years. Different formation scenarios are available to explain the apparent discrepancy between the age of the cluster and the binary. A possibility is that a recent dynamical encounter has played a role in forming the binary or in accelerating the onset of the RLOF phase. If an exchange interaction has taken place in the last 10-100 million years then this would explain why the apparent age of IGR J17480–2446 is so small in comparison with the age of the cluster. Coupled with this, the location of IGR J17480–2446 in the high mass and high central density cluster Terzan 5 suggests that the interaction rate might be particularly high in this environment. Such peculiarities in the work conducted by Patruno et al. (2012) has opened up a whole wealth of new questions regarding the formation of MSPs in the wider Universe.

Journal References:

  • Chakraborty, M.; Bhattacharyya, S.; Mukherjee, A. (2011) Terzan 5 Transient IGR J17480-2446: Variation Of Burst & Spectral Properties. Monthly Notices Royal Astronomical Society, 418 (1): pp.490-499.
  • Boyles, J.; Lorimer, D. R.; Turk, P. J.; Mnatsakanov, R.; Lynch, R. S.; Ransom, S. M.; Freire, P. C.; Belczynski, K. (2011) Young Radio Pulsars in Galactic Globular Clusters. The Astrophysical Journal, 742 (1): Article I.D. #51.
  • Patruno, A.; Alpar, M. A.; van der Klis, M.; van den Heuvel, E.P. J. (2012) The Peculiar Evolutionary History of IGR J17480-2446 In Terzan 5The Astrophysical Journal, 752 (1): Article I.D. #33.

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