Discovery of the Luminous, Decades-long, Extragalactic Radio Transient FIRST J141918.9+394036

In addition to the FIRST and VLASS sky surveys discussed above, J1419+3940 was observed and detected by the NRAO-VLA Sky Survey at 1.4 GHz (NVSS; Condon et al. 1998). We downloaded images from all three VLA surveys and used Aegean (Hancock et al. 2018) to fit a 2D Gaussian to the total intensity counterpart of J1419+3940 in NVSS, designated NVSS J141918+394036 (see Table 2). We also examined the channel-averaged linear polarization data from NVSS, and see no signal in Stokes Q or U down to a 5-σ sensitivity of 1.5 mJy, corresponding to an upper limit on the linearly polarized fraction of 9%.

The most precise localization is measured by FIRST, which finds (R.A. J2000, decl. J2000) = (14:19:18.855, +39:40:36.0) with an uncertainty of 03 (dominated by systematics at 1/20 of beam size; White et al. 1997). We queried the NRAO archive and found that repeated observations toward J1419+3940 within the FIRST and NVSS surveys fell within a one week span. A separate analysis of repeated FIRST observations did not reveal J1419+3940 as variable on that timescale (Thyagarajan et al. 2011).

In the 1980s, the NRAO 91 m telescope was used to conduct sky surveys at 1.4 and 4.85 GHz (Condon & Broderick 1985; Becker et al. 1991). These surveys are interesting because they were relatively sensitive and observed only a few years before the first VLA detection of J1419+3940 in the 1990s. J1419+3940 was not detected in these surveys to limiting flux densities of 100 and 25 mJy at 1.4 and 4.85 GHz, respectively.

Six other sky surveys observed the position of J1419+3940 at frequencies below 1 GHz: the VLA at 74 MHz (VLSSr), the GMRT at 150 MHz (TGSS), the MSRT at 232 MHz (Miyun Survey), WSRT at 325 MHz (WENSS), the Texas Interferometer at 365 MHz (Texas Survey), and the Northern Cross at 408 MHz (Bologna Sky Survey). None of these surveys detected the transient (see Table 2).

WENSS is not only the most sensitive of the low-frequency surveys, but it observed this field within a few months of VLA observations near the (apparent) peak brightness at 1.4 GHz. Assuming the J1419+3940 radio brightness changes slowly, the WSRT and FIRST observations are effectively simultaneous and the WSRT nondetection limits the spectral index near peak flux, α_0.35/1.4 > +0.6.

The Allen Telescope Array (ATA) observed J1419+3940 as part of a 690 deg² region at 1.4 GHz (the ATATS survey; Croft et al. 2010). No counterpart was detected above its 3σ flux density upper limit of 12 mJy.

A search of the WSRT archives revealed a series of observations of NGC 5582 (roughly 10' away) as part of the ATLAS-3D project (Serra et al. 2012). In total, 10 epochs of observing were made in 2008 and 2010 with a bandwidth of 20 MHz centered at 1.415 GHz (T. Oosterloo, R. Morganti and P. Serra, 2018, in preparation). The data, which are in total intensity only, were calibrated and corrected for primary-beam attenuation with MIRIAD (Sault et al. 1995), and images at each epoch detect J1419+3940 with a significance of roughly 20σ.

We modeled the source in FITS images with Aegean and quote peak flux densities in Table 2. The statistical errors are roughly 0.1 mJy, but a joint analysis of all sources in the field suggests that there are significant systematic effects as well. We find that the standard deviation of peak flux for all sources scales with brightness and has a minimum of 0.2 mJy. We add this value to the statistical error in quadrature for all analysis presented here.

The WSRT fluxes for J1419+3940 in Table 2 do not show a purely secular decay, but instead suggest some small level of gradual variability up and down across epochs. To assess the possibility of short-term flux variability, we identified 22 other unresolved sources within the WSRT field of view, each detected at all or almost all epochs, and ranging in flux from 1 to 380 mJy. For each source, we extracted fluxes at each epoch, and computed the mean flux μ, standard deviation σ, and modulation index m ≡ σ/μ. J1419+3940 has m = 0.21, which is larger than for any of the other 22 sources, for which the mean value is m = 0.10 ± 0.05. However, the first flux measurement was made two years prior to the others, so the apparent long-term decay of the flux of J1419+3940 increases the modulation index. If we disregard the flux point from 1998, we find m = 0.17 for J1419+3940, which is comparable to that for other millijansky sources in the field: for example, FIRST J142131.9+392355 has a mean WSRT flux of 1.5 mJy with m = 0.20 and a randomly jittering flux across the WSRT epochs, but has a somewhat different FIRST flux of 2.3 mJy. On the other hand, FIRST J142103.2+392448 has μ = 2.2 mJy and m = 0.15 with a smoothly varying light curve like seen for J1419+3940, but has a similar FIRST flux of 2.3 mJy. Overall, we conclude that there is the suggestive possibility from the data that J1419+3940 exhibits flux variability, but we cannot definitively confirm or rule out such behavior from the WSRT data.

A search of the VLA archives revealed two VLA observing campaigns with useful data toward J1419+3940. The first of these is a legacy VLA observation AB6860, which was observed in 1993 November (about nine months earlier than FIRST), at both 325 MHz and 1.4 GHz.

We analyzed the AB6860 observations in MIRIAD and detected the source with a significance of 40σ at 1.4 GHz (the 325 MHz observations result only in an upper limit). The AB6860 observations did not contain a flux calibrator, so the flux scale is estimated via considering the fluxes of six field sources, all detected as unresolved objects in the FIRST, NVSS, and 10 WRST observations described above. Two of these sources (FIRST J141858.8+394626 and FIRST J142006.4+393503) show significant (>50%) changes in flux between epochs, and are assumed to be variable sources. The remaining four sources (FIRST J142009.3+392738, FIRST J142030.5+400333, FIRST J142120.6+394110, and FIRST J142123.5+393332) have consistent fluxes at 1.4 GHz across all 12 epochs of approximately 35, 12, 15 and 380 mJy, respectively. Setting the fluxes of these four sources to these values in the AB6860 data, we determine a 1.4 GHz flux for J1419+3940 at epoch 1993.87 of 27 mJy. We note though that there is a 4%–5% difference in observing frequency between the AB6860 observations and these other data (see Table 2). Assuming a typical radio galaxy spectral index for these four calibration sources of –0.7, and using the scatter between the flux measurements from FIRST, NVSS, and WSRT as an estimate of the uncertainties, we determine a 1.4 GHz flux for J1419+3940 at this epoch of 26 ± 2 mJy as listed in Table 2. We see no linearly or circularly polarized emission from J1419+3940 in these data above a 5σ limit of ∼4 mJy, corresponding to a fractional upper limit of ∼15%.

The second archival VLA observation was conducted at 1.4 and 3.0 GHz in 2015 (a "Jansky" VLA project designated 15A-033⁸ ). We calibrated and imaged this observation using CASA (McMullin et al. 2007). J1419+3940 is detected in both the 1.4 and 3 GHz bands at a brightness of about 1 mJy with a significance of 10–15σ. The mean flux across the 1.4 and 3 GHz bands implies a spectral index of α_1.4/3 = −0.6 ± 0.2. This late-time spectral index measurement has changed significantly from the early lower limits on the spectral index between 0.35 and 1.4 GHz.

Metzger et al. (2015b) derived general constraints on the light curves of synchrotron radio transients from explosions interacting with a gaseous external medium. They consider the ejection of material with a kinetic energy of E_K = 10⁵¹ E_K,51 erg and an initial velocity of v_i = β_ic (corresponding to an initial Lorentz factor of ${{\rm{\Gamma }}}_{i}={[1-{\beta }_{i}^{2}]}^{-1/2}$) into an ambient medium of constant density n = 1 n₀ cm⁻³. The ejecta transfer their energy to the ambient medium on a deceleration timescale

$$\begin{eqnarray}&&{t}_{\mathrm{dec}}\approx {R}_{\mathrm{dec}}/2c{\beta }_{i}{{\rm{\Gamma }}}_{i}^{2}\approx 115\,\mathrm{days}\,{E}_{K,51}^{1/3}{n}_{0}^{-1/3}{\beta }_{i}^{-5/3}{{\rm{\Gamma }}}_{i}^{-8/3}.\end{eqnarray} \tag{ 2 }$$

where R_dec is the characteristic radius at which the ejecta have swept up a mass ∼1/Γ_i of their own mass. The timescale t_dec defines a lower limit on the rise time to peak for radio transients (see below).

If the observing frequency (ν_obs = 1.4 ν_1.4 GHz) is located above both the synchrotron peak frequency (ν_m) and the self-absorption frequency (ν_a), then the peak brightness is achieved at t_dec, and is given by (Nakar & Piran 2011),

$$\begin{eqnarray}&&{F}_{\nu ,\mathrm{dec}}\approx 70\,\mathrm{mJy}\,{E}_{K,51}{n}_{0}^{4/5}{\epsilon }_{e,-1}^{6/5}{\epsilon }_{B,-2}^{4/5}{\beta }_{i}^{9/4}{\nu }_{1.4}^{-0.6},\end{eqnarray} \tag{ 3 }$$

where we have scaled the distance to that of J1419+3940. We make the standard assumption that electrons are accelerated at the shock front into a power-law distribution in momentum with slope p (as predicted by the theory of Fermi acceleration at shocks; Blandford & Eichler 1987) and that _B = 0.01 _B,−2 and _e = 0.1 _e,−1 are the fractions of post-shock energy in the magnetic field and relativistic electrons, respectively. We have taken p = 2.2 for consistency with the late-time 1.4/3 GHz spectral index of J1419+3940, assuming that it arises from optically thin synchrotron emission with ${F}_{\nu }\propto {\nu }^{-(p-1)/2}$. The rise time can be longer than t_obs, and the peak flux somewhat lower, at observing frequencies below the self-absorption frequency

$$\begin{eqnarray}&&{\nu }_{\mathrm{sa}}({t}_{\mathrm{dec}})\approx 0.75\,\mathrm{GHz}\,{E}_{K,51}^{0.11}{n}_{0}^{0.55}{\epsilon }_{e,-1}^{0.39}{\epsilon }_{B,-2}^{0.34}{\beta }_{i}^{1.23}.\end{eqnarray} \tag{ 4 }$$

Relativistic transients, such as GRBs and jetted TDEs, produce ultra-relativistic ejecta in tightly collimated jets. Less-relativistic transients produce essentially spherical ejecta with isotropic emission at all times. However, relativistic transients viewed in an initial off-axis direction can also be approximated as being spherically symmetric once the shocked matter decelerates to sub-relativistic velocities and spreads laterally into the observer's line of sight (e.g., Zhang & MacFadyen 2009; Wygoda et al. 2011). At this point the radio emission is no longer strongly beamed, and t_dec and F_ν,dec can be approximated using a mildly relativistic value β_i ≈ 0.5.

We focus our analysis of J1419+3940 in the case of non-relativistic ejecta, or an initially relativistic but off-axis jet. At the relatively low observing frequencies of interest, an off-axis viewing angle is the most likely configuration for a volume- or flux-limited sample of radio transients (Metzger et al. 2015b). From Equation (3), we then see that for the fiducial values of the microphysical parameters and an external density n₀ ∼ 1–10 cm⁻¹ typical of the average ISM of a star-forming galaxy or the circumburst environment of a massive star, explaining the peak 1.4 GHz flux ≳20 mJy for J1419+3940 requires an initially relativistic explosion (β_i ≈ 0.5) with a kinetic energy E_K ∼ 3 × 10⁵⁰−3 × 10⁵¹ erg. We can also place an absolute lower limit of E_K ∼ 10⁴⁹ erg for the case of equipartition (_e = _B ∼ 0.5). For such parameters, the predicted rise time for an off-axis event is t_dec ∼ 0.1–1 years (Equation (2)). The predicted self-absorption frequency ν_sa(t_dec) ≳ 0.3–3 GHz (Equation (4)), below which one predicts a spectral turnover to F_ν ∝ ν², is also consistent with the early-time nondetection of J1419+3940 at ν ≤ 350 MHz. The fact that the self-absorption frequency is close to the 1.4 GHz band at the discovery epoch around 1994 also suggests that the transient was detected close to its peak; although a nice consistency check, this makes the earlier archival upper limits (e.g., <25 mJy at 4.85 GHz in 1987) relatively unconstraining.

Alternatively, the peak flux could be consistent with a spherical non-relativistic explosion β_i ≪ 1 with a larger energy ${E}_{{\rm{K}}}\sim 4\,\times {10}^{51}\,\mathrm{erg}\,{n}_{0}^{-4/5}{({\beta }_{i}/0.3)}^{-9/4}$. An example of such a transient is the ejecta from a neutron star merger (β_i ∼ 0.1–0.3), possibly energized by a long-lived magnetar remnant (Metzger & Bower 2014). The rate of neutron star mergers of ${1540}_{-1220}^{+3200}$ Gpc⁻³ yr⁻¹ inferred from the discovery of GW170817 (Abbott et al. 2017) agrees with that of J1419+3940 (see Section 3.4). However, one reason to disfavor this scenario as compared to the long GRB case is the small size of the star-forming host of J1419+3940; the lowest measured host galaxy mass of a short GRB is 10^8.8 M_⊙ (Leibler & Berger 2010), an order of magnitude larger than the host of J1419+3940.

Based on arguments given above, the peak flux and characteristic timescale of J1419+3940 are broadly consistent with arising from a blast wave of energy and external density similar to those that characterize long-duration GRB jets. The long GRB scenario also agrees with the otherwise peculiar host galaxy properties of J1419+3940 and, albeit marginally, estimates of the occurrence rate for off-axis events (Section 3.4).

To explore this connection in greater depth, Figure 4 compares the multi-band light curves of J1419+3940 to theoretical predictions for GRB afterglows based on relativistic hydrodynamical simulations of a relativistic jet interacting with a constant density ISM from the afterglow library models of van Eerten et al. (2012), modified as described by Sironi & Giannios (2013; see below). We explored a range of parameters and tested for consistency with the peak luminosity, timescale of evolution, early-time spectral index, and late-time decay. We find reasonable agreement between radio measurements at 0.35, 1.4, and 3.0 GHz and models for assumed microphysical parameters _e = 0.1, _B = 0.025, electron spectral index p = 2.2, external density n = 10 cm⁻³, and isotropic jet energy E_iso = 2 × 10⁵³ erg, the latter corresponding to a beaming-corrected jet energy of E_K ≈ 10⁵¹ erg. All of these parameters are well within the range of those inferred by modeling the afterglows of on-axis cosmological long GRBs (e.g., Ryan et al. 2015). The implied time of the burst from these models ranges from a few months to a year prior to the first VLA epoch in late 1993.

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The observer viewing angle relative to the jet axis, θ_obs, is not well constrained by the data, as its value is somewhat degenerate with the assumed microphysical parameters; our "best-fit" model shown in the top panel of Figure 4 has θ_obs ≈ 0.6, but, as shown in the bottom panel, a range of other values θ_obs ≲ 1.0 can also work given the uncertainties. The stronger evidence for an off-axis viewing angle comes from the lack of a coincident GRB (Section 3.3), and from the overall much higher geometrical probability of seeing an off-axis event $\propto 1-\cos {\theta }_{\mathrm{obs}}$.

We also compared predictions for X-ray flux for the best afterglow models to the measured flux limit in late 1990. For viewing angles θ_obs ≲ 0.6, the models predict a flux above the measured flux limit and lasting for tens of days. The radio data are most consistent with an explosion two to three years later, so the ROSAT nondetection is consistent with the off-axis GRB model.

One exception to the generally good afterglow fits is the most recent 3 GHz nondetection by VLASS, which requires a ∼50% drop in flux over a baseline of just two years from the prior detection. Such a rapid change is challenging to explain in afterglow models; even large discontinuities in the ISM density produce at most order-unity changes in flux (Mimica & Giannios 2011), but only over timescales comparable to the source age, which in this case is ≳23 years. Alternatively, this flux drop may be a sign of scintillation-induced fluctuation. Reanalysis of VLASS data or new observations are needed to confirm the flux drop.

If J1419+3940 is indeed a GRB afterglow, its nearby distance and long observational baseline as compared to cosmological GRBs enables an unprecedented test of the late-time behavior of afterglow light curves. The standard prediction for late-time decay is (Frail et al. 2000, 2004)

$$\begin{eqnarray}&&{F}_{\nu }\propto {t}^{-3(5p-7)/10}\mathop{\approx }\limits_{p=2.2}{t}^{-1.2}\end{eqnarray} \tag{ 5 }$$

where we have again used p = 2.2 for consistency with the late-time 1.4/3 GHz spectral index. However, these models do not generally include the late-time flattening of the radio light curve, which is expected as the blast wave enters the so-called "deep Newtonian" regime (Sironi & Giannios 2013). In the deep Newtonian regime, which happens after a timescale ${t}_{\mathrm{DN}}\,\approx 2.1\,\mathrm{years}\,{E}_{{\rm{K}},51}^{1/3}{n}_{0}^{-1/3}{\epsilon }_{e,-1}^{5/6}$, the bulk of the shock-accelerated electrons turn non-relativistic and the theory of Fermi acceleration at shocks (Blandford & Eichler 1987) predicts that the electron spectrum should be a power-law distribution in momentum rather than energy. In that case, the light curve is predicted to decay as a shallower power law given by

$$\begin{eqnarray}&&{F}_{\nu }\propto {t}^{-3(p+1)/10}\mathop{\approx }\limits_{p=2.2}{t}^{-0.96},\end{eqnarray} \tag{ 6 }$$

where p is the electron distribution power-law slope.

The bottom panel of Figure 4 shows that, if we were to neglect the physically motivated deep Newtonian correction, our model would underpredict the late-time 1.4 GHz flux measurements by nearly a factor of 2. Our observations of J1419+3940 may thus provide some of the first evidence that GRB afterglows decay at late times in a way that is consistent with predictions for the deep Newtonian regime (Sironi & Giannios 2013).