Detection of the PSR J1741+1351 white dwarf companion with the Gran Telescopio Canarias

TL;DR

This paper reports the detection of the white dwarf companion to the millisecond pulsar J1741+1351 using the Gran Telescopio Canarias, confirming its nature and properties through optical observations and evolutionary models.

Contribution

First detection of the white dwarf companion to PSR J1741+1351 with optical data, confirming its characteristics and consistency with radio timing measurements.

Findings

Companion is likely a He-core white dwarf with ~6000 K temperature.

Companion mass is approximately 0.2 solar masses.

Optical position matches pulsar coordinates.

Abstract

We report detection of the binary companion to the millisecond pulsar J1741+1351 with the Gran Telescopio Canarias. The optical source position coincides with the pulsar coordinates and its magnitudes are g' = 24.84(5), r' = 24.38(4) and i' = 24.17(4). Comparison of the data with the white dwarf evolutionary models shows that the source can be a He-core white dwarf with a temperature of $\approx 6000$ K and a mass of $\approx 0.2$ M$_{\odot}$. The latter is in excellent agreement with the companion mass obtained from the radio timing solution for PSR J1741+1351.

Tables

Table 1. Table 1: Parameters of the PSR J1741+1351 system obtained from [ 10 ] . The distance D p subscript 𝐷 𝑝 D_{p} is derived from the timing parallax of 0.6(1) mas. The dispersion measure distances D NE2001 subscript 𝐷 NE2001 D_{\rm NE2001} and D YMW subscript 𝐷 YMW D_{\rm YMW} were estimated using the YMW16 [ 11 ] and NE2001 [ 12 ] models, respectively. Numbers in parentheses are 1 σ 𝜎 \sigma uncertainties related to the last significant digits quoted.

\brRight ascension $\alpha$ (J2000)	17h41m31${.}^s$144731(2)
Declination $\delta$ (J2000)	+13∘51′44${.}^{\prime \prime }$12188(4)
\mrEpoch (MJD)	56209
Proper motion ${\mu }_{\alpha }=\dot{\alpha }\cos \delta$ (mas yr-1)	$-$8.98(2)
Proper motion ${\mu }_{\delta }$ (mas yr-1)	$-$7.42(2)
\mrSpin period $P$ (ms)	3.747154500259940030(7)
Period derivative $\dot{P}$ (${10}^{-20}$ s s-1)	3.021648(14)
Characteristic age $\tau$ (Gyr)	2
Orbital period $P_b$ (days)	16.335347828(4)
\mrDispersion measure (DM, pc cm-3)	24.2
Distance $D_{\mathrm{NE2001}}$ (kpc)	0.9
Distance $D_{\mathrm{YMW}}$ (kpc)	1.36
Distance $D_p$ (kpc)	${1.8}_{-0.3}^{+0.5}$
\mrPulsar mass $M_p$ (M⊙)	${1.14}_{-0.25}^{+0.43}$
Companion mass $M_c$ (M⊙)	${0.22}_{-0.04}^{+0.05}$
System inclination $i$ (deg)	${73}_{-4}^{+3}$
\br

Full text

1 Introduction

Millisecond pulsars (MSPs) are neutron stars (NSs) that rotate particularly fast, having periods $P<30$ ms. The canonical ‘recycling’ scenario assumes that MSPs are formed in binary systems where they spin up by transfer of mass and angular momentum from the secondary star [1, 2].

Optical observations of MSPs allow one to determine the nature and properties of their companions [3]. This provides additional constraints on binary systems’ parameters which is important for understanding their formation and evolution. In most cases, companions are low-mass white dwarfs (WDs) [4]. These objects are faint and thus hardly detectable. Fortunately, the number of identifications gradually increases thanks to world’s largest sky surveys and ground-based telescopes [5, 6, 7].

The binary MSP J1741+1351 (hereafter J1741) was discovered with the Parkes radio telescope [8] and then detected in $\gamma$-rays by Fermi [9]. It is among the best timed MSPs. High-precision timing using 11-yr data set from the North American Nanohertz Observatory for Gravitational Waves provided measurements of the system inclination and masses of both the pulsar and its companion [10]. The system parameters are presented in table 1.

To study the properties of the J1741 companion, we performed deep optical observations with the Gran Telescopio Canarias (GTC). Here we report the results of the analysis of these data.

2 Observations and data reduction

The observations of the J1741 field111Proposal GTC4-18AMEX, PI A. Kirichenko were carried out in June 2018 in the Sloan $g^{\prime}$, $r^{\prime}$ and $i^{\prime}$ bands with the GTC/OSIRIS222http://www.gtc.iac.es/instruments/osiris instrument. Dithered science frames were taken, with total exposure times of $3$, $3.5$ and $2.76$ ks for the $g^{\prime}$, $r^{\prime}$ and $i^{\prime}$ filters, respectively. A short 20 s exposure was obtained in the $r^{\prime}$ band to avoid saturation of bright stars that were further used for precise astrometry. The formal $rms$ uncertainties of the astrometric fit are 0.05 arcsec in RA and Dec for all bands. The data reduction was performed with the Image Reduction and Analysis Facility (IRAF) package. For the photometric calibration, we used the standard stars SA 112_805 and SA 104_428 observed during the same night as the target. The derived photometric zeropoints are $z_{g^{\prime}}=28.76(2)$, $z_{r^{\prime}}=28.99(1)$ and $z_{i^{\prime}}=28.55(1)$. The values were obtained by comparing the standard star magnitudes from [13] with their instrumental magnitudes, corrected for the finite aperture and atmospheric extinction. We used the atmospheric extinction coefficients333http://www.iac.es/adjuntos/cups/CUps2014-3.pdf $k_{g^{\prime}}=0.15(2)$, $k_{r^{\prime}}=0.07(1)$ and $k_{i^{\prime}}=0.04(1)$.

3 Results and discussion

In figure 1 we show sections of the J1741 field in the $g^{\prime}$, $r^{\prime}$ and $i′$ bands. The pulsar coordinates at the epoch of our observations (MJD 58274, $i^{\prime}$ filter) corrected for its proper motion (table 1) are $\alpha$ = 17h41m31$.\!\!^{\rm s}$141245(8) and $\delta$ = +13*∘51′*44$.\!\!^{\prime\prime}$0799(1). Its position uncertainty is shown in the images by the circle which also accounts for the astrometric referencing uncertainty. In all bands, we detect a starlike source (hereafter Source C) overlapping with the pulsar position.

We obtained the Source C magnitudes $g^{\prime}=24.84(5)$, $r^{\prime}=24.38(4)$ and $i^{\prime}=24.17(4)$. To correct these values for the interstellar extinction, we used the dust map [20] and distances to J1741 (table 1). We got the reddening $E(B-V)$ of $0.10(3)$ and $0.13(2)$ for the minimum and maximum distance estimates. $E(B-V)$ was then transformed to the extinction correction values using coefficients from [21]. For $D_{\rm NE2001}=0.9$ kpc, the resulting intrinsic colours and absolute magnitude are $(g^{\prime}-r^{\prime})_{0}=0.36(14)$, $(r^{\prime}-i^{\prime})_{0}=0.15(10)$ and $M_{r^{\prime}}=14.38(8)$ while for $D_{p}=1.8^{+0.5}_{-0.3}$ kpc – $(g^{\prime}-r^{\prime})_{0}=0.33(11)$, $(r^{\prime}-i^{\prime})_{0}=0.13(8)$ and $M_{r^{\prime}}=12.80^{+0.46}_{-0.59}$.

To check whether Source C is a WD, we compared its colours and magnitudes with the WD cooling tracks from [18, 14, 15, 16, 17]444http://www.astro.umontreal.ca/~bergeron/CoolingModels/ which are shown in figure 2. Indeed, according to the diagrams, Source C is likely a WD with a temperature of about 6000 K. This estimate is appropriate for the whole range of distance estimates to J1741 (table 1) since the reddening does not vary significantly. However, various distances imply different estimates of other parameters of Source C. The minimum distance corresponds to a WD with a mass of $>0.4$ M⊙ and age of 2–5 Gyr and the maximum one – to a WD with a mass of about 0.2–0.3 M⊙ and age of 1–2 Gyr. The former case is not compatible with radio timing measurements of the J1741 companion mass $M_{c}=0.22(5)$ M⊙ while the latter one is in a good agreement with it. This supports the larger estimate of the distance to J1741 and implies that its companion is a DA He-core WD.

\ack

The manuscript is based on observations made with the GTC, installed in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, in the island of La Palma. We thank the NANOGrav collaboration for sharing data products from the 12.5-yr data set. DAZ thanks Pirinem School of Theoretical Physics for hospitality. AYK and SVZ acknowledge support from DGAPA/PAPIIT Project IN100617.