NICER Discovery of Millisecond X-ray Pulsations and an Ultracompact Orbit in IGR J17494-3030
Mason Ng, Paul S. Ray, Peter Bult, Deepto Chakrabarty, Gaurava K. Jaisawal, Christian Malacaria, Diego Altamirano, Zaven Arzoumanian, Keith C. Gendreau, Tolga Güver, Matthew Kerr, Tod E. Strohmayer, Zorawar Wadiasingh, Michael T. Wolff
DDraft version February 3, 2021
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NICER
Discovery of Millisecond X-ray Pulsations and an Ultracompact Orbit in IGR J17494 − Mason Ng, Paul S. Ray, Peter Bult,
3, 4
Deepto Chakrabarty, Gaurava K. Jaisawal, Christian Malacaria,
6, 7
Diego Altamirano, Zaven Arzoumanian, Keith C. Gendreau, Tolga G¨uver,
9, 10
Matthew Kerr, Tod E. Strohmayer,
4, 11
Zorawar Wadiasingh,
4, 12, 13 and Michael T. Wolff MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA Space Science Division, U.S. Naval Research Laboratory, Washington, DC 20375, USA Department of Astronomy, University of Maryland, College Park, MD 20742, USA Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA National Space Institute, Technical University of Denmark, Elektrovej 327-328, DK-2800 Lyngby, Denmark NASA Marshall Space Flight Center, NSSTC, 320 Sparkman Drive, Huntsville, AL 35805, USA Universities Space Research Association, NSSTC, 320 Sparkman Drive, Huntsville, AL 35805, USA School of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, UK Istanbul University, Science Faculty, Department of Astronomy and Space Sciences, Beyazıt, 34119, Istanbul, Turkey Istanbul University Observatory Research and Application Center, Istanbul University 34119, Istanbul, Turkey Joint Space-Science Institute, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA Centre for Space Research, North-West University, Potchefstroom Campus, Private Bag X6001, Potchefstroom 2520, South Africa Universities Space Research Association (USRA), Columbia, MD 21046, USA (Received January 15, 2021; Revised January 29, 2021; Accepted January 31, 2021)
ABSTRACTWe report the detection of 376.05 Hz (2.66 ms) coherent X-ray pulsations in
NICER observationsof a transient outburst of the low-mass X-ray binary IGR J17494 − (cid:39) . M (cid:12) finite-entropy white dwarf composed of He or C/O. The fractionalrms pulsed amplitude is 7.4%, and the soft (1–3 keV) X-ray pulse profile contains a significant secondharmonic. The pulsed amplitude and pulse phase lag (relative to our mean timing model) are energy-dependent, each having a local maximum at 4 keV and 1.5 keV, respectively. We also recoveredthe X-ray pulsations in archival 2012 XMM-Newton observations, allowing us to measure a long-termpulsar spin-down rate of ˙ ν = − . × − Hz s − and to infer a pulsar surface dipole magneticfield strength of (cid:39) G. We show that the mass transfer in the binary is likely non-conservative, andwe discuss various scenarios for mass loss from the system.
Keywords: stars: neutron – stars: oscillations (pulsations) – binaries: close – stars: rotation – X-rays:binaries – X-rays: individual (IGR J17494 − INTRODUCTIONAccreting millisecond X-ray pulsars (AMXPs; see DiSalvo & Sanna 2020, for a recent review) are rapidlyrotating, weakly magnetized ( ∼ G) neutron starsaccreting from a low-mass ( (cid:46) M (cid:12) ) companion in a low-mass X-ray binary (LMXB). Most known AMXPs areX-ray transient systems in which long ( ∼ years) intervalsof X-ray quiescence are punctuated by brief ( ∼ weeks) Corresponding author: Mason [email protected] outbursts of enhanced X-ray emission. These transientoutbursts are understood to arise from a thermal in-stability in the accretion disk around a neutron star orblack hole LMXB primary, analogous to “dwarf nova”optical outbursts in accreting white dwarfs (see Lasota2001; Hameury 2020, and references therein).The X-ray transient IGR J17494 − l = 359 . ◦ , b = − . ◦ ; hereafter called IGRJ17494) was first discovered in a 2012 March outburst inthe 3–80 keV hard X-ray band (IBIS and JEM-X) in an INTEGRAL survey of the Galactic center region (Bois-say et al. 2012). Soft X-ray (0.5–10 keV) monitoring ob- a r X i v : . [ a s t r o - ph . H E ] F e b Ng et al. servations with
Swift showed that the outburst lastedapproximately one month (Armas Padilla et al. 2013)before fading into quiescence (Chakrabarty et al. 2013).
XMM-Newton
IN-TEGRAL in 2020 October (Ducci et al. 2020), lead-ing to a more precise X-ray localization with
Chandra (Chakrabarty & Jonker 2020) and the identification ofa 4.5 GHz radio counterpart with the VLA (van denEijnden et al. 2020).Soft X-ray observations of the 2020 outburst with the
Neutron Star Interior Composition Explorer (NICER) revealed the presence of coherent 376 Hz pulsationsmodulated by a 75 minute binary orbit, establishing thesystem as a millisecond pulsar (neutron star) in an ul-tracompact binary (Ng et al. 2020). In this Letter, wefirst outline the
NICER and
XMM-Newton observationsand data processing. We then present results from tim-ing and spectral analyses of the
NICER observations,as well as from a timing analysis of the archival 2012
XMM-Newton observations. Finally, we constrain thepossible nature of the donor in the IGR J17494 systemand discuss further implications of the source. OBSERVATIONS AND DATA PROCESSING2.1.
NICERNICER is an X-ray telescope mounted on the
Inter-national Space Station ( ISS ) since 2017 June.
NICER has 56 aligned pairs of X-ray concentrator optics andsilicon drift detectors (52 detectors are usually active on
NICER ). NICER is capable of fast-timing observationsin the 0.2–12.0 keV band, with timing accuracy of time-tagged photons to better than 100 ns (Gendreau et al.2012; LaMarr et al. 2016; Prigozhin et al. 2016).
NICER observed IGR J17494 from 2020 October 27to November 4 for a total exposure time of 32 . . Theseobservations were available through the public NASAHEASARC data archive. There were additional NICER observations, to which we did not have access, duringthis interval for a proprietary guest observer investiga-tion (PI: A. Sanna; shown as the shaded region in the toppanel of Figure 1). The events were barycenter-correctedin the ICRS reference frame, with source coordinates The source became unobservable due to Sun-angle constraintsaround November 5. During the course of the observations, several detectors wereturned off for scheduled maintenance. Detectors 01, 02, 10, 13,34, 43, and 44 were affected. In all observations, 46–48 detectorswere active. Detectors 11, 20, 22, and 60 have been inactive sincelaunch.
R.A. = 267 . ◦ and Decl.= − . ◦ (equinoxJ2000.0) obtained from a recent Chandra observation(Chakrabarty & Jonker 2020), using barycorr from
FTOOLS with the JPL DE405 solar system ephemeris(Standish 1998).The
NICER observations were processed with
HEASoft version 6.28 and the
NICER
Data AnalysisSoftware ( nicerdas ) version 7.0 (2020-04-23 V007a).The following criteria, which we note are relaxed com-pared to standard filtering criteria as the latter weretoo restrictive and resulted in no events, were imposedin the construction of the good time intervals (GTIs):no discrimination of events when
NICER (on the
ISS )was inside or outside of the South Atlantic Anomalyduring the course of the observations; ≥ ◦ for thesource-Earth limb angle ( ≥ ◦ for the Sun-illuminatedEarth); ≥
38 operational Focal Plane Modules (FPMs);undershoot (dark current) count-rate range of 0–400 perFPM ( underonly range ); overshoot (saturation fromcharged particles) count-rate range of 0–2 per FPM( overonly range and overonly expr ); pointing offsetis < . ◦ from the nominal source position.We analyzed spectral data using XSPEC v12.11.1 (Ar-naud 1996).
NICER data were selected in the range1–10 keV, to avoid contamination from optical load-ing and significant interstellar absorption at lower en-ergy. The spectra were rebinned to have at least 25counts per bin. Background spectra were extracted us-ing nibackgen3C50 version 6 from the official
NICER tools . Standard response files made available by the NICER team were used to perform spectral analysis .2.2. XMM-NewtonXMM-Newton performed a 43 ks observation of IGRJ17494 on 2012 March 31 (ObsID 0694040201). The
EPIC-PN camera was operated in timing mode, yield-ing a time resolution of 29.56 µ s, which is sufficient to al-low us to search for the presence of coherent pulsations.We processed these data using SAS version 18.0 and thelatest version of the calibration files . Applying stan-dard screening criteria, we retained only those eventswith photon energies in the 0.4–10 keV range, with pattern ≤ flag = 0. Source eventswere extracted from rawx columns [34 : 42], while back-ground events were extracted from rawx [51 : 59]. Con-structing a 32-s resolution light curve of the source and https://heasarc.gsfc.nasa.gov/docs/nicer/tools/nicer bkg esttools.html. https://heasarc.gsfc.nasa.gov/docs/heasarc/caldb/data/nicer/xti/index.html. GR J17494 − − to 1 ct s − . Additionally, we fil-tered out an episode of background flaring that occurredbetween 15750 s and 21500 s after the start of the obser-vation. Finally, we applied barycentric corrections tothe cleaned event data, again using the JPL DE405 so-lar system ephemeris and the source coordinates quotedpreviously. RESULTS3.1.
NICER
The
NICER tbabs(powerlaw+bbodyrad) in XSPEC ), with ab-sorption column density n H = 2 . × cm − , pho-ton index Γ = 1 . kT =0 . R bb = 2 . d km,where d is the source distance in units of 10 kpc.The uncertainties are reported at the 90% confidencelevel. The reduced χ ( χ ν ) of the fit was 1 .
14 for849 degrees of freedom. The spectrum softened duringthe late decay phase of the outburst, where the sametwo-component model fit yielded Γ = 4 . +0 . − . and n H is assumed to be unchanged throughout the observa-tions. The peak absorbed 1–10 keV flux we observed was1 . × − erg s − cm − on MJD 59149 .
4, correspond-ing to an unabsorbed flux of 1 . × − erg s − cm − .The lowest absorbed flux we measured was 1 . × − erg s − cm − on MJD 59157 .
5, corresponding toan unabsorbed flux of 3 . × − erg s − cm − . Thisis roughly a factor of 3 fainter than the minimum fluxdetected by XMM-Newton at the end of the 2012 out-burst (Armas Padilla et al. 2013). A more detailed X-rayspectral analysis will be reported elsewhere.We first detected X-ray pulsations with a data anal-ysis pipeline that employs multiple techniques for X-ray pulsation searches, including averaged power spec-tral stacking with Bartlett’s method (Bartlett 1948)and acceleration searches with PRESTO (Ransom et al.2002). The initial detection was made through
PRESTO ,an open-source pulsar timing software package designedfor efficient searches for binary millisecond pulsars. We https://github.com/masonng-astro/nicerpy xrayanalysis, partic-ularly with Lv3 incoming.py and scripts therein https://github.com/scottransom/presto Days since MJD 59149 R a t e ( c o un t s / s / d e t e c t o r ) D a t a U n a v a il a b l e Orbital phase -10.000.0010.00 P u l s e a rr i v a l t i m e d e l a y ( m s ) Pulse Phase N o r m a li z e d R a t e -0.200.000.20 R e s i d u a l s ( m s ) Figure 1.
Top: NICER
NICER data was unavailable to us.
Middle:
Pulse arrival time delayas a function of orbital phase relative to the ascending node.The crosses are our measurements, and the solid curve is ourbest-fit model. The squares are the fit residuals, plotted on a30 × magnified scale. Bottom:
Pulse profiles in the 1–3 keV(solid red) and 3–7 keV (dashed blue) bands. The 1–3 keVprofile contains a significant second harmonic. ran a Fourier-domain acceleration search scheme withthe accelsearch task over the range 1–1000 Hz, andposited that the Doppler motion would cause the possi-ble signal to drift over a maximum of 100 bins in Fourierfrequency space. This yielded a strong (cid:39) .
05 Hz pul-sation candidate (trial-adjusted significance of 3 . σ ) inthe 2–12 keV range. Ng et al.
After initial identification of the candidate in the 2–12 keV range, we optimized the pulse significance byadjusting the energy range to maximize the Z statistic,where Z = 2 N N (cid:88) j =1 cos 2 πνt j + N (cid:88) j =1 sin 2 πνt j , (1)where t j are the N photon arrival times (Buccheri et al.1983). We found that an optimal energy range of 1.01–7.11 keV yielded Z = 1915 .
41. Our subsequent timinganalyses were carried out over 1–7 keV.The acceleration searches indicated that the pulsationfrequency is modulated by a binary orbit. We used theacceleration data to estimate an initial timing modelwith a provisional circular orbit. We then used this ini-tial model to construct 35 pulse times of arrival (TOAs)with the photon toa.py tool in the
NICERsoft dataanalysis package, using a Gaussian pulse template andensuring an integration time of 500 s for each TOA (withminimum exposure time of 200 s). We then used theseTOAs to compute corrections to our initial orbit modelusing weighted least-squares fitting with the PINT pulsardata analysis package (Luo et al. 2020). Our best-fit or-bit ephemeris is shown in Table 1, and the orbital decaycurve is shown in the middle panel of Figure 1. Using ourbest-fit timing model, pulsations were detected through-out the entire outburst. At the end of the observations,we were able to detect the pulsations in observationsfrom MJD 59154–59157 (November 1–4) by combiningall the data. The mean unabsorbed flux over this 4-day interval was 8 . × − erg s − cm − (1–10 keV).We did not have sufficient sensitivity to detect the pulsa-tions in individual pointings from these dates. The time-averaged fractional root-mean-squared (rms) pulsed am-plitude was 7.4% (1–7 keV). Examining the lower andhigher energies separately, we found amplitudes of 7.2%in the 1–3 keV band and 8.7% in the 3–7 keV band.The soft and hard X-ray pulse profiles are shown in thebottom panel of Figure 1. The 1–3 keV profile showsthe presence of a second harmonic; this component isnot significantly detected in the 3–7 keV profile. To fur-ther examine the energy dependence of the pulse wave-form, we adaptively binned the timing data in energy.We required the energy bins to contain a multiple of5 pulse-invariant (PI) energy channels (0.05 keV), suchthat each bin contained at least 5000 counts. For eachof these energy bins, we then folded the data using our https://github.com/paulray/NICERsoft A m p li t u d e ( % r m s ) P h a s e l a g ( c y c l e s ) Figure 2.
Top:
Fractional rms pulsed amplitude as a func-tion of energy, as measured by
NICER . Bottom:
Pulse phaselag as a function of energy, as measured by
NICER . The lagis measured relative to the best-fit timing model in Table 1. best-fit timing solution and measured the background-corrected fractional rms pulsed amplitude and the pulsephase offset relative to the model. The resulting energydependencies are shown in Figure 2. The pulsed ampli-tude has a local maximum of 11% at 4 keV, while thepulse phase lag has a local maximum of +0 .
05 cycles(130 µ s) at around 1.5 keV.3.2. XMM-Newton
The uncertainty in our P orb value does not allow usto coherently extrapolate our timing model back to the2012 outburst. Thus, we searched for pulsations in the XMM-Newton data by constructing a grid of trial T asc values around the local epoch that spanned one orbitalperiod. The grid resolution was set to 50 s, which isequivalent to 4 ◦ in orbital longitude. For each trialephemeris, we then demodulated the event data andcomputed the Z statistic (see Eq. 1). We evaluatedthis statistic for pulse frequencies in a ± NICER ,adopting a frequency resolution of 1 /T , with T the du-ration of the XMM-Newton observation. The best can-
GR J17494 − Z = 89,which converts to a trial-adjusted pulse detection signif-icance of 8 σ .Adopting the best T asc and pulse frequency fromthe grid search as a provisional model, we performeda phase-coherent pulse analysis. We divided the lightcurve into ≈ NICER values.The best-fit values were ν = 376 . T asc , = MJD 56017 . NICER measurement, we find ∆ ν ≡ ν − ν = − . ± . ν = − . × − Hz s − . Owing to the uncertainty in exact orbitalcycle count between the 2012 and 2020 epochs, we areunable to use these T asc measurements to further refinethe orbital period.The XMM-Newton data also showed an energy-dependent trend in pulse phase lag similar to that ob-served in the
NICER data. We were unable to mea-sure an energy-dependence in the pulsed amplitude with
XMM-Newton , but the results from the two data setswere consistent within the measurement uncertainties. DISCUSSIONThe discovery of coherent millisecond X-ray pulsa-tions from IGR J17494 − µ = 5 . × (cid:0) α (cid:1) − / × (cid:18) I g cm (cid:19) / G cm , (2)where α is the angle between the magnetic and spinaxes, and I is the neutron star moment of inertia. Thecorresponding surface dipole field strength of (cid:39) Gis on the high end of the distribution inferred for otherAMXPs (Mukherjee et al. 2015).We found that the fractional rms pulsed amplitudeand the pulse phase of IGR J17494 vary as a function ofphoton energy. Both the amplitude and the phase lagreach a local maximum at a (different) characteristic en-ergy of 4 and 1.5 keV, respectively. Energy-dependentvariations of the pulse waveform are ubiquitous among AMXPs, although the location of these local maximavaries greatly from source to source (Gierli´nski et al.2002; Gierli´nski & Poutanen 2005; Falanga et al. 2005;Patruno et al. 2009; Falanga et al. 2012). The behaviorcan be understood through a two-component emissionmodel, with thermal emission originating from the stel-lar surface and scattered Compton emission originatingfrom some height above the surface (Gierli´nski et al.2002; Wilkinson et al. 2011). Accounting for the differ-ence in geometry and emission patterns, such a modelcan self-consistently explain the energy dependence ofboth the phase lags and the pulsed amplitudes (Pouta-nen & Gierli´nski 2003).Our measurement of a 75 min binary orbit allows usto constrain the nature of the mass donor in this sys-tem. The vast majority of Roche-lobe–filling LMXBsand cataclysmic variables contain hydrogen-rich donorstars, and they all have binary periods P orb (cid:38)
80 min(Paczynski & Sienkiewicz 1981; Rappaport et al. 1982).The so-called ultracompact binaries ( P orb (cid:46)
80 min)have H-depleted donors (Nelson et al. 1986; Pylyser& Savonije 1988, 1989; Nelemans et al. 2010). IGRJ17494 has the longest known period for an ultracom-pact LMXB and lies near the period boundary, making ita particularly interesting case. We also note the recentdiscovery of the rotation-powered millisecond gamma-ray pulsar PSR J1653 − shortest orbitalperiod known for a rotation-powered binary pulsar, andthis “black widow” system is believed to have evolvedfrom an ultracompact LMXB after mass transfer ended.From our measured orbital parameters, the binarymass function of IGR J17494 is f m ≡ ( M d sin i ) ( M ns + M d ) = 4 π ( a x sin i ) GP ≈ . × − M (cid:12) , (3)where M ns is the neutron star mass, M d is the donormass, a x sin i is the projected semimajor axis, and thebinary inclination i is defined as the angle between theline of sight and the orbital angular momentum vector.For a given value of M ns , we can use Equation 3 to calcu-late M d as a function of i (see top panel of Figure 3). As-suming M ns = 1 . . M (cid:12) , the minimum donor mass(for an edge-on binary with i = 90 ◦ ) is 0.014 (0.018) M (cid:12) . For a random ensemble of binaries, the proba-bility distribution of cos i is uniformly distributed andPr( i < i ) = 1 − cos i . Thus, the donor mass is likelyto be very low, with a 90% confidence upper limit of M d < .
033 (0 . M (cid:12) for M ns = 1 . . M (cid:12) . Ng et al.
Table 1.
IGR J17494 − α (J2000) 267 . ◦ Declination, δ (J2000) − . ◦ Position epoch (TT) MJD 59156 . ν (Hz) 376 . | ˙ ν | (Hz/s) < . × − Spin epoch, t (TDB) MJD 59149 . P orb (s) 4496 . a x sin i (lt-ms) 15 . T asc (TDB) MJD 59149 . e < .
006 (2 σ )Spin frequency derivative (long-term), ˙ ν (Hz/s) − . × − Note —Source coordinates adopted for the barycentering were determined by Chakrabarty & Jonker (2020). The spinfrequency derivative quoted here is during the 2020 outburst.
Assuming a Roche-lobe–filling donor, we can calcu-late the donor radius R d as a function of M d (Eggleton1983); this is shown in the bottom panel of Figure 3 for M ns = 1 . M (cid:12) . For comparison, the figure also showsthe mass-radius relations for different types of low-massstars: cold white dwarfs (WDs; Zapolsky & Salpeter1969; Rappaport & Joss 1984; Nelemans et al. 2001);hot (finite-entropy) WDs composed of either He, C, orO (Deloye & Bildsten 2003); and low-mass H-rich stars,including brown dwarfs (Chabrier et al. 2000). We seethat cold WD models are inconsistent with our mea-sured mass-radius constraint, indicating that thermalbloating is likely important. Moderately hot He WDswith central temperature T c = 2 . × K or C/O WDswith T c = 5 × K are consistent with our constraint athigh binary inclination. Hotter WDs and moderately old(cool) brown dwarfs are also consistent, but the requiredinclinations have low a priori probability. Finally, H-richdwarfs above the mass-burning limit are also possible,but only for extremely low (improbable) inclinations.We conclude that the donor is likely to be a (cid:39) . M (cid:12) finite-entropy He or C/O white dwarf.The angular momentum evolution of the binary is de-scribed by (Verbunt 1993; Verbunt & van den Heuvel1995) − ˙ JJ = − ˙ M d M d f ML , (4)where ˙ J is the rate of change of the orbital angular mo-mentum J due to effects other than mass loss from thesystem, ˙ M d ( <
0) is the rate of change of the donormass, and the dimensionless factor f ML is given by f ML = 56 + n − βq − (1 − β )( q + 3 α )3(1 + q ) , (5) where q = M d /M ns (cid:28) β isthe fraction of ˙ M d that accretes onto the neutron star( β = 1 for conservative mass transfer), n = d (ln R d ) d (ln M d ) (6)denotes how the donor radius R d changes with massloss, and α is the specific angular momentum of any(non-conservative) mass lost from the system in unitsof the donor star’s specific angular momentum. Thus, α parameterizes the site of any mass ejection from thesystem, where α (cid:39) α (cid:39) q for mass loss close to the pulsar. Masstransfer in ultracompact binaries is primarily driven byangular momentum loss due to gravitational radiationfrom the binary orbit (see Rappaport et al. 1982, andreferences therein); for a circular orbit, this loss is givenby (Landau & Lifshitz 1989; Peters 1964) − (cid:32) ˙ JJ (cid:33) GW = 32 G c M ns M d ( M ns + M d ) a , (7)where a is the binary separation. Inserting this into theleft-hand side of Equation 4, we can then calculate thegravitational-wave–driven mass transfer rate from thedonor into the accretion disk as˙ M GW = − ˙ M d = 32 G c (cid:18) π G (cid:19) / M / q (1 + q ) / P / f ML ≈ . × − (cid:18) M ns . M (cid:12) (cid:19) / × (cid:18) M d . M (cid:12) (cid:19) (cid:18) f ML . (cid:19) − M (cid:12) yr − . (8) GR J17494 − i (degrees)0.00 0.20 0.40 0.60 0.80 1.00 cos i D o n o r M a ss , M d ( M ) IGR J17494-30300.02 0.04 0.06 0.08 0.10
Donor Mass, M d ( M ) D o n o r R a d i u s , R d ( R ) Cold WDsBrown Dwarfs I G R J - HotWDsWarmWDsO OC CHe He
Figure 3.
Top:
Donor star mass M d as a function of bi-nary inclination i , assuming M ns = 1 . M (cid:12) . The a prioriprobability distribution is uniform in cos i , so low massesare likeliest. Bottom:
Mass-radius constraints for the donorstar. The thick solid black curve is the mass-radius con-straint for a Roche-lobe–filling donor from our orbital mea-surements. The dashed black line shows cold WD models.The blue and red lines show representative “warm” and hotWD models, respectively, with He (dotted), C (dashed), andO (dash-dotted) compositions. These models take T c = 2 . T c = 5 and 10 MK for C/O. Thesolid cyan curves show brown dwarf models for ages 0.1, 0.5,1.0, 5.0, and 10.0 Gyr (from top to bottom). The likeliestdonor is a warm (cid:39) . M (cid:12) He or C/O WD.
Our scaling value of f ML = 0 .
66 corresponds to n = − / β = 1.Although accretion onto the neutron star is mediatedby episodic outbursts, mass continuity requires that thelong-term average accretion luminosity reflect ˙ M GW ifthe mass transfer is conservative. Our observations arenot ideal for examining this, since we did not observethe early (brightest) part of the 2020 outburst with NICER . However, the unabsorbed 0.5–10 keV X-ray flu-ence in the 2012 outburst was 1 . × − erg cm − (Ar-mas Padilla et al. 2013). Assuming that the 2012 out-burst was typical, that the long-term average accretionrate is dominated by the outbursts, and that there wereno intervening outbursts between 2012 and 2020, theoutburst separation of ≈ F x, avg = 3 . × − erg s − cm − (0.5–10 keV). We can then write the accretion luminos-ity as GM ns β ˙ M GW R ns = (cid:18) ∆Ω4 π (cid:19) πd f bol F x, avg , (9)where R ns is the neutron star radius, d is the distance tothe source, f bol is the bolometric correction (accountingfor accretion luminosity outside the 0.5–10 keV band-pass), and ∆Ω is the solid angle into which the accretionluminosity is emitted. Based on the INTEGRAL hardX-ray observations in 2012 (Boissay et al. 2012), we es-timate f bol ≈ .
7. Assuming R ns = 10 km and taking β = 1 and ∆Ω = 4 π , we obtain an implausibly largedistance of 20 kpc. Although it is not impossible thatthe source lies on the far side of the Galaxy, a locationnear the Galactic center is far more likely given the lineof sight. There are several reasons that our distance es-timate might be significantly inflated. Obtaining a moreplausible distance of 8 kpc would require1 β (cid:18) ∆Ω4 π (cid:19) (cid:18) f bol . (cid:19) (cid:18) f ML . (cid:19) × (cid:18) M ns . M (cid:12) (cid:19) − / (cid:18) M d . M (cid:12) (cid:19) − × (cid:18) F x, avg . × − erg s − cm − (cid:19) ≈ . (10)Some combination of these factors may be different thanwhat we assumed above. However, a heavier neutronstar ( M ns > . M (cid:12) ), a heavier mass donor (equivalentto a lower binary inclination), or significant beaming(∆Ω < π ) would further inflate the distance estimate.Also, our estimate of f bol is fairly robust, given the broadX-ray coverage of the INTEGRAL data. It is possiblethat we have underestimated F x, avg . This could happenif we missed accretion outbursts that occurred between2012 and 2020, or if the quiescent (non-outburst) flux isas high as ∼ − erg s − cm − . The former possibilitycan be explored through a careful analysis of archival X-ray monitoring data, while the latter possibility couldbe checked through sensitive X-ray observations of thesource in quiescence.The factor f ML may be somewhat larger than we as-sumed. Although we calculated it using the usual valueof n = − / n values in the range of − . − . f ML by more than a factor of (cid:39) . Ng et al.
Non-conservative mass transfer ( β <
1) is a morepromising avenue. The radio detection of IGR J17494(van den Eijnden et al. 2020) points to the likelihood ofa collimated jet ejection during the outburst. Moreover,a similar distance conundrum was invoked to infer non-conservative mass transfer in the ultracompact LMXBpulsar XTE J0929 −
314 (Marino et al. 2017) as well asseveral other AMXPs (Marino et al. 2019). Also, therewas evidence found for an outflow in the ultracompactLMXB pulsar IGR J17062 − M c (cid:46) . M (cid:12) ) and “redback” ( M c (cid:38) . M (cid:12) )systems, where M c is the companion mass (see, e.g., Ro-mani et al. 2016, and references therein). One possibilityis black-widow–like ablation of the companion, driven byrotation-powered gamma-ray emission from the pulsar(Ginzburg & Quataert 2020). Such ablation could alsobe driven by particle heating via the rotation-poweredpulsar wind (see Harding & Gaisser 1990, and referencestherein). Hard X-rays and gamma-rays from the intrabi-nary shock observed in many black widow systems couldsignificantly affect the mass loss rate (Wadiasingh et al.2018). Another possibility is that the pulsar wind coulddrive an outflow from the inner Lagrange ( L ) point byovercoming the ram pressure of accreting material (Bur-deri et al. 2001; Di Salvo et al. 2008).As an example, we consider the case of gamma-ray ab-lation. If we assume the gamma-ray luminosity is (cid:39) (cid:39) × erg s − based onour long-term ˙ ν measurement) as typically seen in blackwidow systems (Abdo et al. 2013), this would imply acompanion mass loss rate of ∼ − M (cid:12) / yr (Ginzburg& Quataert 2020). For a source distance of 8 kpc and as-suming that gravitational wave losses dominate in Equa-tion 4, this implies β ≈ .
04 and α ≈ .
4, suggestingthat the mass ejection occurs somewhere between thepulsar and the L point ( α ≈ . β and increase α even furtherin our case.All of the X-ray–quiescent mechanisms mentionedabove rely on the system entering a rotation-poweredradio pulsar state during X-ray quiescence. We notethat a growing class of so-called transitional millisecondpulsars (tMSPs) has been identified that switch betweenLMXB and radio pulsar states (see Papitto & de Mar-tino 2020, for a review). The known tMSPs would beclassified as redback systems in their radio pulsar state.If IGR J17494 is a tMSP, then its low companion masswould make it a black widow system in its rotation-powered state. We note that the X-ray properties ofIGR J17494 correspond to those of the so-called veryfaint X-ray transients (VFXTs; Wijnands et al. 2006),whose low outburst luminosities and long-term accre-tion rates are difficult to understand. Our observationssupport the suggestion that some VFXTs may also betMSPs (Heinke et al. 2015). The distinction betweenVFXTs and ordinary LMXBs may somehow relate tothe level of non-conservative mass transfer.ACKNOWLEDGMENTSM.N. and this work were supported by NASA un-der grant 80NSSC19K1287 as well as through the NICER mission and the Astrophysics Explorers Pro-gram.
NICER work at NRL is also supported by NASA.D.A. acknowledges support from the Royal Society. Thisresearch has made use of data and/or software providedby the High Energy Astrophysics Science Archive Re-search Center (HEASARC), which is a service of theAstrophysics Science Division at NASA/GSFC and theHigh Energy Astrophysics Division of the SmithsonianAstrophysical Observatory.
Facilities:
NICER, XMM
Software: astropy (Astropy Collaboration et al.2013), NumPy and SciPy (Oliphant 2007), Matplotlib(Hunter 2007), IPython (Perez & Granger 2007), tqdm(Da Costa-Luis et al. 2020), NICERsoft, PRESTO (Ran-som et al. 2002), PINT (Luo et al. 2020), HEASoft 6.28REFERENCES
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