Radio Observations of X-ray Binaries : Solitary and Binary Millisecond Pulsars

1. Radio Observations of X-ray Binaries:Solitary and BinaryMillisecond Pulsars Jeong-Sook Kim1 & Soon-Wook Kim2 Department of Space Science and Astronomy in KyungHee University Korea Astronomy and Space Science Institute

2. Abstract Radio study of millisecond pulsars has recently been active with numerous, newly discovered sources in globular clusters, and a few detected Galactic ones. We discuss recent radio observations of two classes of millisecond pulsars, solitary and binary, emphasizing perspectives in low frequency observations and its importance.

3. Introduction (1) • Binary millisecond Pulsar: Most millisecond pulsars are members of low-mass X-ray binary system in which the companion is a degenerate (white) dwarf, or supergiant and planets in a few cases. These system are generally believed to evolve from low-mass X-ray binary system in which a neutron star is spun up to millisecond becoming visible as a radio pulsar at the end of accretion phase (e.g., Bahttacharya & Van den heuvel 1991 for a review). Formation of millisecond Pulsar (Lorimer 2005 Science 307, 855)

4. Introduction (2) • Isolated (Solitary) millisecond Pulsar : Two commonly invoked variants of the spin-up model to explain isolated millisecond pulsars are • the binary system was disrupted at the time of the second supernova explosion, or • the companion from which the neutron star received its spin angular momentum has since been ablated by the pulsar wind (Lorimer, et al.,1995ApJ439,933) • Spectra are of great importance to understand the mechanism of pulsar radio emission. • Millisecond pulsars are believed to be a special population of pulsars, which are distinguished from “normal” pulsars by period, period derivative, magnetic field strength, age and evolutionary history.

5. Single and Binary MSPs:Similarity & Difference • In many respects, the properties of single and binary millisecond pulsars (MSPs) look similar. • For example, their period distributions are not significantly different. In addition, characteristic age and spin-down luminosity are dependent on the period derivative (e. g., Lorimer 2005). • However, there are arguments that solitary MSPs are less luminous than when they are in binaries. • How about their spectral index at the lower, poorly explored radio frequencies ? We here present this issue.

6. Low Frequency Radio Spectra of Binary & Isolated MSPs (1) binary MSPs single MSPs

7. Low Frequency Radio Spectra of Binary & Isolated MSPs (2) • As shown in the figures we present, the spectral indices for 100-1700 MHz observations look similar to each other. • Only a handful of sources, however, are available in particular at the low frequency range, down to ~400-500 and ~100MHz for single and binary pulsars, respectively. Therefore it is not enough to discuss realistic spectral slopes and “turnover” frequencies. In the case of normal pulsars, the turnovers have been observed at ~100MHz or lower, with higher fluxes than that in MSPs (e. g., Izvekova et al. 1981). • As shown in the right hand side of presented figures, there are a few solitary pulsars with their spectral slopes become flatter. Further exploration of lower frequencies below 400MHz might results in its turnover.

8. Low Frequency Radio Spectra of Binary & Isolated MSPs (3) • A irregularity in the spectral distribution can occur in a few hundreds MHz, as in the case of PRS J1022+1001. • This small turnover could be related to the system or accretion geometries. • Therefore, the detailed multi-wavelength observations within limited low frequency ranges are also interesting. Binary MSP PRS J1022+1001 log

9. Low Frequency Radio Spectra of Binary & Isolated MSPs (4) • There is an argument whether the spectral indices in MSPs are much steeper than those in normal pulsars. • Similarly, there also is the same argument for binary MSPs to the indices for solitary MSPs. • The number of MSPs discovered still is not enough to make desirable statistics: at most, less than a hundred including all binary and solitary MSPs. Observations at the frequencies of ~100MHz or lower are in particular rare. • The low frequency radio emission in binary MSPs is closely related not only to the magnetic field or system geometry itself, but to the possible existence of survived. truncated accretion disk around the pulsars (e. g., E k s i & Alpha 2005).

10. Perspectives for Low Frequency Observations of MSPs • Low flux, multi-wavelength observations at the low frequencies below ~100 MHz is currently not easily available. • In the sense of the multi-wavelength facility, newly launched Korean VLBI project, KVN, is not appropriate for the pulsar research. Its expected flux sensitivity, a few hundred mJy, in particular is far from the observed flux of ~mJy for MSPs. • Sing-dishes like the Parkes, or “giant” Arercibo Telescope and array systems like VLA or ACTA are only available for above ~400MHz. The lowest possible radio frequencies currently available for pulsar research may be ~100MHz from the Large Pulsed Array BSA radio telescope in Pushchino Radio Astronomy Observatory (Kuzmin et al.). • Fortunately, starting from this year, 2006, the low frequency array, LOFAR, for 30-200MHz range, expected to do its full operation in 2008, will be partly available with ~40 components. In addition, the Square Meter Array, SKA, for 100MHz-25GHz, will be also accessible partly from ~2013.

11. LOFAR: Low Frequency Array • LOFAR will be an ideal facility to detect the dim, low frequency MSPs with ~mJy detectability and ~30MHz waveband. • The possible “turnover” frequencies at ~100MHz, or lower, and its implication to the physics of accretion in binary MSPs will be discussed in detail in Kim & Kim (2006), in preparation.