A white dwarf companion to the relativistic pulsar PSR J1141−6545*

J. Antoniadis et al. 0 Centre of Astrophysics Research, University of Hertfordshire, College Lane , Hatfield AL10 9AB 1 Jodrell Bank Centre for Astrophysics, The University of Manchester , Alan Turing Building, Manchester M13 9PL 2 Max-Planck-Institut fur Radioastronomie , Auf dem Hugel 69, 53121 Bonn, Germany A B S T R A C T Pulsars with compact companions in close eccentric orbits are unique laboratories for testing general relativity and alternative theories of gravity. Moreover, they are excellent targets for future gravitational wave experiments like LISA and they are also highly important for understanding the equation of state of superdense matter and the evolution of massive binaries. Here we report on optical observations of the 1.02 M companion to the pulsar PSR J11416545. We detect an optical counterpart with apparent magnitudes V = 25.08(11) and R = 24.38(14), consistent with the timing position of the pulsar. We demonstrate that our results are in agreement with a white dwarf companion. However the latter is redder than expected and the inferred values are not consistent with the theoretical cooling tracks, preventing us from deriving the exact age. Our results confirm the importance of the PSR J11416545 system for gravitational experiments. 1 I N T R O D U C T I O N The value of relativistic binaries is highly recognized, as their study can provide insight into some of the holy grails of fundamental physics. Among them are tests of general relativity and alternative theories of gravity, the detection of gravitational waves, the study of the equation of state of superdense matter and tests of evolutionary scenarios for heavy stars (for a complete review see Lorimer & Kramer 2004). The sample of relativistic binaries discovered so far is dominated by double neutron stars (NSs), covering a wide range of orbital parameters. Another substantial fraction consists of white dwarf neutron star binaries, most of them in almost perfectly circular orbits (e.g. review by van Kerkwijk et al. 2005). These systems are the result of the evolution of a massive primary which evolves fast, explodes as a supernova and becomes a NS; and of a lighter secondary which evolves slower and eventually becomes a white dwarf (WD) (Driebe et al. 1998). During the final interaction phase, the NS is spun up to very short rotation periods and becomes a millisecond pulsar. Any eccentricity (primordial or resulting from the supernova kick) is dampened by tidal interaction before the secondary becomes a WD. A significant exception to the preceding is the binary PSR B2303+46 (Stokes, Taylor & Dewey 1985). In that system, the WD (van Kerkwijk & Kulkarni 1999) orbits the non-recycled pulsar in a highly eccentric orbit. Investigations into possible formation scenarios for this type of binary (Tauris & Sennels 2000; Davies, Ritter & King 2002; Church et al. 2006) have shown that they most likely originate from a binary system of massive stars with nearly equal mass. When the initially more massive star reaches the red giant phase, the secondary star accretes sufficient mass to surpass the Chandrasekhar limit, allowing it to eventually evolve into a NS. The primary star, however, loses sufficient mass to end up as a heavy WD. Hence, in the resulting system, the WD is expected to be older than the pulsar. The only other promising candidate for this category is the PSR J11416545 binary system, initially discovered in a Parkes survey (Kaspi et al. 2000). PSR J11416545 is a 0.2-d binary in an eccentric orbit (e 0.17, Bhat, Bailes & Verbiest 2008). The primary is a relatively young 394 ms pulsar (characteristic age 1.4 Myr), orbited by a compact object of unknown nature. Bhat et al. (2008) derived Mc = 1.02(1) M for the mass of the companion by applying the relativistic DDGR orbital model (Damour & Deruelle 1986) to their timing measurements. The latter is consistent with both a heavy WD and a light NS with the former case being more favoured by statistical evidence (Tauris & Sennels 2000). Jacoby et al. (2006) included the system in an optical survey but found no optical counterpart down to R = 23.4. This paper reports on optical observations of the companion star in the PSR J11416545 binary system. Our main scientific rationale for this study is that in the case of a positive WD confirmation, the system would be of great importance for gravitational tests. In particular, because of its gravitational asymmetry, PSR J11416545 would be one of the most constraining systems known for general relativity in the strong field regime as it is expected to emit strong dipolar gravitational radiation in a wide range of scalartensor theories (Will 1993; Esposito-Farese 2005; Bhat et al. 2008). The structure of the text is as follows. In Section 2 we describe the observations and the data reduction process while in Section 3 we present our results. Finally, in Section 4 we discuss our findings and comment on their astrophysical consequences and their importance in gravitational tests. 2 O B S E RVAT I O N S A N D D ATA R E D U C T I O N We have obtained optical images in the V-band and R-band filters, of the field containing PSR J11416545 using the FORS1 instrument mounted at the UT2 of the Very Large Telescope (VLT). Both filters resemble the standard JohnsonCousins filters but have slightly higher sensitivity in the red, sharper cut offs and higher throughput. The observations were conducted in service mode during the night of 2008 April 06. The conditions were photometric and the average seeing of the night was 0.7 arcsec. The total exposure time was 600 s in V and 1500 s in R. In order to minimize potential problems with cosmic rays and guiding errors and avoid saturation of bright stars, the exposures were split in three sub-exposures of 200 s in the V-band and three sub-exposures of 500 s in the R-band. For the data reduction we used the FORS1 pipeline provided by ESO. Each image was first bias corrected and flat-fielded using twilight flats. Bad pixels and cosmic ray hits in all frames were replaced by a median over their neighbours. The resulting frames were then sky subtracted, registered and combined in one averaged frame for each filter. 2.1 Photometry We performed point spread function (PSF) photometry on the average frame of each filter using DAOPHOT II (Stetson 1987) inside the Munich Image Data Analysis System (MIDAS). The PSF was determined following a slightly modified version of the recipe in Stetson (1987). First, we selected 100 bright, unsaturated stars (40 000 ADUs) located within 1arcmin distance from our target. Then we fitted their PSFs with a Moffat function and through an iterative process we rejected fits with root mean square (rms) residuals greater than 1 per cent. The stars in the vicinity of the PSF template stars were then removed with the SUBTRACT routine of DAOPHOT II and the PSF was determined again on the subtracted image, improving the rms of the fit by a factor of (...truncated)