EM Counterparts from Long-Lived BNS Merger Remnants 2/8 Product of BNS Mergers

Home , Pulsar wind nebula

THE UNIVERSITY IDENTITY The design of the Columbia identity incorporates the core elements of well- Pantone 286 thought-out branding: name, font, color, and visual mark. The logo was designed using the official University font, Trajan Pro, and features specific proportions of type height in relation to the visual mark. The official Colum- bia color is Columbia Blue, or Pantone Black 290. On a light color background, the logo can also be rendered in black, grey (60% black), Pantone 280, or Pantone 286; on a darker color background, the logo can be rendered in Pantone 290, 291, or 284, depending on which color works best with the overall design of your product, the media in which it will 4-color Process be4D.M.S reproduced, and its intendedIEGEL use.&R.CIOLFI 100% Cyan magnetosphere. Via dipole spin-down, the NS starts power- ing a highly relativistic, Poynting-flux dominated outflow of charged particles (mainly electrons and positrons; see Sec- 72% Magenta tion 4.2.1) or ‘pulsar wind’ at the expense of rotational energy. This occurs at a time t = tpul,in and marks the beginning of Phase II. The pulsar wind inflates a PWN behind the less rapidly expanding ejecta, a plasma of electrons, positrons and photons EM counterparts from long-lived BNS (see Section 4.3.1 for a detailed discussion). As this PWN is highly overpressured with respect to the confining ejecta en- velope, it drives a strong hydrodynamical shock into the fluid, which heats up the material upstream of the shock and moves radially outward at relativistic speeds, thereby sweeping up all the material behind the shock front into a thin shell. During this phase the system is composed of a NS (henceforth “pulsar” in Phase II and III) surrounded by an essentially baryon- merger remnants free PWN and a layer of confining ejecta material. The prop- agating shock front separates the ejecta material into an in- ner shocked part and an outer unshocked part (cf. Figure 1 and 2). While the shock front is moving outward across the ejecta, the unshocked matter layer still emits thermal radiation with increasing luminosity as the optical depth decreases. 49 Initially, the expansion of the PWN nebula is highly relativistic and decelerates to non-relativistic speeds only when ⌘ =0.5 Bp the shock front encounters high-density material in the outer ˙ 3 Min = 10 M ejecta layers. The total crossing time for the shock front is 48 t =0.1s typically t = t t t , where t dr shock shock,out pul,in ⌧ pul,in shock,out tdr = 10 s denotes the time when the shock reaches the outer surface. At 2 1 this break-out time, a short burst-type non-thermal EM signal  = 10 cm g

] 47 could be emitted that encodes the signature of particle accel- 1 fcoll =0.1, eration at the shock front. B¯ =3 1016 G ⇥ Phase III starts at t = tshock,out. At this time, the entire ejecta fcoll,PI =0.1 [erg s 46 material has been swept up into a thin shell of thickness ej (which we assume to be constant during the following evo- obs

L lution) that moves outward with speed vej (cf. Figure 2). In White or Pantone 290 general, this speed is higher than the expansion speed of the log 45 baryon-loaded wind in Phase I (vej,in), as during shock prop- (Columbia Blue) agation kinetic energy is deposited into the shocked ejecta. Rotational energy is extracted from the pulsar via dipole spin- Background:44 down and it is reprocessed in the PWN via various radiative processes in analogy to pair plasmas in compact sources, such as active galactic nuclei (see Section 4.3.1 for a detailed dis- Pantone 286 cussion). Radiation escaping from the PWN ionizes the ejecta 43 material, which thermalizes the radiation due to the optical 0 1 2 3 4 5 6 7 depth still being very high. Only at much later times the ejecta log t [s] layer eventually becomes transparent to radiation from the For photographs, nebula, which gives rise to a transition from predominantly Figure 1. Evolution of the system according to the proposed scenario (with thermal to non-thermal emission spectra. We note that for use theincreasing logo spatial scale). in A BNS merger (top left) forms a differentially ro- reasons discussed in Section 5.6, the total luminosity of the tating NS that emits a baryon-loaded wind (Phase I). The NS eventually set- 2 system shows the characteristic t behavior for dipole tles down to uniform rotation and inflates a pulsar wind nebula (or simply / white‘nebula’) against that sweeps up a all the ejecta material into a thin shell (Phase II). spin-down at late times t tsd, where tsd is the spin-down Spin-down emission from the NS continues while the nebula and the ejecta timescale. However, when restricted to individual frequency shell keep expanding (Phase III). bands, the late time behavior of the luminosity can signifi- 2 darker area, posi- cantly differ from a t power law. Daniel M. Siegel ally expanding winds is expected to be predominantly ther- As the NS is most/ likely not indefinitely stable against grav- tioningmal, due it to theeither very high at optical depths at these early times. itational collapse, it might collapse at any time during the evo- However, because of the high optical depth, radiative energy lution outlined above (see Section 4.4). If the NS is supramas- top lossleft/right is still rather inefficient. or sive, the collapse is expected to occur within timescales of As differential rotation is being removed on the timescale the order of t , for the spin-down timescale represents the Center for Theoretical Physics & Columbia Astrophysics Laboratory ⇠ sd tdr, the NS settles down to uniform rotation. Mass loss is time needed to remove a significant fraction of the rotational bottomsuppressed left/right. and while the ejected matter keeps moving out- energy from the NS and thus of its rotational support against ward the density in the vicinity of the NS is expected to collapse. For typical parameters, the collapse occurs in Phase drop on roughly the same timescale. In the resulting essen- III. However, if the NS is hypermassive at birth and does not Columbia University tially baryon-free environment the NS can set up a pulsar-like migrate to a supramassive configuration thereafter, it is ex-

Einstein Fellows Symposium, Harvard-Smithsonian Center for Astrophysics, Oct 19, 2016 3 EM counterparts to NS mergers

The Astrophysical Journal,746:48(15pp),2012February10 Metzger & Berger with specific stellar populations). Because merger counterparts • Short gamma-ray bursts (SGRBs) Jet ISM Shock (Afterglow) are predicted to be faint, obtaining a spectroscopic redshift Optical (hours days) “Standard” afterglows: is challenging (cf. Rowlinson et al. 2010), in which case Radio (weeks years) spectroscopy of the host galaxy is the most promising means Ejecta ISM Shock • X-ray of obtaining the event redshift. Radio (years) • UV/optical It is important to distinguish two general strategies for con- obs • radio necting EM and GW events. One approach is to search for a GRB GW signal following an EM trigger, either in real time or at (t ~ 0.1 1 s) Berger 2014, Kumar & Zhang 2015 apost-processingstage(e.g.,Finnetal.1999; Mohanty et al. Kilonova “Non-standard” X-ray afterglows: 2004). This is particularly promising for counterparts predicted Optical (t ~ 1 day) to occur in temporal coincidence with the GW chirp, such as (revealed by Swift) j Merger Ejecta short-duration gamma-ray bursts (SGRBs). Unfortunately, most Tidal Tail & Disk Wind other promising counterparts (none of which have yet been • Extended Emission v ~ 0.1 0.3 c independently identified) occur hours to months after coales- BH • X-ray plateaus cence.6 Thus, the predicted arrival time of the GW signal will remain uncertain, in which case the additional sensitivity gained • X-ray flares from this information is significantly reduced. For instance, if Rowlinson+ 2013, Gompertz+ 2013,2014, Lue+ 2015 the time of merger is known only to within an uncertainty of hours (weeks), as we will show is the case for optical (radio) Interaction of dynamical ejecta with ISM (radio) ∼ • counterparts, then the number of trial GW templates that must Hotokezaka & Piran 2015 be searched is larger by a factor 104–106 than if the merger time is known to within seconds, as∼ in the case of SGRBs. Figure 1. Summary of potential electromagnetic counterparts of NS–NS/ NS–BH mergers discussedMetzger in & this Berger paper, 2012 as a function of the observer angle,• radioactively powered kilonova/macronova A second approach, which is the primary focus of this paper, θobs.Followingthemergeracentrifugallysupporteddisk(blue)remainsaround is EM follow-up of GW triggers. A potential advantage in this the central compact object (usually a BH). Rapid accretion lasting !1s Li & Paczynski 1998, Rosswog 2005, Metzger+ 2010, case is that counterpart searches are restricted to the nearby powers a collimated relativistic jet, which produces a short-duration gamma- Barnes & Kasen 2013, Piran+ 2013, Tanaka & Hotokezaka 2013 universe, as determined by the ALIGO/Virgo sensitivity range ray burst (Section 2). Due to relativistic beaming, the gamma-ray emission is restricted to observers with θobs ! θj ,thehalf-openingangleofthejet. (redshift z ! 0.05–0.1). On the other hand, the large error Non-thermal afterglow emission results from the interaction of the jet with regions are a significant challenge, which are estimated to be theDaniel surrounding Siegel circumburst medium (pink).EM Optical counterparts afterglow emissionfrom long-lived is BNS merger remnants 1/8 tens of square degrees even for optimistic configurations of GW observable on timescales up to days–weeks by observers with viewing angles ∼ detectors (e.g., Gursel¨ & Tinto 1989;Fairhurst2009;Wen& of θobs ! 2θj (Section 3.1). Radio afterglow emission is observable from all viewing angles (isotropic) once the jet decelerates to mildly relativistic speeds Chen 2010;Nissankeetal.2011). Although it has been argued on a timescale of weeks–months, and can also be produced on timescales of that this difficulty may be alleviated if the search is restricted years from sub-relativistic ejecta (Section 3.2). Short-lived isotropic optical to galaxies within 200 Mpc (Nuttall & Sutton 2010), we stress emission lasting few days (kilonova; yellow) can also accompany the merger, powered by the radioactive∼ decay of heavy elements synthesized in the ejecta that the number of galaxies with L " 0.1 L∗ (typical of SGRB host galaxies; Berger 2009, 2011)withinanexpectedGWerror (Section 4). region is 400, large enough to negate this advantage for most (A color version of this figure is available in the online journal.) search strategies.∼ In principle the number of candidate galaxies error regions is required, whether the aim is to detect optical could be reduced if the distance can be constrained from the or radio counterparts. Even with this approach, the follow- GW signal; however, distance estimates for individual events up observations will still require large field-of-view (FOV) are rather uncertain, especially at that low of S/Ns that will telescopes to cover tens of square degrees; targeted observations characterize most detections (Nissanke et al. 2010). Moreover, of galaxies are unlikely to substantially reduce the large amount current galaxy catalogs are incomplete within the ALIGO/Virgo of time to scan the full error region. volume, especially at lower luminosities. Finally, some mergers Our investigation of EM counterparts is organized as follows. may also occur outside of their host galaxies (Berger 2010; We begin by comparing various types of EM counterparts, each Kelley et al. 2010). Although restricting counterpart searches to illustrated by the schematic diagram in Figure 1. The first is an nearby galaxies is unlikely to reduce the number of telescope SGRB, powered by accretion following the merger (Section 2). pointings necessary in follow-up searches, it nevertheless can Even if no SGRB is produced or detected, the merger may still substantially reduce the effective sky region to be searched, be accompanied by relativistic ejecta, which will power non- thereby allowing for more effective vetoes of false positive thermal afterglow emission as it interacts with the surrounding events (Kulkarni & Kasliwal 2009). medium. In Section 3 we explore the properties of such “or- At the present there are no optical or radio facilities that can phan afterglows” from bursts with jets nearly aligned toward provide all-sky coverage at a cadence and depth matched to Earth (optical afterglows; Section 3.1)andforlargerviewing the expected light curves of EM counterparts. As we show in angles (late radio afterglows; Section 3.2). We constrain our this paper, even the Large Synoptic Survey Telescope (LSST), models using the existing observations of SGRB afterglows, with a planned all-sky cadence of four days and a depth of coupled with off-axis afterglow models. We also provide a re- r 24.7mag,isunlikelytoeffectivelycapturetherangeof alistic assessment of the required observing time and achiev- expected≈ EM counterparts. Thus, targeted follow-up of GW able depths in the optical and radio bands. In Section 4 we consider isotropic optical transients powered by the radioac- 6 Predicted EM counterparts that may instead precede the GW signal include tive decay of heavy elements synthesized in the ejecta (referred emission powered by the magnetosphere of the NS (e.g., Hansen & Lyutikov to here as “kilonovae,” since their peak luminosities are pre- 2001;McWilliams&Levin2011; Lyutikov 2011a, 2011b), or cracking of the dicted to be roughly one thousand times brighter than those NS crust due to tidal interactions (e.g., Troja et al. 2010;Tsangetal.2011), during the final inspiral. However, given the current uncertainties in these of standard novae). In Section 5 we compare and contrast the models, we do not discuss them further. potential counterparts in the context of our four Cardinal Virtues.

2 What is a promising EM counterpart?

bright isotropic long lasting high fraction smoking gun for BNS SGRBs standard afterglows BNS post-merger transients (this talk) … … … … … dynamical ejecta, ISM kilonovae

Daniel Siegel EM counterparts from long-lived BNS merger remnants 2/8 Product of BNS mergers

SMNS / HMNS long-lived NS

BNS

BH - torus BH - torus prompt collapse

sim. & vis.: W. Kastaun

• observationally: M TOV & 2M Demorest+ 2010, Antoniadis+ 2013 • progenitor masses peak around 1 . 3 1 . 4M remnant NS mass typically 2.3M 2.4M Belczynski+ 2008 ⇡ • supramassive to hypermassive limit at 1.2MTOV & 2.4M Lasota+ 1996 ⇡ the most likely outcome should be a long-lived (supramassive) NS

Daniel Siegel EM counterparts from long-lived BNS merger remnants 3/8 1072 A. Rowlinson et al.

1072 A. Rowlinson et al. I ‘Time-reversal’ scenario X-rays SGRB ‘Time-reversal’ scenario

ejecta X-rays Post-merger evolution IEGEL IOLFI jet 1072 A. Rowlinson et al. 4D.M.SII III &R.C a X-raysmagnetosphere. Via dipole spin-down, the NS starts power- ing a highly relativistic, Poynting-fluxBH-torus dominated outflow of X-rays chargedIII particlesnebula (mainly electrons andSGRB positrons; see Sec- tion 4.2.1) or ‘pulsar wind’ at the expense of rotational en- II Ciolfi & Siegel 2015 General Phenomenology for BNSI mergers leadingNS ejecta X-raysergy. This occurs at a time t = tpul,in and marks the beginningX-rays X-rays ejecta of Phase II. ‘Time-reversal’ scenario shocked shocked ejecta The pulsar wind inflates a PWN behind the less rapidly ex- to a long-lived (>100ms) remnant NS: ejecta panding ejecta,ejecta a plasma of electrons, positrons and photons I shocked ejecta (seeX-rays Section 4.3.1III for a detailed discussion). As this PWN is ejecta ‘Time-reversal’ highlyscenario overpressurednebula with respect to the confiningjet ejecta en- II shock nebula velope, it drives a strong hydrodynamicalX-rays shock into the fluid, Phase I (baryonic wind phase, ~1s): shockednebula ejecta which heats up the material upstream of the shock and moves radially outwardshocked at relativistic speeds, thereby sweeping up all SGRB • hot, differentially rotating NS ‘Time-reversal’nebula scenariothe materialejecta behind the shock front into aBH-torus thinX-raysSGRB shell. During this phase the system is composed of a NS (henceforth “pul- 1072 A. Rowlinson et al. • baryon pollution due to dynamical ejecta, shock NSNSNS gamma rays tafterglowNS X-rays sar” in Phase II and III) surrounded by anCiolfi essentially & Siegel baryon- 2015a neutrino and magnetically driven winds free PWN and a layer of confining ejecta material. The prop- ejecta X-rays III agating shock frontnebula separates the ejecta material into an in- II gammaner shocked raysshocked part and an outer unshocked part (cf. Figure 1 shock and 2).ejecta While the shock frontjet is moving outward across theX-rays ‘Time-reversal’ scenarioX-rays jet X-raysgamma raysejecta, the unshocked matter layer still emits thermal radia- Phase II (Pulsar ‘ignition’ and pulsar wind shock ~sec-min): tion with increasing luminosity as the optical depth decreases. I II III delayX-raysSGRB ejecta Initially, thenebula expansion of the PWN nebula is highly rela-a • cold, uniformly rotating NS X-rays tivistic and decelerates to non-relativistict NS BH-torus speeds only when afterglow 15 I ejecta X-rays t 12/ NS ejecta ejecta the shock front encounters high-densityBH-torus material in theX-rays outer • baryon pollution suppressedRowlinson+2013 spin-down emission, ejecta layers. The total crossing time for the shock front is II shocked & shocked ejecta typically tshock = tshock,out Ciolfitpul,inCiolfi &t pul,in Siegel&delay ,Siegel where 2015 t2015shock,out pulsar wind inflates nebula, drives shock through ejecta ⌧ Daniel Siegel ejectatafterglow NS shocked ejecta denotesejecta the time when the shock reaches thet outerNS surface. At shocked ejecta X-raysthis break-outIII time, a short burst-type non-thermal EM signal X-rays could be emittednebula that encodes the signaturejet of particle accel- shock nebulanebula eration atdelay the shock front. a Phase III (Pulsar wind nebula phase ~min-days): Model calculation: nebula tNS X-rays Phaseshocked III starts at t =generally:tshock,out. At this time, the entire ejecta NS material has been swept up intoX-rays a thin shell of thickness ej • swept-up material provides cavity for a pulsar (whichejecta we assume to be constant during theSGRB following evo- NS shock NSNS delay BH-torus t gammalution) rays that moves outward with speed vej (cf. Figure 2). In I tafterglowEM counterparts to the GW signal of compact binary mergers NS A. Rowlinson et al. wind nebula (PWN) in analogy to CCSNe “time-reversal”shock scenario compatible with observationsgeneral, this speed is higher than the expansion speed of the ejecta baryon-loaded wind in Phase I (v Ciolfi), as during & Siegel shock prop-2015a nebuladelay ej,int generally:agationshocked kinetic energytNS is deposited& intoafterglow the shocked ejecta. afterglow generally: gammaRotational rays energy is extracted from the pulsar via dipole spin- 1072 t Model calculation: ejecta X-rays • NS may collapse to a BH at any time down and it is reprocessed in the PWNjet via various radiative A. Rowlinson et al. Rowlinson+2013 gamma raysprocesses in analogy to pair plasmas in compact sources, such NS Modelt delay calculation:as active galactic nuclei (see Section 4.3.1 for a detailed dis- 1072 • EM emission: reprocessed spin-down energy “time-reversal” scenarioNS cussion).compatible Radiation escaping with from theobservations PWN ionizes the ejecta delay X-rays “time-reversal”material, which thermalizes scenario theBH-torus radiationcompatible due to the opticalwith observations12/15 model predicts broad-band spectrum from radio to gamma rays & t NS Rowlinson+2013 EM counterparts to the GW signal of compact binarydeptht still mergers being very high. Only at much later times the ejecta Daniel Siegel Siegel & Ciolfi 2016alayerafterglow eventually becomes transparentCiolfi & Siegel to radiation 2015 from the t nebula, which gives rise to a transition from predominantlytafterglow Daniel Siegel afterglow Figure 1. Evolution of the system according to the proposed scenario (with thermal to non-thermal emission spectra. We note that for Daniel Siegel EMShort counterparts gamma-ray frombursts long-lived in the “time-reversal” BNS mergerincreasing remnantsscenario spatial scale). A BNS merger (top left) forms a differentially ro- reasons4/8 discussed in Section delay5.6, the total& luminosity of the tating NS that emits a baryon-loaded wind (Phase I). The NS eventually set- 2 15 tles down to uniform rotation and inflates a pulsar windEM nebula counterparts (or simply tosystem the GW shows signal the characteristic of compactt NS binaryt behavior mergers for dipole 12/ ‘nebula’) that sweeps up all the ejecta material into a thin shell (Phase II). spin-down at lateX-rays times t t ,/ where t is the spin-downa Model calculation: delay sd sd Spin-downRowlinson+2013 emission from the NS continues while the nebula and the ejecta timescale.tNS However, when restricted to individual frequency shell keep expanding (Phase III). bands, the late time behavior of the luminosity can signifi- afterglow generally: 2 Figure 8 t delay cantly differ from a t power law. ally expanding winds is expected to be predominantlyt ther- As the NS is most/ likely not indefinitely stable against grav- EM counterparts“time-reversal” to the GW signal of compactscenario binary compatible mergers NS with observations 12/ – continued mal, due to the very high optical depths at these early times. itational collapse, it might collapse at any time during the evo- However, because of the high optical depth, radiative energy lution outlined above (see Section15 4.4). If the NS is supramas- loss is still rather inefficient.generally: sive, the collapsedelay is expected to occur within timescales of AsModel differential rotationcalculation: is being removed on the timescalegenerally:the order oft tsd, for the spin-downt timescale represents the Daniel Siegel Model calculation: NS⇠ & afterglow Rowlinson+2013 tdr, the NS settles down to uniform rotation. Mass loss is time needed to remove a significant fraction of the rotational suppressed and while the ejected matter keeps movingdelay out- energy from the NS and thus of its rotational support against ward the density in the vicinity of“time-reversal” the NS ist expected to scenariocollapse. For typical compatible parameters, the collapse with occurs observations in Phase drop on roughly“time-reversal” the same timescale. In thescenario resultingNS essen- compatibleIII. However, if thewith NS is hypermassive observations at birth and does not tially baryon-free environment the NS can set up a pulsar-like& migrate to a supramassive configuration thereafter, it is ex- t Daniel Siegel EM counterparts to the GW signal of compact binary mergersafterglow 12/15 Rowlinson+2013 EM counterparts to the GW signal of compact binary mergers

Figure 8 Daniel Siegel 12/ – continued 15

Figure 8 – continued

continued –

Figure 8

Figure 8 – continued

– continued Figure 8 The Astrophysical Journal Letters,785:L6(6pp),2014April10 Siegel, Ciolﬁ, & Rezzolla

Figure 1. Snapshots of the magnetic field strength (color-coded in logarithmic scale and Gauss) and rest-mass density contours in the (x,z)planeatrepresentative times for model dip-60.Magneticfieldlinesaredrawninredintheleftpanel.TheleftmostinsetshowsamagnificationoftheHMNS,theotheronesshowa horizontal cut at z 120 km. = (A color version of this figure is available in the online journal.)

1694 DESSART ET AL. Vol. 690

Outﬂows from BNS merger remnants Figure 2. Same as Figure 1,butformodeldip-6. (A color version of this ﬁgure is available in the online journal.)

wind wind

wind

2 1 1 Figure 13. Colormaps of the log of the mass-loss rate per steradian (d M/dt dΩ,inunitsofM s− str− ) for the no-spin BNS merger model at 10 ms (top left), 30 ms (top right), 60 ms (bottom left), and 100 ms (bottom right) after the start of the VULCAN/2D⊙ Figure simulation, 3. andSame depicting as Figure the mass1,butformodel loss associated withrand the. initial 2 Dessart+ 2009 10 3 Siegel+ 2014 transient, followed by the neutrino-driven wind. The displayed region covers 2000 2000(A km color. Regions version that are of infalling this figure or denser is available than 10 ingcm the− onlineare shown journal.) in × 1 red, and velocity vectors, overplotted in black, have a length saturated at 7% of the width of the display for a magnitude of 30,000 km s− . Note the concomitant mass loss from the poles down to midlatitudes (the wind) and the expansion of BNS merger material at near-equatorial latitudes. (A color version of this figurefield is available geometry in the online and journal.) could be absent if the field is randomly Defining the isotropic luminosity as Siegel+ 2017 distributed.52 Fernández & Metzger 2013, Just+ 2015 is on the order of 2 10 erg in the torus disk, regions with den- Oechslin et al. (2007), using a conformally flat approximation EM 11× In14 all3 of the configurations considered, the mag- r sities between 10 and 10 gcm− .Similarconditionsinthe to GR and an SPH code, find that BNS mergers of theL typeEM dΩ √ g (T ) t , (2) core-collapse context yieldnetized powerful, baryon-loaded magnetically (and outflow ther- hasdiscussed rest-mass here and densities modeled with the Shen EOS avoid the ≡− r Rd − 8 9 3 ! = mally) driven explosions (LeBlanc10 –10 &gcm Wilson− 1970and; Bisnovatyi-is ejected fromgeneral-relativisticneutrino-driven the star with velocities gravitational wind instability for many tens ofmagnetically driven wind delayed outflows Kogan et al. 1976; Akiyama∼v/c et al.0.1,2003 in; the Ardeljan isotropic et al. 2005 part,; andmillisecondsv/c 0.3, after in the the neutron colli- stars firstwhere comedΩ intois contact. the solid-angle element, g is the determinant ! (from! hot remnant NS) (from remnantEM NS) (from accretion disks) Moiseenko et al. 2006mated;Obergaulingeretal. part. 2006;Burrows Baumgarte et al. (2000), and more recentlyof the Morrison spacetime et al. metric, and T is the EM part of the et al. 2007a; Dessart et al. 2007). Rotation dramatically en- (2004), Duez et al. (2004, 2006), and Shibata et al. (2006), µν hances the rate of mass ejection by increasing the density using GR (and(~ms-1s) for some using a polytropic EOS), find that (~ms-1s) (~1s) rather than the velocity of the flow, even possibly halting ac- imposing even modest levels of differential3 rotation yields a cretion and inhibiting the formation of a black hole (Dessart significant increase by up to 50% in the maximum mass that can et al. 2008). In the present context, the magneto-rotational ˙be supported stably,4 in particular3 pushing this value1 beyond that˙ 3 2 1 3 2 effects, which we do not include here, would considerably Mofin the merger(10 remnant10 mass after)M coalescence.s Surprisingly,Min (10 10 )M s Mtot . 10 10 M enhance the mass flux of the neutrino-driven wind. Impor- Baiotti⇠ et al. (2008), using a full GR treatment but with a ⇠ tantly, the loss of differential rotational energy needed to fa- simplified (and soft) EOS, find prompt black hole formation cilitate the gravitational instability is at the same time de- in such high-mass progenitors. Despite this lack of consensus, laying it through the enhanced mass loss it induces. Work is the existence of neutron stars with a gravitational mass around needed to understand the systematics of this interplay, and how 2 M favors a high incompressibility of nuclear matter, such ⊙ much rotational energy the back hole is eventually endowed as in the Shen EOS, and suggests that SMNSs formed through with. DanielBNS mergerSiegel events may survive for tens of millisecondsEM counterparts before from long-lived BNS merger remnants 4/8 1072 A. Rowlinson et al.

1072 A. Rowlinson et al. I ‘Time-reversal’ scenario X-rays SGRB ‘Time-reversal’ scenario

Figure 8 Daniel Siegel 12/ – continued 15

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Figure 8

Figure 8 – continued

– continued Figure 8 1072 A. Rowlinson et al.

1072 A. Rowlinson et al. I ‘Time-reversal’ scenario X-rays SGRB ‘Time-reversal’ scenario

ejecta X-rays Post-merger evolution: evolution equations IEGEL IOLFI jet 1072 A. Rowlinson et al. 4D.M.SII III &R.C a magnetosphere. Via dipole spin-down, the NS starts power- X-raysing a highly relativistic, Poynting-flux dominated outflow of nebula BH-torusSGRB dRej X-rays chargedIII particles (mainly electrons and positrons; see Sec- = v (R (t),t) tion 4.2.1) or ‘pulsar wind’ at the expense of rotational en- Phase I: dt w ej II Ciolfi & Siegel 2015 I NS ejecta X-raysergy. This occurs at a time t = tpul,in and marks the beginningX-rays dE dE X-rays ejecta of Phase II. th = L (t)+ th,NS L (t) ‘Time-reversal’ scenario shocked EM rad shocked ejecta The pulsar wind inflates a PWN behind the less rapidly ex- dt dt ejecta panding ejecta,ejecta a plasma of electrons, positrons and photons dR I shocked ejecta (seeX-rays Section 4.3.1III for a detailed discussion). As this PWN is ej ejecta ‘Time-reversal’ highlyscenario overpressured with respect to the confiningjet ejecta en- Phase II: = vw(Rej(t),t) nebula dt II shock nebula velope, it drives a strong hydrodynamicalX-rays shock into the fluid, dR shockednebula ejecta which heats up the material upstream of the shock and moves sh set of coupled ODEs radially outwardshocked at relativistic speeds, thereby sweeping up all = vsh(t) SGRB dt ‘Time-reversal’nebula scenariothe materialejecta behind the shock front into aBH-torus thinX-raysSGRB shell. During this phase the system is composed of a NS (henceforth “pul- 1072 A. Rowlinson et al. dRn dRsh dsh shock NSNSNS gamma rays = tafterglowNS X-rays sar” in Phase II and III) surrounded by an essentially baryon- a dt dt dt free PWN and a layer of confining ejectaCiolfi material. & The Siegel prop- 2015 dE dE dE dE ejecta X-rays agating shock frontnebula separates the ejecta material into an in- th,sh sh th,vol PWN III shocked = + + Lrad,in(t) II gammaner shocked rays part and an outer unshocked part (cf. Figure 1 dt dt dt dt shock and 2).ejecta While the shock frontjet is moving outward across theX-rays ‘Time-reversal’ scenarioX-rays jet dEth,ush dEth,vol X-raysgamma raysejecta, the unshocked matter layer still emits thermal radia- = Lrad(t) tion with increasing luminosity as the optical depth decreases. dt dt I II III delayX-raysSGRB ejecta Initially, thenebula expansion of the PWN nebula is highly rela-a dE dE dE X-rays tivistic and decelerates to non-relativistict NS BH-torus speeds only when afterglow 15 th th,sh th,ush I ejecta X-rays t 12/ = + NS ejecta ejecta the shock front encounters high-densityBH-torus material in theX-rays outer dt dt dt Rowlinson+2013 ejecta layers. The total crossing time for the shock front is II shocked & dEnth Enth dRn dEPWN shocked ejecta typically tshock = tshock,out Ciolfitpul,inCiolfi &t pul,in Siegel&delay ,Siegel where 2015 t2015shock,out = + L (t)+⌘ [L (t)+L (t)] ⌧ Daniel Siegel rad,in ejectaTStafterglowsd NSrad,pul shocked ejecta denotesejecta the time when the shock reaches thet outerNS surface. At dt Rn dt dt shocked ejecta X-raysthis break-outIII time, a short burst-type non-thermal EM signal dEB X-rays could be emittednebula that encodes the signaturejet of particle accel- = ⌘ [L (t)+L (t)] shock nebula delay dt Bn sd rad,pul nebulanebula eration att the shockX-rays front. a Model calculation: Phase IIINS starts at t = tshock,out. At this time, the entire ejecta materialshocked has been sweptgenerally: up into a thin shell of thickness dvej NS X-rays ej = a (t) (whichejecta we assume to be constant during theSGRB following evo- Phase III: ej NS shock NSNS delay BH-torus dt t gammalution) rays that moves outward with speed vej (cf. Figure 2). In I tafterglowEM counterparts to the GW signal of compact binary mergers NS A. Rowlinson et al. dRej 1 ejecta “time-reversal”shock scenario compatible with observationsgeneral, this speed is higher than the expansion speed of the = vej(t)+ aej(t)dt baryon-loaded winddelay in Phase I (vej,inCiolfi), as during & Siegel shock prop-2015a dt 2 nebula t generally:agationshocked kinetic energytNS is deposited& intoafterglow the shocked ejecta. dR dR afterglow generally: gammaRotational rays energy is extracted from the pulsar via dipole spin- 1072 n ej t Model calculation: ejecta X-rays = down and it is reprocessed in the PWNjet via various radiative A. Rowlinson et al. dt dt Rowlinson+2013 gamma raysprocesses in analogy to pair plasmas in compact sources, such dEth dEPWN NS Modelt delay calculation:as active galactic nuclei (see Section 4.3.1 for a detailed dis- 1072 =[1 fej(t)] Lrad(t) Lrad,in(t) “time-reversal” scenarioNS cussion).compatible Radiation escaping with from theobservations PWN ionizes the ejecta dt dt delay X-rays “time-reversal”& material, which thermalizes scenario theBH-torus radiationcompatible due to the opticalwith observations12/15 dEB depth still beingt veryNS high. Only at much later times the ejecta = ⌘B [Lsd(t)+LRowlinson+2013rad,pul(t)] EM counterparts to the GW signal of compact binarytafterglow mergers dt n Daniel Siegel Siegel & Ciolfi 2016alayer eventually becomes transparentCiolfi & Siegel to radiation 2015 from the t nebula, which gives rise to a transition from predominantlytafterglow Daniel Siegel afterglow Figure 1. Evolution of the system according to the proposed scenario (with thermal to non-thermal emission spectra. We note that for Daniel Siegel EMShort counterparts gamma-ray frombursts long-lived in the “time-reversal” BNS mergerincreasing remnantsscenario spatial scale). A BNS merger (top left) forms a differentially ro- reasons5/8 discussed in Section delay5.6, the total& luminosity of the tating NS that emits a baryon-loaded wind (Phase I). The NS eventually set- 2 15 tles down to uniform rotation and inflates a pulsar windEM nebula counterparts (or simply tosystem the GW shows signal the characteristic of compactt NS binaryt behavior mergers for dipole 12/ ‘nebula’) that sweeps up all the ejecta material into a thin shell (Phase II). spin-down at lateX-rays times t t ,/ where t is the spin-downa Model calculation: delay sd sd Spin-downRowlinson+2013 emission from the NS continues while the nebula and the ejecta timescale.tNS However, when restricted to individual frequency shell keep expanding (Phase III). bands, the late time behavior of the luminosity can signifi- afterglow generally: 2 Figure 8 t delay cantly differ from a t power law. ally expanding winds is expected to be predominantlyt ther- As the NS is most/ likely not indefinitely stable against grav- EM counterparts“time-reversal” to the GW signal of compactscenario binary compatible mergers NS with observations 12/ – continued mal, due to the very high optical depths at these early times. itational collapse, it might collapse at any time during the evo- However, because of the high optical depth, radiative energy lution outlined above (see Section15 4.4). If the NS is supramas- loss is still rather inefficient.generally: sive, the collapsedelay is expected to occur within timescales of AsModel differential rotationcalculation: is being removed on the timescalegenerally:the order oft tsd, for the spin-downt timescale represents the Daniel Siegel Model calculation: NS⇠ & afterglow Rowlinson+2013 tdr, the NS settles down to uniform rotation. Mass loss is time needed to remove a significant fraction of the rotational suppressed and while the ejected matter keeps movingdelay out- energy from the NS and thus of its rotational support against ward the density in the vicinity of“time-reversal” the NS ist expected to scenariocollapse. For typical compatible parameters, the collapse with occurs observations in Phase drop on roughly“time-reversal” the same timescale. In thescenario resultingNS essen- compatibleIII. However, if thewith NS is hypermassive observations at birth and does not tially baryon-free environment the NS can set up a pulsar-like& migrate to a supramassive configuration thereafter, it is ex- t Daniel Siegel EM counterparts to the GW signal of compact binary mergersafterglow 12/15 Rowlinson+2013 EM counterparts to the GW signal of compact binary mergers

Figure 8 Daniel Siegel 12/ – continued 15

Figure 8 – continued

continued –

Figure 8

Figure 8 – continued

– continued Figure 8 Post-merger EM emission

⌘Bp =0.5 3 M˙ in = 10 M 48 tdr =0.1s

tdr = 10 s 2 1  = 10 cm g

] 47

1 fcoll =0.1, B¯ =3 1016 G ⇥ fcoll,PI =0.1 [erg s 46 obs L

log 45

43 0 1 2 3 4 5 6 7 log t [s] Siegel & Ciolﬁ 2016b Fig.: Reconstructed X-ray lightcurves (0.3-10 keV)

• hot ejecta (continuous heating by nebula): emission is in the X-rays • delayed onset of strong X-ray radiation ~1-10s after merger (high optical depth at early times) • bright, isotropic, long-lasting X-ray signal peaking at ~102-104s after merger (L~1046-1048erg s-1)

Daniel Siegel EMShort counterparts gamma-ray frombursts long-lived in the “time-reversal” BNS merger remnantsscenario 6/8 Post-merger EM emission

49 0.5

⌘ =0.5 Bp soft X-rays 3 M˙ in = 10 M 48 tdr =0.1s 0.4

tdr = 10 s 2 1  = 10 cm g

] 47

1 fcoll =0.1, B¯ =3 1016 G 0.3 ⇥ fcoll,PI =0.1 [erg s 46 [keV] ↵ obs e T L 0.2

log 45

0.1 44

43 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 log t [s] log t [s] Siegel & Ciolﬁ 2016b Fig.: X-ray light curves and effective temperature evolution (example)

• at timescale of peak brightness, predominantly thermal emission in the X-rays (continuous heating by the nebula) • heating by r-process nucleosynthesis typically subdominant up to t ~ 1d

• degree of ionization of ejecta matter important: if low, peak might be shifted toward lower frequencies

Daniel Siegel EMShort counterparts gamma-ray frombursts long-lived in the “time-reversal” BNS merger remnantsscenario 6/8 Post-merger EM emission: EM counterpart to GWs

⌘Bp =0.5 3 M˙ in = 10 M 48 tdr =0.1s

tdr = 10 s 2 1  = 10 cm g

] 47

1 fcoll =0.1, B¯ =3 1016 G ⇥ fcoll,PI =0.1 [erg s 46 obs L

log 45

43 0 1 2 3 4 5 6 7 log t [s] Siegel & Ciolﬁ 2016b Fig.: Reconstructed X-ray lightcurves (0.3-10 keV)

• bright, isotropic, long-lasting X-ray signal peaking at ~102-104s after merger (L~1046-1048erg s-1) smoking gun for BNS merger event timescale well suited for EM follow up of GW event X-ray signal represents ideal EM counterpart

Daniel Siegel EMShort counterparts gamma-ray frombursts long-lived in the “time-reversal” BNS merger remnantsscenario 6/8 What is a promising EM counterpart?

bright isotropic long lasting high fraction smoking gun for BNS SGRBs standard afterglows BNS post-merger X-ray transients (this talk) !!! dynamical ejecta, ISM kilonovae

according to the model: BNS post-merger X-ray transients represent ideal EM counterpart

Daniel Siegel EM counterparts from long-lived BNS merger remnants 7/8 1072

A. Rowlinson et al.

I Conclusions

4D.M.SIEGEL &R.CIOLFI

magnetosphere. Via dipole spin-down, the NS starts power- • ing a highly relativistic, Poynting-flux dominated outflow of majority of BNS mergers should lead to long-lived NSs charged particles (mainly electrons and positrons; see Sec- tion 4.2.1) or ‘pulsar wind’ at the expense of rotational energy. This occurs at a time t = tpul,in and marks the beginning of Phase II. The pulsar wind inflates a PWN behind the less rapidly expanding ejecta, a plasma of electrons, positrons and photons • proposed post-merger phenomenology and detailed (see Section 4.3.1 for a detailed discussion). As this PWN is highly overpressured with respect to the confining ejecta en- numerical model for those events velope, it drives a strong hydrodynamical shock into the fluid, ‘Time-reversal’ scenario which heats up the material upstream of the shock and moves radially outward at relativistic speeds, thereby sweeping up all the material behind the shock front into a thin shell. During this phase the system is composed of a NS (henceforth “pul- ejecta general model to compute broad band EM emission sar” in Phase II and III) surrounded by an essentially baryon- free PWN and a layer of confining ejecta material. The prop- agating shock front separates the ejecta material into an in- (radio to gamma rays) ner shocked part and an outer unshocked part (cf. Figure 1 and 2). While the shock front is moving outward across the ejecta, the unshocked matter layer still emits thermal radia- X-rays tion with increasing luminosity as the optical depth decreases. bridges the gap between numerical relativity simulations Initially, the expansion of the PWN nebula is highly relativistic and decelerates to non-relativistic speeds only when the shock front encounters high-density material in the outer and the observational timescales of EM transients ejecta layers. The total crossing time for the shock front is typically tshock = tshock,out tpul,in tpul,in, where tshock,out NS denotes the time when the shock reaches⌧ the outer surface. At this break-out time, a short burst-type non-thermal EM signal reveals strong thermal transient (peaking in the X-rays, could be emitted that encodes the signature of particle accel- II eration at the shock front. but also UV and optical counterparts at later times), Phase III starts at t = tshock,out. At this time, the entire ejecta material has been swept up into a thin shell of thickness ej (which we assume to be constant during the following evo- promising counterpart for GW astronomy lution) that moves outward with speed vej (cf. Figure 2). In general, this speed is higher than the expansion speed of the baryon-loaded wind in Phase I (vej,in), as during shock prop- agation kinetic energy is deposited into the shocked ejecta. together with NS component masses from GW signal can Rotational energy is extracted from the pulsar via dipole spin- down and it is reprocessed in the PWN via various radiative tightly constrain EOS (using supramassive NS assumption) processes in analogy to pair plasmas in compact sources, such as active galactic nuclei (see Section 4.3.1 for a detailed dis- t cussion). Radiation escaping from the PWN ionizes the ejecta afterglow Ciolfi & Siegel (2015), ProcSci (SWIFT 10)108 material, which thermalizes the radiation due to the optical depth still being very high. Only at much later times the ejecta layer eventually becomes transparent to radiation from the natural explanation for combined phenomenology of Swift X- nebula, which gives rise to a transition from predominantly ejecta Figure 1. Evolution of the system according to the proposed scenario (with thermal to non-thermal emission spectra. We note that for shocked ejecta increasing spatial scale). A BNS merger (top left) forms a differentially ro- reasons discussed in Section 5.6, the total luminosity of the tating NS that emits a baryon-loaded wind (Phase I). The NS eventually set- 2 ray lightcurves (not this talk), and late-time kilonova emission system shows the characteristic t behavior for dipole shock tles down to uniform rotation and inflates a pulsar wind nebula (or simply / ‘nebula’) that sweeps up all the ejecta material into a thin shell (Phase II). spin-down at late times t tsd, where tsd is the spin-down Spin-down emission from the NS continues while the nebula and the ejecta timescale. However, when restricted to individual frequency Daniel Siegel shell keep expanding (Phase III). bands, the late time behavior of the luminosity can signifi- makes very specific predictions that can be tested 2 cantly differ from a t power law. Rowlinson+2013 Siegel D.M.ally expanding & Ciolfi winds is expected R. (2016a), to be predominantly ApJ ther- 819As, 14 the NS is most/ likely not indefinitely stable against grav- observationally mal, due to the very high optical depths at these early times. itational collapse, it might collapse at any time during the evo- nebula However, because of the high optical depth, radiative energy lution outlined above (see Section 4.4). If the NS is supramas- Siegel D.M.loss is still & rather Ciolfi inefficient. R. (2016b), ApJ sive,, the15 collapse is expected to occur within timescales of see also “time-reversal” scenario As differential rotation is being removed on the timescale 819the order of tsd, for the spin-down timescale represents the ⇠ tdr, the NS settles down to uniform rotation. Mass loss is time needed to remove a significant fraction of the rotational Ciolfi & Siegel (2015), ApJL 798, L36 suppressed and while the ejected matter keeps moving out- energy from the NS and thus of its rotational support against X-rays ward the density in the vicinity of the NS is expected to collapse. For typical parameters, the collapse occurs in Phase drop on roughly the same timescale. In the resulting essen- III. However, if the NS is hypermassive at birth and does not NS Daniel Siegel EMShort counterparts gamma-ray frombursts long-lived in the “time-reversal” BNS merger remnantsscenario tially baryon-free environment the NS can set up a pulsar-like migrate8/ to8 a supramassive configuration thereafter, it is ex- III

Model calculation: gamma rays Figure 8 EM counterparts to the GW signal of compact binary mergers shocked ejecta nebula SGRB

– “time-reversal” scenario compatible with observations continued

jet

X-rays BH-torus Ciolﬁ & Siegel 2015 generally: t delay NS X-rays

t delay a NS

& t afterglow

12/ 15