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arXiv:1902.11150v1 [astro-ph.SR] 28 Feb 2019 cetdXX eevdYY noiia omZZZ form original in YYY; Received XXX. Accepted ste pnu o rcce’ u oaceino mat- of which to NS ( due a secondary of ‘recycled’) into the massive (or turns from ter spun-up more star then initially components is the binary two systems: the binary in formed MNRAS whr ta.2018 al. et Swihart 2014 al. et Bassa ⋆ (see process recycling MSP the in canonical be the to of found stages recently was late J1417.7–4402 1FGL nary ( high a that fact the by 1974 1982 in discovered was B1937+21, PSR ( MSP, first The .M Beronya M. D. J0740+6620? companion PSR dwarf of white helium-atmosphere ultracool The © .A Zyuzin A. D. n ewe h oainpwrdadaccretion-powered ( and states rotation-powered switch- the MSPs several between of discoveries ing by and companions has pca ucaso the on subclass special ml pndw ae ( rates spin-down small ilscn usr MP)aenurnsas(S)with (NSs) neutron periods rotation are short (MSPs) Millisecond INTRODUCTION 1 ouaino nra’plas(e.g., pulsars ‘normal’ of population 5 4 3 2 1 akre l 1982 al. et Backer eatmnod srfıia nvria eL aua La Laguna, La Ten de 38200, Astrof´ısica, Universidad Laguna de Departamento La Pol Canarias, University, Astrof´ısica de Polytechnic de Petersburg Instituto St. Great the Peter nttt eAtoo´a nvria ainlAut´onoma Nacional Astronom´ıa, 19 Universidad de Petersburg, Instituto St. 26, Politekhnicheskaya Institute, Ioffe -al [email protected] E-mail: 09TeAuthors The 2019 pnproso Ssaeetariaiysal which stable extraordinarily are MSPs of periods Spin ; la ta.1982 al. et Alpar 000 , 1 – 7 rhbl ta.2009 al. et Archibald ; L MNRAS using Compiled 2019 March 1 Preprint (2019) n eeecstherein). references and , o ta.2015 al. et Roy .I sblee htteeojcsare objects these that believed is It ). 1 u .Shibanov A. Yu. , 1 P ; ⋆ P yial eo 0m.Te have They ms. 30 below typically , Û P ars2011 Tauris .V Karpova V. A. , > . − 50 invtiKgn&Komberg & Bisnovatyi-Kogan P 10 Û − e et rcino MSPs of fraction cent) per iga,dsic rmthe from distinct diagram, ftedtce triesuc onie ihtepla coordinat pulsar the with coincides source starlike detected the of erpr eeto ftelkl opno ftebnr ilscn pu millisecond binary the of companion the likely in Canarias the Telescopio Gran of the with detection J0740+6620 report We ABSTRACT netit of uncertainty oprn h aawt h ht wr oln rcssget th suggests tracks cooling the dwarf with dwarf white white the helium-atmosphere with ultracool data the Comparing age ilscn usr.Dtcino h orei te ad ol eu be words: would Key bands other nature. in dwa and source white properties the known its of among source Detection reddest pulsars. the millisecond is This effects. kinematic 19 .Mroe,teN bi- NS the Moreover, ). s s & ; .Ti ssupported is This ). − acetr2017a Manchester ait ta.2013 al. et Papitto 1 5 n osiuea constitute and ) y.Teaei ossetwt h usrcaatrsi g corre age characteristic pulsar the with consistent is age The Gyr. iais eea usr:idvda:PRJ0740+6620 PSR individual: pulsars: – general binaries: ≈ 1 , 3 1 . rsc t antdsare magnitudes Its arcsec. 0.2 .Y.Kirichenko Yu. A. , n .Cabrera-Lavers A. and 01 Russia 4021, tknceky 9 t eesug 921 Russia 195251, Petersburg, St. 29, itekhnicheskaya eMeio po otl87 aaClfri,M´exico, 228 California, Baja 877, Postal M´exico, Apdo. de rf,Spain erife, ). aua326 eeie Spain Tenerife, 38206, Laguna ; a erltdt hi eketra antcfields magnetic external weak their to related ( be may et fgnrlrltvt ( relativity unprecedented general out carry of to tests one allow J0337+1715 PSR tem 2013 st of characteristics physical ( obtain clusters to lar used also is timing 2018 aoye l 2005 al. et Jacoby ( interiors superdens NSs the role of in key (EoS) a matter equation-of-state plays on which constraints ac- masses in most NSs for of allow mediu determination MSP interstellar a curate the containing systems of Binary evo properties (ISM). stellar reveal and also binary and of lution exploration for essential are MSPs spacecra ( and de- waves navigation for gravitational proposed low-frequency was Array’) of Timing tection ‘Pulsar (or MSPs of set double-pulsar the B1913+16, J0737 PSR PSR pulsar system binary the st instance, of For astrophysics. ies of areas different in portant nsses tews,adtoa pia bevtosar observations optical additional otherwise, systems; on h esrmn ftegnrlrltvsi hpr de- ( observations Shapiro timing relativistic radio through general pulsar precision the from high lay a of with measurement inferred the be can companion acetr2017a Manchester nabnr ytmmse fbt h Sadits and NS the both of masses system binary a In ). ; eseg&Hag2016 Huang & Weisberg riee l 2017 al. et Freire acetr2017b Manchester 2 , 1 .Hwvr h ffc ssrn o edge- for strong is effect the However, ). r − .Tepro tblt ae Ssim- MSPs makes stability period The ). .V Zharikov V. S. , ′ 09/ n seilytetil sys- triple the especially and 3039A/B = 4 26 , 5 . atmr2012 Lattimer 51 r rmr2018 Kramer ′ ; ± rgre l 2017 al. et Prager and n eeecsteen.A therein). references and , ; 0 . ekre l 2018 al. et Becker 17 i ′ . and ad.Teposition The bands. 3500 swti the within es ; i fcmain of companions rf A ; ′ T noidse al. et Antoniadis ti a ean be can it at 2 = rhbl tal. et Archibald E eu oclarify to seful n cooling and K 00 tl l v3.0 file style X , hpr 1964 Shapiro 25 .Suisof Studies ). . 49 tdfor cted .MSP ). ± 0 lsar . ud- 15 1 el- m σ ft e e - . ; 2 D. M. Beronya et al.

Table 1. Parameters of the J0740 system (from Table 2. Log of the GTC observations. The mean airmass and Arzoumanian et al. (2018)). Current values are available in seeing values are presented. the NANOGrav Data Repositorya. The dispersion measure distances DYMW and DNE2001 were estimated using the YMW16 Filter Exposure time, s Airmass Seeing, arcsec (Yao, Manchester & Wang 2017) and NE2001 (Cordes & Lazio ′ 2002) models for the distribution of free electrons in the , r 15 × 1 1.32 0.9 respectively. The distance Dp corresponds to the timing 120 × 31 1.29 0.9 of 2.3(6) mas. Numbers in parentheses are 1σ uncertainties i′ 15 × 1 1.26 0.8 related to the last significant digits quoted. 120 × 20 1.26 0.8

Right ascension α (J2000) 07h40m45s.79492(2) Declination δ(J2000) +66◦20′33′′. 5593(2) Galactic longitude l (deg) 149.7 Lynch et al. (2018) suggested that the pulsar companion Galactic latitude b (deg) 29.6 is a He WD. Using the lower limits from the Epoch (MJD) 57017 PanSTARRS 3π Steradian Survey bands, they constrained −1 µα = αÛcosδ (mas yr ) −10.3(2) the WD temperature < 4200 K and age > 3.2 Gyr. −1 Proper motion µδ (mas yr ) −31.0(2) To find the optical counterpart of the pulsar compan- Spin period P (ms) 2.88573641085019(2) ion, we performed deep optical observations with the Gran −20 −1 Period derivative PÛ (10 s s ) 1.2184(4) Telescopio Canarias (GTC). Here we report a likely iden- Characteristic age τ (Gyr) 3.75 tification of the pulsar companion. The details of the ob- Û −1 34 Spin-down E (erg s ) 2.0 × 10 servations and data reduction are described in Sect. 2, the

Orbital period Pb (days) 4.766944619(1) analysis and results are presented in Sect. 3 and discussed Projected semi-major axis (lt-s) 3.9775602(2) in Sect. 4. Eccentricity (10−6) 4.9(1)

b Mass function fM (M⊙) 0.003 b Minimum companion mass Mc,min (M⊙) 0.2 2 OBSERVATIONS AND DATA REDUCTION −3 Dispersion measure (DM, pc cm ) 15.0 The observations of the J0740 field were performed on De- Distance D (kpc) 0.93 YMW cember 26, 20171 in the Sloan r′ and i′ bands using the Distance DNE2001 (kpc) 0.68 2 +0.2 GTC/OSIRIS instrument. The conditions were photomet- Distance Dp (kpc) 0.4 −0.1 ric and seeing varied from 0.8 to 1.3 arcsec. The instrument ahttps://data.nanograv.org/ field of view (FOV) was 7.8 arcmin × 7.8 arcmin with the b Parameters are provided by Lynch et al. (2018). Mc,min is pixel scale of 0.254 arcsec. The pulsar was exposed on the calculated assuming the inclination angle i = 90 deg and the CCD2 chip of the two OSIRIS CCD chips. The dithered pulsar mass Mp = 1.4 M⊙. science frames of 120 s were taken, resulting in the total exposure times of 3.7 and 2.4 ks for the r′ and i′ filters, re- spectively. The short 15 s exposures were obtained in each necessary. The latter are also crucial for determination of band to avoid saturation of bright stars that were further an MSP companion nature (e.g. van Kerkwijk et al. 2005). used for precise astrometry. The observing log is presented Low-mass white dwarfs (WDs) are the most frequent MSP in Table 2. companions (Tauris 2012). In such a case, comparison be- The data reduction and analysis were carried out within tween the optical data and WD evolutionary sequences can the Image Reduction and Analysis Facility (iraf) package. provide the type, temperature, cooling age and mass of The raw data included 10 bias frames and 5 sky flats for each the WD. Knowing the companion mass, the mass function filter, which were used to create the master bias and master (from pulsar timing observations) and the mass ratio (from flats frames. The bias subtracted and the flat-fielded science the optical phase-resolved spectroscopy), one can estimate frames were obtained with the ccdproc routine, aligned the pulsar mass and the system inclination. Unfortunately, to the frame with the best seeing, and then stacked for due to the weakness of the optical counterparts, identifica- each filter using the imcombine task. Accounting for known tions of MSP companions are not numerous. Nevertheless, OSIRIS geometrical distortions and background gradients in increasing the number of such identifications is important the images towards the FOV boundaries3, we ignored the ar- for understanding the nature and origin of these systems. eas outside 2.5 arcmin from the target to avoid the influence The situation has been improving thanks to the world’s of these effects on the calibration and further results. largest telescopes and deep sky surveys (e.g., Bassa et al. To perform the astrometric transformation, we used 2016; Dai et al. 2017; Karpova et al. 2018; Kirichenko et al. the iraf ccmap task and 9 bright non-saturated refer- 2018). ence stars detected with a high signal-to-noise ratio in The subject of this paper, the binary MSP J0740+6620 the r′-band short-exposure frame. The best coordinates (hereafter J0740), was discovered in the radio in the frame of the stars are contained in the Gaia DR2 catalogue of the Green Bank North Celestial Cap Pulsar Survey (Stovall et al. 2014; Lynch et al. 2018). The pulsar was Fermi also identified in gamma-rays with the telescope 1 Proposal GTCMULTIPLE2A-17BMEX, PI A. Kirichenko (Laffon et al. 2015; Guillemot et al. 2016). Table 1 sum- 2 http://www.gtc.iac.es/instruments/osiris/ marises the parameters of the pulsar system. Basing on 3 http://www.gtc.iac.es/instruments/osiris/media/OSIRIS- the computed minimum companion mass of ≈ 0.2 M⊙, USER-MANUAL v3 1.pdf

MNRAS 000, 1–7 (2019) The ultracool WD companion of PSR J0740+6620 3

41.8 41.8

36.4 36.4

Declination 66:20:31.0 Declination 66:20:31.0

25.6 25.6

47.1 46.2 7:40:45.4 44.5 47.1 46.2 7:40:45.4 44.5

Right ascension Right ascension

Figure 1. 16 arcsec × 16 arcsec GTC/OSIRIS Sloan r′ (left) and i′-band (right) images of the J0740 field. The images are smoothed with the Gaussian kernel of 3 pixels, compatible with the seeing value. The white circle shows the 3σ pulsar position uncertainty of ≈ 0.24 arcsec, which accounts for the optical astrometric referencing errors and the radio timing position uncertainties, corrected for the proper motion. The region of 2.3 × 2.3 arcsec within the black box in the right panel is enlarged and shown in the inset at the right-top corner of the image. The solid and dashed circles are 1σ position uncertainties of the pulsar and its possible binary companion, respectively.

(Gaia Collaboration et al. 2016, 2018) and have the cata- positions in both bands. The errors account for the position logue conservative accuracy of ≈ 0.7 mas (Lindegren et al. uncertainties of the object in the frames, the difference in the 2018). The resulting 1σ rms uncertainties of the astrometric positions in the two bands, and the astrometric referencing fit are 0.08 arcsec in RA and Dec. The Gaia catalogue uncer- uncertainty. The object coordinates coincide with the pulsar tainties are substantially smaller and can be neglected. The position at the 1σ significance level. This is demonstrated in resulting WCS solution was applied to the combined images the inset of the right panel of Figure 1 where the 1σ position in the r′ and i′ bands. Selecting other reference stars did not circles for the pulsar and the object overlap. change the fit result. For photometry of this faint object, we used the circular For the photometric calibration, we used the Landolt source aperture with ≈ 0.76 arcsec (3 pixels) radius, which is standard star G163-50 (Landolt 1992) observed during the consistent with the seeing value and provides the maximum same night as the target. The derived photometric zero- signal-to-noise ratio on the source. To minimise possible con- points for the CCD2 chip are zr′ = 28.88 ± 0.01 and zi′ = tributions from wings of nearby sources, backgrounds were 28.52 ± 0.02. The values were obtained by comparing the taken from a narrow annulus centred on the source with the standard star magnitudes from Smith et al. (2002) with its inner and outer radii of ≈ 0.8 and 1.5 arcsec. As a result, instrumental magnitudes, corrected for the finite aperture we obtained the source magnitudes r′ = 26.51 ± 0.17 and and atmospheric . We used the atmospheric ex- i′ = 25.49 ± 0.15 corrected for the finite aperture and the at- 4 tinction coefficients kr′ = 0.07 ± 0.01 and ki′ = 0.04 ± 0.01. mospheric extinction. Alternatively, for both bands we con- The 3σ detection limits on the resulting images are r′ < 27.5 structed the point spread functions (PSFs) using about a and i′ < 26.5. dozen of unsaturated isolated field stars and the daophot package, and performed the PSF-fitting photometry of field sources applying the allstar routine with the same source 3 RESULTS aperture but a wider background annulus with the inner and outer radii of ≈ 2.5 and 5 arcsec5. This yielded the same ob- ′ ′ Figure 1 shows the sections of the r and i images, contain- ject magnitudes with only marginally smaller errors. Our ob- ing the J0740 position. The radio coordinates of the pulsar ject is perfectly PSF-subtracted without any residuals that corrected for the proper motion (Table 1) to the epoch of our could indicate a possible presence of fainter sources hidden h m s observations (58114 MJD) are RA=07 40 45.7898(1) and in its profile. Finally, we performed the aperture photometry ◦ ′ ′′ Dec=+66 20 33. 4661(6). Accounting for the astrometric on the images where all neighbors, excluding the object of referencing uncertainty, the radius of the J0740 1σ position interest, were PSF-subtracted. Independently of the annulus error circle on the optical images is ≈ 0.08 arcsec. In the main width this led again to the results consistent with the above panels of Figure 1 we show the 3σ circle for convenience. A aperture and PSF magnitude measurements. We thus adopt faint starlike source is detected in both bands, which over- the above values as the most conservative ones. laps with the circle. Its coordinates RA=07h40m45s.76(1) and Dec=+66◦20′33′′. 4(2) were derived as a mean of the source 5 The latter is appropriate as the routine iteratively subtracts brighter neighbors in a source group before the photometry of 4 http://www.iac.es/adjuntos/cups/CUps2014-3.pdf fainter ones.

MNRAS 000, 1–7 (2019) 4 D. M. Beronya et al.

0.2 Gyr BB 0.5 Gyr DA DB 10 1 Gyr 1.5 2 Gyr J0348+0432 5 Gyr J0614−3329 10 Gyr J0621+2514 12 20000 K J1012+5307 1.0 14000 K

− 0 J1231 1411 ) 10000 K ′ r 8000 K

J2317+1439 − ′ ′ 6000 K r g

14 J0751+1807 (

M 4000 K 0.5 WD 0346+246 3000 K WD J1102 J0740+6620 16 0.0

18

0.0 0.5 1.0 1.5 ′ ′ (r − i )0

20 0.0 0.5 1.0 1.5 Figure 3. Colour-colour diagram with various WD evolutionary r′ − i′ ( )0 sequences and the data for the same WDs as in Fig. 2. The model predictions presented in Fig. 2 are labelled by the same symbols Figure 2. Colour-magnitude diagram with various WD evolu- and colours. WD are indicated by different symbols. tionary sequences and the data for different WDs listed in the The location range of the likely J0740 companion is shown by the legend. Dashed grey and black solid lines show the cooling tracks light-blue stripe. for WDs with hydrogen (DA) and helium (DB) atmospheres, re- spectively (Holberg & Bergeron 2006; Kowalski & Saumon 2006; Tremblay et al. 2011; Bergeron et al. 2011) with masses 0.2 – 1.0 magnitude measurements. For the above distance range, the M⊙ (spaced by 0.2 M⊙) increasing from upper to lower curves. Cooling ages are indicated by different symbols. The dash-dotted Mr′ varies from 16.4 to 19.2. The specific values for the timing parallax distance and for the maximum red line demonstrates the track for a WD with the mass of 0.4 + ′ = . 0.8 M⊙ emitting the blackbody spectrum. The location of the J0740 model depended DM distance of 930 pc are Mr 18 4−1.1 + presumed companion is marked by the black crosses: the up- and 16.6 0.2, respectively. + −0.2 ′ = 0.2 per one (Mr 16.6−0.2) is for the maximum distance estimate As it has been suggested, the J0740 companion is + = ′ = 0.8 DYMW 0.93 kpc and the lower one (Mr 18.4−1.1) is for the a WD (Lynch et al. 2018). Therefore, we compared the = +0.2 minimum distance estimate Dp 0.4−0.1 kpc, derived from the derived colour and magnitudes of the likely companion timing parallax. The light-blue rectangle encompasses the whole with the cooling sequences of hydrogen (DA) and helium distance uncertainty range. The data points for WDs that likely (DB) white dwarfs. The corresponding colour-magnitude have pure helium or mixed atmospheres are shown in blue. and colour-colour diagrams are presented in Figures 2 and 3, respectively. Cooling tracks for WDs with hydrogen and helium atmospheres (known as Bergeron models: We estimated the probability of detecting a random ob- Holberg & Bergeron 2006; Kowalski & Saumon 2006; ject at the pulsar position using the Poisson distribution Tremblay et al. 2011; Bergeron et al. 2011) are shown P = 1 − exp(−σS), where σ is the surface number density of by different line types6. The data points of compan- objects within the GTC field of view and S is the area of the ions to some other MSPs are presented: J0348+0432 pulsar error ellipse. As a result, we found the probability of (Antoniadis et al. 2013), J0614−3329 (Bassa et al. −3 ≈ 2 × 10 for an accidental coincidence of the pulsar and an 2016), J0621+2514 (Karpova et al. 2018), J0751+1807 unrelated object with a brightness of > 20 mag. This indi- (Bassa et al. 2006), J1012+5307 (Nicastro et al. 1995), cates the source is a likely optical counterpart to the J0740 J1231−1411 (Bassa et al. 2016) and J2317+1439 (Dai et al. binary companion. 2017). We also include two ultracool isolated WDs: SDSS To obtain intrinsic colours and magnitudes of the pre- J110217.48+411315.4 (hereafter WD J1102; Hall et al. 2008; sumed companion we used the empirical correlation be- Kilic et al. 2012) and WD 0346+246 (Oppenheimer et al. tween the distance D and the interstellar reddening E(B −V) 2001). For WD 0346+246 and the PSR J0751+1807 based on the Pan-STARRS 1 and 2MASS photometric data companion, the UBVRI magnitudes were converted to the (Green et al. 2018). The expected E(B − V) in the pulsar Sloan Digital Sky Survey (SDSS) system using transforma- direction is ≈ 0.04 ± 0.02 for the most conservative dis- tions by Lupton7. The magnitudes of other objects were tance range of 0.3 – 0.9 kpc following from Table 1. This adopted either from the cited articles or the SDSS cata- value was transformed to the extinction correction values logue (Abolfathi et al. 2018). The corresponding values of = = Ar′ 0.09 ± 0.05 and Ai′ 0.07 ± 0.03 using the conver- interstellar reddening E(B − V) of the sources were obtained sion coefficients presented by Schlafly & Finkbeiner (2011). The resulting intrinsic magnitudes and colour index are ′ = . . ′ = . . ′ ′ = . . 6 r0 26 42 ± 0 18, i0 25 42 ± 0 15, and (r − i )0 1 0 ± 0 2, http://www.astro.umontreal.ca/∼bergeron/CoolingModels where the errors include uncertainties of the reddening and 7 https://www.sdss.org/dr14/algorithms/sdssUBVRITransform/

MNRAS 000, 1–7 (2019) The ultracool WD companion of PSR J0740+6620 5 using the models by Drimmel et al. (2003) and Green et al. Lobe detachment a proto-WD goes through the contraction (2018) together with the distances from DM and parallax phase until it reaches its cooling track (Istrate et al. 2014, measurements or model-predicted WD distances. For PSR 2016). The duration of this phase increases as the mass of the J2317+1439, the parallax information was updated by proto-WD decreases, and may last as long as ∼ 2 Gyr. The Arzoumanian et al. (2018), and for this reason the redden- J0740 characteristic age of 3.75 Gyr (Table 1) is smaller than ing and absolute magnitude from Dai et al. (2017) were the WD age estimate. However, the observed pulsar period recalculated. For PSR J0621+2514, we combined the results derivative and consequently its characteristic age can be bi- for two DM distance estimates (Karpova et al. 2018). ased by kinematic effects, i.e. the effects of the pulsar proper The J0740 presumed companion is the reddest among motion (Shklovskii effect; Shklovskii 1970), the acceleration sources shown in the diagrams (Figures 2,3). The Berg- towards the Galactic plane and the acceleration due to dif- eron models presented here are constructed for CO-core ferential Galactic rotation (Nice & Taylor 1995). Using the WDs. However, it is generally believed that most of the J0740 proper motion value from Table 1, the ’s galac- MSP companions are He-core WDs, due to stripping a com- tocentric velocity and the distance (240 km s−1 and 8.34 panion envelope during the mass-transfer binary phase (see kpc, respectively; Reid et al. 2014), we calculated these cor- −21 e.g. Rivera-Sandoval et al. 2015; Cadelano et al. 2015). For rections to the pulsar period derivative: PÛS = 3.0 × 10 , −22 −23 hydrogen-atmosphere WDs, the comparison of the Berg- PÛG,⊥ = −1.6 × 10 , PÛG,p = 3.8 × 10 for the minimum and −21 −22 −23 eron models with those of the He-core ones created by PÛS = 6.9 × 10 , PÛG,⊥ = −2.2 × 10 , PÛG,p = 8.9 × 10 for Althaus et al. (2013) shows that the presumed J0740 com- the maximum pulsar distance estimates. The correspond- panion cannot certainly belong to DA WDs, considering any ing intrinsic characteristic ages are τi ∼ 5 Gyr and ∼ 8.5 ′ ′ core composition. This is because the upper limit for (r −i )0 Gyr, which are compatible with the cooling age8. Thus, the < indices of the hydrogen atmosphere sequences is ∼ 0.5 for considered binary system indeed can be very old and the both core types and all DA tracks turn towards blue colours presumed WD can be ultracool. This is not a unique sit- at high (Mr′ & 16) absolute magnitudes/low temperatures uation. There are other examples of the objects with sim- (see also Bassa et al. 2016). At the same time, the colour in- ilar characteristics. One of them is the WD companion of dex of the presumed companion is about one (Figure 2). On PSR J0751+1807 (Bassa et al. 2006). Its colours (see Fig- the other hand, for cool WDs with pure helium atmospheres ure 2 and 3) indicate that the WD has a pure helium or colours continue to redden with decreasing temperatures. mixed H/He atmosphere with a temperature T ∼ 3500 – 4300 Unfortunately, we have not found respective He-core mag- K. Other examples are isolated ultracool white dwarfs WD nitude sequences for these WDs. Nevertheless, calculations J1102 (Hall et al. 2008; Kilic et al. 2012) and WD 0346+246 show that the luminosity of a He-core WD at a given age (Oppenheimer et al. 2001). These WDs have temperatures has to be significantly higher than that of the CO-core WD of about 3650 and 3300 K, respectively, and are best ex- with the same mass (e.g., van Oirschot et al. 2014). This plained by the mixed atmosphere models (Gianninas et al. is due to the fact that the global specific heat capacity of 2015). the former is larger than that of the latter. However, at ages It is clear that a single colour index is not enough to > ∼ 1 Gyr the luminosity difference for low mass WDs becomes confidently assert that the source is the optical counterpart less than a half of magnitude which could not be resolved of the pulsar companion. Although the probability of the ac- within a factor of four larger intrinsic magnitude uncertainty cidental coincidence is low, however, it cannot be excluded. of the presumed companion. This is mainly caused by the As seen from the colour-magnitude diagram presented in large distance uncertainty (light-blue rectangle in Figure 2). Figure 4, the presumed companion is not very distinct from Moreover, the spectral energy distributions of cool helium- the distribution of other stellar sources detected within the atmosphere WDs become close to those of blackbodies (BBs) GTC FOV. It is located near the red locus of the main se- (e.g. Kilic 2011). As an example, in the diagrams we present quence distribution. Below we consider possible alternative the cooling sequence for a 0.4 M⊙ WD with the BB spec- interpretations of the source nature. trum, describing the cooling from 6000 to 2000 K. One can Firstly, the source colour is also in agreement with that see that the presumed J0740 companion position in the di- of red/brown dwarfs which are cool low-mass (M ∼ 0.08 agrams is in agreement with this track. Therefore, it most – 0.6 M⊙) main-sequence stars. According to the correla- likely belongs to the class of ultracool WDs with helium at- tion between the colour and the spectral type, based on the mospheres and has a temperature of . 3500 K and a cooling SDSS sample of M, L, and T dwarfs (Hawley et al. 2002), age of & 5 Gyr. the source can be a M0–M3V or a T-type . The corresponding distance of a red dwarf is & 8 kpc, and the object with b = +29.6 deg (see Table 1) must 4 DISCUSSION AND CONCLUSIONS be located at & 4 kpc above the Galactic disk. A T-type brown dwarf would be very close to the Sun at a distance We detected the likely companion to the MSP J0740. The . 50 pc. Both values are not consistent with the distance to comparison of the photometry results with the WD evolu- tionary sequences showed that the source is most probably an ultracool WD with a pure helium atmosphere. It can be the reddest WD among the known MSP companions. The 8 It is well known that the characteristic age is just an estimate WD-core composition remains unclear due to the large dis- of the pulsar true age and can significantly deviate from it (e.g. tance and magnitude uncertainties. Lorimer & Kramer 2012). However, it is usually the only available The presumed WD cooling age is & 5 Gyr. If it has a estimate which is the case of J0740. The pulsar true age can be low mass, this age limit can be larger since after the Roche- even larger than τi if the braking index is < 3.

MNRAS 000, 1–7 (2019) 6 D. M. Beronya et al.

18 by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. This work has made use 20 of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, 22 https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national insti- ′ r tutions, in particular the institutions participating in the 24 Gaia Multilateral Agreement. DAZ thanks Pirinem School of Theoretical Physics for hospitality.

26 REFERENCES

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