Near-infrared observations of Rotating Radio Transients

arXiv:1003.2085v2 [astro-ph.GA] 17 May 2010 Mon. Not. R. Astron. Soc. 000, 1–8 (2009) Printed 26 October 2018 (MN LATEX style file v2.2) Near-infrared observations of Rotating Radio Transients N. Rea1,2⋆, G. Lo Curto3, V. Testa4, G. L. Israel4, A. Possenti5, M. McLaughlin6,7, F. Camilo8, B.M. Gaensler9, M. Burgay5 1 Institut de Ciencies de l’Espai (ICE-CSIC, IEEC), Campus UAB, Fac. de Ciencies, Torre C5-parell, 2a planta, 08193 Barcelona, Spain 2 University of Amsterdam, Astronomical Institute “Anton Pannekoek”, Postbus 94249, 1090 GE, Amsterdam, The Netherlands 3 European Southern Observatory, Av. Alonso de Cordova 3107, Vitacura, Santiago, Chile 4 INAF - Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone, Italy 5 INAF - Osservatorio astronomico di Cagliari, Poggio dei Pini, Strada 54, 09012 Capoterra (CA), Italy 6 Department of Physics, West Virginia University, Morgantown, WV 26506, USA 7 National Radio Astronomy Observatory, Green Bank, WV 24944, USA 8 Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA 9 Sydney Institute for Astronomy, School of Physics, The University of Sydney, NSW 2006, Australia 26 October 2018 ABSTRACT We report on the first near-infrared observations obtained for Rotating RA- dio Transients (RRATs). Using adaptive optics devices mounted on the ESO Very Large Telescope (VLT), we observed two objects of this class: RRAT J1819–1458 and RRAT J1317–5759. These observations have been performed in 2006 and 2008, in the J, H and Ks bands. We found no candidate infrared counterpart to RRAT J1317–5759 down to a limiting magnitude of Ks ∼21. On the other hand, we found a possible can- didate counterpart for RRAT J1819–1458 having a magnitude of Ks = 20.96±0.10. In particular, this is the only source within a 1σ error circle around the source’s accurate X-ray position, although given the crowded field we cannot exclude that this is due to a chance coincidence. The infrared flux of the putative counterpart to the highly magnetic RRAT J1819–1458 is higher than expected from a normal radio pulsar, but consistent with that seen from magnetars. We also searched for the near-infrared coun- terpart to the X-ray diffuse emission recently discovered around RRAT J1819–1458, but we did not detect this component in the near-infrared band. We discuss the lu- minosity of the putative counterpart to RRAT J1819–1458 in comparison with the near-infrared emission of all isolated neutron stars detected to date in this band (5 pulsars and 7 magnetars). Key words: stars: pulsars: general — pulsar: individual: RRAT J1819–1458, RRAT J1317–5759 1 INTRODUCTION Rotating RAdio Transients (RRATs) are a recently discov- ered class of neutron stars (McLaughlin et al. 2006, 2009; Keane et al. 2010) characterized by dispersed radio bursts with flux densities (at a wavelength of 20 cm) ranging from ∼100 mJy to 4 Jy, durations from 2 and 30 ms, and aver- age intervals between repetition from 4 minutes to 3 hours. Periodicities have been inferred through the study of arrival times of these bursts, ranging between 0.1–7 s. The timing solutions derived from the radio bursts, i.e. their periods and period derivatives, indicate that they are neutron stars (McLaughlin et al. 2006, 2009). ⋆Ramon y Cajal Research Fellow; rea@ieec.uab.es. The detection of such sources is rather difficult, mainly due to the very tiny duty cycle of their radio bursts (0.1– 1 s of radio emission per day). Detailed population studies show that there may be more RRATs than canonical rota- tional powered radio pulsars in our Galaxy (McLaughlin et al. 2006, 2009; Keane & Kramer 2008). Determining the nature of the emission from these ob- jects and how many RRATs are present in our Galaxy is of paramount importance for pulsar emission theories, as well as for neutron star population studies. Up to now many hypotheses on the nature of these objects have been put for- ward, based on the comparison with the radio pulsar class: e.g. that the RRATs may be neutron stars near the radio “death line” (Zhang et al. 2006), or that the sporadicity of the RRATs is due to the presence of a circumstellar as- 2 Rea et al. RRAT J1819-1458 Date Filter FWHM(pix) Exp.Time Mag. Lim.(3σ) zero point 2006 Jun. 26 Ks 2.9 2280 21.4 22.52±0.06 2006 Jun. 26 J 5.0 1620 22.8 23.27±0.18 2006 Jun. 28 H 3.2 1620 21.8 23.12±0.07 2006 Jul. 21 H 3.1 1800 22.4 23.43±0.07 2006 Jul. 31 J 10.0 1800 22.9 23.67±0.06 2008 Jul. 18 Ks 4.0 1080 20.7 22.40±0.08 RRAT J1317-5759 Filter Date FWHM(pix) Exp.Time Mag. Lim.(3σ) zero point 2006 Apr. 02 Ks 3.2 2280 20.90 22.48±0.06 2006 Apr. 02 J 6.2 2520 22.70 23.14±0.07 2006 Apr. 27 H 3.9 1320 20.90 22.55±0.06 2006 Apr. 28 H 3.9 2280 22.80 23.06±0.06 Table 1. Summary of the ESO-VLT observations of RRAT J1819–1458 and RRAT J1317–5759 . The NACO pixel size corresponds to 0.027′′. teroid belt (Cordes & Shannon 2008; Li 2006) or a radia- tion belt such as seen in planetary magnetospheres (Luo & Melrose 2007). Note that

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