Neutron Star Planets: Atmospheric processes and habitability

Of the roughly 3000 neutron stars known, only a handful have sub-stellar companions. The most famous of these are the low-mass planets around the millisecond pulsar B1257+12. New evidence indicates that observational biases could still hide a wide variety of planetary systems around most neutron stars. We consider the environment and physical processes relevant to neutron star planets, in particular the effect of X-ray irradiation and the relativistic pulsar wind on the planetary atmosphere. We discuss the survival time of planet atmospheres and the planetary surface conditions around different classes of neutron stars, and define a neutron star habitable zone. Depending on as-yet poorly constrained aspects of the pulsar wind, both Super-Earths around B1257+12 could lie within its habitable zone.Comment: Submitted to A&

Radio pulsars around intermediate mass black holes in super stellar clusters

We study accretion in binaries hosting an intermediate mass black hole (IMBH) of 1000 solar masses, and a donor star more massive than 15 solar masses. These systems experience an active X-ray phase characterized by luminosities varying over a wide interval, from <10^36 erg/s up to a few 10^40 erg/s typical of the ultra luminous X-ray sources (ULXs). Roche lobe overflow on the zero-age main sequence and donor masses above 20 solar masses can maintain a long-lived accretion phase at the level required to feed a ULX source. In wide systems, wind transfer rates are magnified by the focusing action of the IMBH yielding wind luminosities around 10^38 erg/s. These high mass-IMBH binaries can be identified as progenitors of IMBH-radio pulsar (PSR) binaries. We find that the formation of an IMBH-PSR binary does not necessarely require the transit through a ULX phase, but that a ULX can highlight a system that will evolve into an IMBH-PSR, if the mass of the donor star is constrained to lie within 15 to 30 solar masses. We show that binary evolution delivers the pre-exploding helium core in an orbit such that after explosion, the neutron star has a very high probability to remain bound to the IMBH, at distances of 1-10 AU. The detection of an IMBH-PSR binary in the Milky Way has suffered, so far, from the same small number of statistics limit affecting the population of ULXs in our Galaxy. Ongoing deeper surveys or next generation radio telescopes like SKA will have an improved chance to unveil such intriguing systems. Timing analysis of a pulsar orbiting around an IMBH would weigh the black hole in the still uncharted interval of mass around 1000 solar massesComment: 10 pages, 8 figures, accepted for publication in MNRAS. Few comments added (minor revision

Ultraluminous X-ray Sources forming in low metallicity natal environments

In the last few years multiwavelength observations have boosted our understanding of Ultraluminous X-ray Sources (ULXs). Yet, the most fundamental questions on ULXs still remain to be definitively answered: do they contain stellar or intermediate mass black holes? How do they form? We investigate the possibility that the black holes hosted in ULXs originate from massive (40-120 $M_\odot$) stars in low metallicity natal environments. Such black holes have a typical mass in the range $\sim 30-90 M_\odot$ and may account for the properties of bright (above $\sim 10^{40}$ erg s$^{-1}$) ULXs. More than $\sim 10^5$ massive black holes might have been generated in this way in the metal poor Cartwheel galaxy during the last $10^7$ years and might power most of the ULXs observed in it. Support to our interpretation comes from NGC 1313 X-2, the first ULX with a tentative identification of the orbital period in the optical band, for which binary evolution calculations show that the system is most likely made by a massive donor dumping matter on a $50-100 M_\odot$ black hole.Comment: 4 pages. To appear in the Proceedings of the Conference "X-Ray Astronomy 2009: Present Status, Multiwavelength Approach and Future Perspectives", Bologna, Italy, September 2009, Eds. A. Comastri, M. Cappi, L. Angelini, 2010 AIP (in press)

The Peculiar Evolutionary History of IGR J17480-2446 in Terzan 5

The low mass X-ray binary (LMXB) IGR J17480-2446 in the globular cluster Terzan 5 harbors an 11 Hz accreting pulsar. This is the first object discovered in a globular cluster with a pulsar spinning at such low rate. The accreting pulsar is anomalous because its characteristics are very different from the other five known slow accreting pulsars in galactic LMXBs. Many features of the 11 Hz pulsar are instead very similar to those of accreting millisecond pulsars, spinning at frequencies >100 Hz. 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. We first verify that the neutron star (NS) in IGR J17480-2446 is indeed spinning up by carefully analyzing X-ray data with coherent timing techniques that account for the presence of timing noise. We then study the present Roche lobe overflow epoch and the two previous spin-down epochs dominated by magneto dipole radiation and stellar wind accretion. We find that IGR J17480-2446 is very likely a mildly recycled pulsar and suggest that it has started a spin-up phase in an exceptionally recent time, that has lasted less than a few 10^7 yr. We also find that the total age of the binary is surprisingly low (<10^8 yr) when considering typical parameters for the newborn NS and propose different scenarios to explain this anomaly.Comment: Accepted by ApJ, in pres

The Optical Counterpart to the Accreting Millisecond X-ray Pulsar SAX J1748.9-2021 in the Globular Cluster NGC 6440

We used a combination of deep optical and Halpha images of the Galactic globular cluster NGC 6440, acquired with the Hubble Space Telescope, to identify the optical counterpart to the accreting millisecond X-ray pulsar SAX J1748.9-2021during quiescence. A strong Halpha emission has been detected from a main sequence star (hereafter COM-SAX J1748.9-2021) located at only 0.15" from the nominal position of the X-ray source. The position of the star also agrees with the optical counterpart found by Verbunt et al. (2000) during an outburst. We propose this star as the most likely optical counterpart to the binary system. By direct comparison with isochrones, we estimated that COM-SAX J1748.9-2021 has a mass of 0.70 Msun - 0.83 Msun, a radius of 0.88 pm 0.02 Rsun and a superficial temperature of 5250pm80 K. These parameters combined with the orbital characteristics of the binary suggest that the system is observed at a very low inclination angle (~8 deg -14 deg) and that the star is filling or even overflowing its Roche Lobe. This, together with the equivalent width of the Halpha emission (~20 Ang), suggest possible on-going mass transfer. The possibile presence of such a on-going mass transfer during a quiescence state also suggests that the radio pulsar is not active yet and thus this system, despite its similarity with the class of redback millisecond pulsars, is not a transitional millisecond pulsar.Comment: 8 pages, 6 figures. Accepted for publication in Ap

Type I X-ray bursts, burst oscillations and kHz quasi-periodic oscillations in the neutron star system IGR J17191-2821

We present a detailed study of the X-ray energy and power spectral properties of the neutron star transient IGR J17191-2821. We discovered four instances of pairs of simultaneous kilohertz quasi-periodic oscillations (kHz QPOs). The frequency difference between these kHz QPOs is between 315 Hz and 362 Hz. We also report on the detection of five thermonuclear type-I X-ray bursts and the discovery of burst oscillations at ~294 Hz during three of them. Finally, we report on a faint and short outburst precursor, which occurred about two months before the main outburst. Our results on the broadband spectral and variability properties allow us to firmly establish the atoll source nature of IGR J17191-2821.Comment: 9 pages, 7 figures - accepted for publication in MNRA

Timing the accretion flow around accreting millisecond pulsars

At present, ten years after they were first discovered, ten accreting millisecond pulsars are known. I present a study of the aperiodic X-ray variability in three of these systems, which led to the discovery of simultaneous kHz quasi periodic oscillations in XTE J1807-294 and extremely strong broadband noise at unusually low variability frequencies in IGR J00291+5934. Furthermore, we classified SWIFT J1756.9-2508 as an atoll source and measured in its 2007 outburst spectral and variability properties typical of the extreme island state. I also give detailed estimates of the total fluence during the studied outbursts.Comment: To be published in the proceedings of the workshop 'A Decade of Accreting Millisecond X-ray pulsars' (Amsterdam 14-18 April 2008; Eds. Wijnands et al.

Modelling the outburst profile of X-ray powered millisecond pulsars

The outbursts of low mass X-ray binaries are prolonged relative to those of dwarf nova cataclysmic variables as a consequence of X-ray irradiation of the disc. We show that the time-scale of the decay light curve and its luminosity at a characteristic time are linked to the radius of the accretion disc. Hence a good X-ray light curve permits two independent estimates of the disc radius. In the case of the millisecond pulsars SAX J1808.4-3658 and XTE J0929-314 the agreement between these estimates is very strong. Our analysis allows new determinations of distances and accretion disc radii. Our analysis will allow determination of accretion disc radii for sources in external galaxies, and hence constrain system parameters where other observational techniques are not possible. We also use the X-ray light curves to estimate the mass transfer rate. The broken exponential decay observed in the 2002 outburst of SAX J1808.4-3658 may be caused by the changing self-shadowing of the disc.Comment: To be published in the proceedings of the workshop 'A Decade of Accreting Millisecond X-ray pulsars' (Amsterdam 14-18 April 2008; Eds. Wijnands et al.

A Chandra observation of the millisecond X-ray pulsar IGR J17511-3057

IGR J17511-3057 is a low mass X-ray binary hosting a neutron star and is one of the few accreting millisecond X-ray pulsars with X-ray bursts. We report on a 20ksec Chandra grating observation of IGR J17511-3057, performed on 2009 September 22. We determine the most accurate X-ray position of IGR J17511-3057, alpha(J2000) = 17h 51m 08.66s, delta(J2000) = -30deg 57' 41.0" (90% uncertainty of 0.6"). During the observation, a ~54sec long type-I X-ray burst is detected. The persistent (non-burst) emission has an absorbed 0.5-8keV luminosity of 1.7 x 10^36 erg/sec (at 6.9kpc) and can be well described by a thermal Comptonization model of soft, ~0.6keV, seed photons up-scattered by a hot corona. The type-I X-ray burst spectrum, with average luminosity over the 54sec duration L(0.5-8keV)=1.6 x 10^37 erg/sec, can be well described by a blackbody with kT_(bb)~1.6keV and R_(bb)~5km. While an evolution in temperature of the blackbody can be appreciated throughout the burst (average peak kT_(bb)=2.5(+0.8/-0.4)keV to tail kT_(bb)=1.3(+0.2/-0.1)keV), the relative emitting surface shows no evolution. The overall persistent and type-I burst properties observed during the Chandra observation are consistent with what was previously reported during the 2009 outburst of IGR J17511-3057.Comment: 6 pages, 4 figures, accepted for publication in ApJ (2012-06-08