Redback/Black Widow Systems as progenitors of the highest neutron star masses and low-mass Black Holes
J. E. Horvath, A. Bernardo, L.S. Rocha, R. Valentim, P.H.R.S. Moraes, M.G.B. de Avellar
*Corresponding author (email: [email protected])
Redback/Black Widow Systems as progenitors of the highest neu-tron star masses and low-mass Black Holes
J. E. Horvath , A. Bernardo , L.S. Rocha , R. Valentim , P.H.R.S. Moraes and M.G.B. de Avellar Universidade de São Paulo, Department of Astronomy IAG-USP. R. do Matão 1226, 05508-090, Cidade Universitária, São Paulo SP, Brazil; Departamento de Física, Instituto de Ciências Ambientais, Químicas e Farmacêuticas (ICAQF). R. São Nicolau 210, 09913-030, Diadema, SP, Brazil
The long-standing problem of the maximum mass that can be achieved by these compact objects, with clear implica-tions for the equation of state of matter above the nuclear saturation density (see [1] and references therein), is re-ceiving a new twist with the latest data from binary systems and NS-NS merging events. Long ago, double neutron stars systems (DNS) data installed the idea of a "canonical'' mass of ~ 1.4 𝑀 ʘ , but later work provided evidence for heavier objects with increasing degree of confidence, and it became clear that at least a second ``mass scale'' had to be present [2]. This second mass scale contains NSs born massive plus the set of substantially accreted objects. On the other hand, the celebrated event widely explained as the fusion of a DNS GW170817, was readily interpreted as evidence for the formation of a transitory state, most likely a hypermassive NS (HMNS), or a supermassive NS (SMNS), eventually forming a black hole (BH), or even a stable NS (SNS) [4]. Simple physical assumptions were later used to connect this transient state to the maximum mass [3,4,5] of a static NS sequence. The group of "spiders" ("black widow'' systems like PSR1957 + 20 and their redback "cousins''), are strongly interacting relativistic systems harboring one pulsar. They have been modeled as the result of two unusual ingredients, not present in ordinary LMXBs: the back illumination onto the donor and the later ablation of the donor by the pulsar wind, showing an evolutionary connection between the two groups [6,7]. In fact the observation of high-masses in "spiders" stems from the accretion history of the systems: starting from the formation of the NS (highly variable ac-cretion rates ≤ 10 −9 𝑀 ʘ ) , the accretion times until the do-nor starts to enter the degenerate regime are very long , in the ballpark of (Ref. [6], Fig. 2) and allow the growth of the NS mass even for moderate efficiencies of accretion ( 𝛽 ≥ 0.1 ), as required for the systems to evolve towards the ``black widow'' region in the orbital-donor mass plane within a Hubble time. Figure 1
The masses of 17 Redback/Black Widows NSs. The circles denote Redback systems and the squares Black Widow ones. Lower limits on the masses of J1622-0315, J1306-40, J1816+4510 and J1048+2339 are indicated with the arrows. The dotted vertical line is the maximum mass derived by Magalit and Metzger [4], the dashed-dotted line the upper limit of the range quoted by Ruiz, Shapiro and Tsokaros [5] and the dashed line the value of Ai, Gao and Zhang [3] for the SMNS formation case in the GW170817 merger (full data of the direct observations and references can be found in Ö zel and Freire [8]). The hatched region on the right marks the region of BHs, beyond the Rhoades-Ruffini limit, although objects pushed above 𝑀 𝑚𝑎𝑥 will become BHs before the vertical line. Fig. 1 depicts the NS masses of available "spider'' systems. Our interpretation is that this is a distribution of accreted masses, starting with the NS masses at birth ( not restricted to be ~ 1.4 𝑀 ʘ ). The data is building a tension with some of the limits for the static maximum mass derived from the GW170817 merger event (vertical lines). Thus, higher sta-tistics of "spider'' systems has the potential of revealing di-rectly the long-sought quantity 𝑀 𝑚𝑎𝑥 , if a statistically significant pile-up of masses happens at some limit > 2 𝑀 ʘ . Direct formation of ≥ 2 𝑀 ʘ NSs is certainly very proble-matic from the theoretical point of view. Another important point is that some of the initially "spider'' systems could have pushed their NSs over the TOV limit, forming a (small) number of low-mass black holes. This would be an alternative channel for BHs, not subject to con-straints of gravitational collapse dynamics behind the "mass gap''. The observed spiders clearly cannot host a BH, but the prospects for detection of the "collapsed spiders", the ones that accreted sufficient mass for the NS to be "pushed" over the 𝑀 𝑚𝑎𝑥 limit, can be enhanced by looking at accreting binary systems containing compact object candidates and companion stars complying with the features of a previous redback progenitor. One possible candidate is the system VLA J2130+12 [9], a low-luminosity source having a low-mass ʘ star as a companion and a very short orbital period of . It should be noted that the common assumption of a "standard'' BH mass of
10 𝑀 ʘ made in that work automatically rules out the possibility of identification of the object as a "collapsed spider", and a reanalysis is in order. There is a recent detection by LIGO (see https://gracedb.ligo.org/superevents/S200316bj/view/) which is being considered as a candidate for BHs ``filling the gap'', with a preliminary probability of 0.9957 in favor of this ʘ range . This would indicate the possibility of forming low-mass BHs in other channels, not only in the "collapsed spider'' systems suggested here. J.L. Lattimer and M. Prakash, Phys. Rept. , 127 (2016) 2
J.E.Horvath and R.Valentim, in
Handbook of Supernovae , edited by A.W. Alsabti and P. Murdin (Springer International Publishing AG, London, 2017) p. 1317 and references therein 3
S. Ai, H. Gao and B. Zhang, arXiv:1912.06369 (2019) 4
B. Margalit and B. Metzger, ApJLett , L19 (2017) 5
M. Ruiz, S.L. Shapiro and A.Tsokaros, Phys. Rev. D , 021501 (2018) 6 O.G. Benvenuto, M.A. De Vito and J.E. Horvath, ApJLett , L33 (2012) 7
O.G. Benvenuto, M.A. De Vito and J.E. Horvath, ApJLett , L7 (2014) 8
F. Ö zel and P.C.C. Freire, Annu. Rev. Astron. Astrophys. , 401 (2016) 9 B.E. Tetarenko, A. Bahramian, R.M. Arnason, J.C.A. Miller-Jones, S. Repetto, C.O. Heinke, T.J. Maccarone, L. Chomiuk, G.R. Sivakoff, J. Strader, F. Kirsten and W. Vlemmings, ApJ , 10 (2016), 10 (2016)