GEMINI SPECTROSCOPY OF THE SHORT-HARD GAMMA-RAY BURST GRB 130603B AFTERGLOW AND HOST GALAXY

Short gamma-ray bursts (SGRBs) are historically identified based on the duration of their gamma-ray emission (T₉₀ ≲ 2 s⁸) and their hard spectrum (e.g., Mazets et al. 1981; Norris et al. 1984; Kouveliotou et al. 1993). Recently, based on a statistical approach, several attempts have been made to improve this classification (Řípa et al. 2009; Veres et al. 2010; Bromberg et al. 2013). Short-GRBs also differ from the "long" GRBs class for their redshift distribution and, likely, their progenitors (see, for example, Li & Paczyński 1998; O'Shaughnessy et al. 2008; Zhang et al. 2009; Kann et al. 2011, and references therein).

The optical afterglows of SGRBs are on average two orders of magnitude less optically luminous than their long duration counterparts (Kann et al. 2011), making broadband follow-up, and optical spectroscopy in particular, quite challenging. Nevertheless, the Swift satellite (Gehrels et al. 2004) has enabled the localization of a modest sample in X-rays and a smaller set have been detected in optical/near-IR passbands (e.g., Fox et al. 2005; Bloom et al. 2006; Hjorth et al. 2005; Berger 2007; Nakar 2007; Fong et al. 2013a).

In contrast, long GRBs (T₉₀ ≳ 2 s) present brighter afterglows allowing accurate localization and spectroscopic follow-up hours after the events occur. Robotic facilities and Target of Opportunity (ToO) programs have provided a plethora of photometric and spectroscopic data in support of theoretical models of long GRB progenitors and the host galaxies they live in. These data first established conclusively the extragalactic nature of the events (Metzger et al. 1997) and, eventually, analysis of the Lyα forest for high-z events (e.g., Salvaterra et al. 2009; Tanvir et al. 2009; Cucchiara et al. 2011) and fine-structure transitions at lower redshift provided unambiguous physical associations to their hosts. Optically bright long GRBs seem exclusively hosted by star-forming galaxies with high specific star-formation and sub-L* luminosities, indicating massive stars as likely progenitors of long GRBs (see Levesque 2013, and reference therein), while "dark" GRBs (Jakobsson et al. 2004) seem to be harbored, on average, in more massive and highly star-forming galaxies (3 × 10¹⁰ M_☉ at z ≈ 2, Perley et al. 2013).

Thanks to a concerted community effort, of the ∼70 short GRBs detected by Swift ∼1/3 have been localized to within a few arcseconds accuracy, and, with a a posteriori probabilistic arguments, have been securely associated with nearby galaxies (see, e.g., Fong et al. 2013a; Fong & Berger 2013). The number of these events, however, is still in the few dozens (Fong et al. 2013a). The few well-observed SGRBs have been associated primarily to a population of galaxies very similar to field galaxies at similar redshifts with moderate to negligible star formation rate (SFR), lending credence to the idea that at least some SGRBs explode with delay times ≳ 1 Gyr consistent with the progenitor model of compact mergers (e.g., neutron star binaries, or neutron star–black hole; Lee et al. 2005, 2010; Hjorth et al. 2005). Recently, using deep observations from space and from the ground it has been possible to quantify the relative fraction short-duration GRB hosts: 20%–40% early- and 60%–80% late-type galaxies (Berger 2010; Fong et al. 2013a).

From the afterglow perspective, there has yet to be a bona fide short duration GRB afterglow for which we have measured a redshift from absorption features in the optical spectrum. Among these it is important to note the long GRB 090426, for which the duration of the prompt emission (t₉₀ = 1.2 s) and the properties of the host made its classification uncertain (Antonelli et al. 2009; Levesque et al. 2010). Another debated short GRB for which an afterglow+host galaxy spectrum has been obtained is GRB 051221A (Soderberg et al. 2006): also in this case the non-collapsar nature of this event has been recently questioned based on probabilistic arguments and an accurate analysis of the instrumental biases which may lead one to mistakenly associate collapsar events like this one to short-GRBs (Bromberg et al. 2013). Finally, GRB 100816A, despite having t₉₀ ≈ 2 s, has been associated with the SGRB class based on lag analysis (Norris et al. 2010) and its afterglow (or a combination of afterglow and host) has been spectroscopically observed (Tanvir et al. 2010; Gorosabel et al. 2010). The lack of a large sample of SGRB afterglow spectra has made it impossible to conduct analogous studies of the host galaxy ISM and circumburst environment, as are routinely achieved for long-duration events.

Finally, the lower rate of space-based localization (compared to the long GRB class) and their faintness demand a very rapid response by large area telescopes to reach the quickly fading afterglows and obtain similar high-quality data for a larger sample of SGRBs. Such a collection will serve two main goals: (1) provide unambiguously, based, e.g., on absorption-line diagnostics like fine-structure transition, the redshift of these events and their association with an underlying host galaxy; (2) directly allow the characterization of the interstellar and circumstellar environment from 200 to 300 pc up to the host halo (Chen et al. 2005; Prochaska et al. 2006; Berger et al. 2006; Vreeswijk et al. 2013) and, then, provide clues about the progenitor itself.

In this paper we present our prompt optical spectroscopic and photometric observations of the short GRB 130603B, obtained at the Gemini South telescope. Based on the optical and near infrared emission at late time (∼9 days post-burst) derived by deep Hubble Space Telescope (HST) observations which allowed to clearly resolve the GRB-host complex, it has been proposed that this event might resemble the expected emission due to the decay of radioactive species produced and initially ejected during the merging process of a neutron star's binary system, referred to as a "kilonova" (Li & Paczyński 1998; Metzger & Berger 2012; Berger et al. 2013; Tanvir et al. 2013).

Thanks to our ToO program, we observed this event within the first day, when the afterglow dominated the host flux despite their small projected separation. This provides constraints on the event redshift and the properties of the GRB explosion site, in particular, in comparison to the overall SGRB host galaxy population.

The paper is divided as follows: in Section 2 we present our observing campaign; in Section 3 we present our spectral analysis, and in Section 4 we discuss our results and compare them with previous studies on SGRBs and their host galaxies. Finally, in Section 5 we present some implications on the nature of GRB 130603B and the possibilities offered by rapid response facilities for SGRB studies. Throughout the paper, we will use the standard cosmological parameters, H₀ = 70 km s⁻¹ Mpc⁻¹, Ω_m = 0.27, and Ω_Λ = 0.73.

On 2013 June 3, at T₀ = 15:49:14 UT, the Swift satellite (Gehrels et al. 2004) triggered on GRB 130603B (Melandri et al. 2013). The onboard Burst Alert Telescope (BAT; Barthelmy et al. 2005) detected a single bright peak with a duration of 0.4 s, placing this event unambiguously in the short-GRB category (Norris et al. 2013). After slewing to the source location, the X-ray Telescope (XRT; Burrows et al. 2005), began observing 59 s after the trigger, and detected a fading X-ray counterpart at α = 11^h28^m4816, δ = +17°04'188 (with an uncertainty of 27; Evans et al. 2013). No optical counterpart was found in the UV/Optical Telescope Roming et al. (2005) prompt data (Melandri et al. 2013), while a counterpart was detected when more data became available (de Pasquale & Melandri 2013).

At T₀ + 5.8 hr, using the William Herschel Telescope, Levan et al. (2013) identified a point-like source inside the XRT error circle, which they determined to be the optical counterpart of the short GRB 130603B. This position lies in the outskirts of a galaxy present in the archival Sloan Digital Sky Survey (SDSS) DR9 (Ahn et al. 2012). Other subsequent follow-up observations were performed on GRB 130603B yielding a spectroscopic redshift for the afterglow and/or the host galaxy (Thoene et al. 2013; Xu et al. 2013; Sánchez-Ramírez et al. 2013; Foley et al. 2013; Cucchiara et al. 2013). The afterglow was also later detected at radio wavelengths (Fong et al. 2013b). Unfortunately, no observations to date have been providing detections of fine-structure lines, which are connected to the GRB radiation itself (leaving still some uncertainty on the actual GRB-host association).

Using our ToO program (GS-2013A-Q-31; PI Cucchiara) we performed a series of photometric observations with the GMOS camera (Hook et al. 2004) on Gemini South in the g', r', and i' filters for a total of 8x180 s exposures per band from T = T₀ + 7.19 h to T = T₀ + 9.1 h. The data were analyzed using the standard GEMINI/GMOS data analysis packages within the IRAF⁹ environment. The afterglow is detected at a projected distance of ≈0.92 ± 010 from the center of a bright, neighboring galaxy (see the false-color image in Figure 1). At the galaxy's redshift (see next section), this corresponds to 4.8 (±0.5) kpc in projected distance.

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Subsequently, we obtained a spectroscopic sequence with the same instrument: we obtained 2 × 900 s spectra, using the R400 grism with the 1'' slit (resolution of about 5.5 Å) centered at 6000 Å, covering wavelengths 3900–8100 Å. We reduced the spectroscopic data with standard techniques, performing flat-fielding, wavelength calibration with CuAr lamp spectra, and cosmic ray rejection using the lacos_spec package (van Dokkum 2001). A sky region close in the spatial direction, but unaffected by the spectral trace, was used for sky subtraction. The two-dimensional spectra were then coadded. Figure 2 presents the processed data which reveal the spectrum of the extended galaxy exhibiting a faint continuum and a series of emission lines. Clearly visible superimposed on the galaxy light is an unresolved (spatially) trace that coincides with the expected position of the afterglow based on our imaging data.

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Using the IRAF/APALL routine, we extracted a spectrum corresponding to the entire detected trace, therefore including both the galaxy and the GRB afterglow signal. We then extracted a second spectrum with the aperture restricted to the spatial location of the GRB afterglow. Variance spectra were extracted in both cases evaluating sky contribution in two regions unaffected by the galaxy or the GRB light (plotted in gray in Figure 3 and with dash lines in Figure 2).

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Flux calibration was performed using an observation of the spectrophotometric standard star Feige 110 taken with the same instrument configuration. An air-to-vacuum correction was applied to the 1D wavelength solution. Using the measured optical brightness of the GRB+host we also corrected for a small slit-loss (≲ 10%).

On the following day, starting at T₀ + 1.3 days after the burst, we observed again the field with Gemini South obtaining 3 × 180 s imaging exposures with the GMOS camera in the g', r', i' bands and a single 900 s spectrum with the same configuration used the first night. The afterglow had faded considerably in comparison with the host galaxy. The reduced spectrum revealed no trace from the GRB, but only a faint continuum from the host, with identical emission lines superimposed.

We also obtained an optical spectrum of the host galaxy of GRB 130603B on 2013 June 6 UT with the Deep Imaging Multi-Object Spectrograph (DEIMOS; Faber et al. 2003) mounted on the 10 m Keck II telescope. The instrument was configured with the 600 lines mm⁻¹ grating, providing spectral coverage over the region λ = 4500–9500 Å with a spectral resolution of 3.5 Å. The spectra were optimally extracted (Horne 1986), and the rectification and sky subtraction were performed following the procedure described by Kelson (2003). The slit was oriented at an angle such to include the host nucleus and the GRB location. Flux calibration was performed relative to the spectrophotometric standard star BD+262606.

Finally, on June 16 UT, we imaged the field with the Gemini-South telescope in g', r', i', and z' bands to estimate the galaxy contribution at a time when the afterglow was expected to have faded well beyond our detectability.

Figure 3 presents extractions associated with the combined afterglow and galaxy and for an aperture restricted to the afterglow location. In the combined spectrum, we identify a series of nebular emission lines (e.g., [O ii] λ3727, [O iii] λλ4959, 5007, and Hβ), at a common redshift z = 0.3568 ± 0.0005. We associate these lines to H ii regions near the center of the galaxy, spatially offset from the afterglow location (Figure 2). The only significant absorption features present in the spectrum are Ca ii H+K features, despite the fact that the flux and signal-to-noise ratio (S/N) are lower (see Section 4.3)

The narrow aperture centered on the afterglow location shows a largely featureless continuum (lower panel of Figure 3). At wavelengths λ ≳ 6000 Å the two-dimensional spectrum and our imaging indicate the afterglow dominates the flux. We measure a spectral slope in the optical of $\nu f_\nu \propto \nu ^{-\beta _o}$ with β_o = 0.62  ±  0.17.

Examining the GRB afterglow spectrum at the wavelengths of the galaxy's nebular lines we detect little or no emission (see zoom-in regions in Figure 3). Upper limits on the line fluxes are given in Table 1.

Table 1. Properties of the Host and Afterglow

	Value
αHost	11:28:48.227
δHost	+17:04:18.42
αOT	11:28:48.168
δOT	+17:04:18.06
z	0.3568 ± 0.0005
Separationa (δr)	085 W, 036 S
P(⩽δr)	0.00064b
$F_{{\rm [O\,\scriptsize{II}]} \lambda 3727}$	21.5 (± 0.4)c
$F_{{\rm GRB},\; {\rm [O\,\scriptsize{II}]}\; \lambda 3727}$d	⩽5.8c
F Hβ	9.36 (± 0.3)c
FGRB,  Hβd	⩽1.44c
$F_{{\rm [O\,\scriptsize{III}]}\; \lambda 4960}$	1.47 (± 0.12)c
$F_{{\rm GRB},\; {\rm [O\,\scriptsize{III}]}\; \lambda 4960}$d	⩽1.45c
$F_{{\rm [O\,\scriptsize{III}]}\; \lambda 5008}$	6.19 (± 0.23)c
$F_{{\rm GRB},\; {\rm [O\,\scriptsize{III}]}\; \lambda 5008}$d	⩽2.31c
F Hα	40.2 (± 0.2)c
$F_{{\rm N\,\scriptsize{II}}\; \lambda 6584}$	10.4 (± 0.3)c
MHost, B	−20.96 (± 0.07)e
SFRHost	1.84 M☉ yr−1
12 + log(O/H)	8.7 ± 0.2
$\mathcal {M}$	5 × 109 M☉
AV	1.3 mag

Notes. Summary of the Afterglow and Host galaxy properties as derived from our photometric and spectroscopic observations. The nebular lines fluxes reported here are the raw values as measured from our spectra (see text for the final values and derived quantities). ^a092 (±0.10) projected or 4.8 ± 0.5 kpc. ^bProbability to find such a host galaxy at projected distance ⩽δr. ^cIn units of 10⁻¹⁷ erg cm⁻² s⁻¹. ^dUpper limit on the galaxy line-flux measured from the GRB afterglow spectrum. ^eRest-frame B magnitude derived from observed r'.

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Figure 4 shows a section of the two-dimensional spectrum centered on the location of the Ca ii doublet (dashed lines represent the variance spectrum). In the outset we highlight this region in the corresponding extracted spectra for the afterglow only and the afterglow+host: at the location of the redshifted Ca ii H&K we see a decrement in flux in the two-dimensional spectrum and in both extracted spectra. Using the variance spectra we can determine the pixel-by-pixel flux error. At the expected location we detect a redshifted Ca ii K-line at ∼4σ significance level in the afterglow+galaxy spectrum (2.5σ in the afterglow only), while the Ca ii H-line only at 2σ (and ≲ 1σ). This result, in combination with the emission lines detected, places strong constraints on the GRB redshift (z ≳ 0.3568) and suggests a likely association of the GRB with the galaxy.

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4.1. Properties of the GRB and its Afterglow

4.2. Properties of the Host Galaxy

At z = 0.3568 the r' band magnitude samples the rest-frame B-band luminosity of the host. Therefore, using our second epoch observations we derive r'= 20.76 ± 0.06 mag for the GRB host and we estimate a rest-frame, k-corrected, absolute B-band magnitude (AB) of M_B = −20.96. Despite the brightness of this galaxy (∼L*; Zucca et al. 2009), the derived luminosity is not unusual among the short-GRB hosts population (Figure 5; Berger 2009). Using the late-time multiband photometry and the IDL package kcorrect (Blanton & Roweis 2007) in a similar fashion to Werk et al. (2012) we estimate the mass of the host galaxy as $\mathcal {M}\approx 5.0 \times 10^9 \,M_{\odot }$.

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The GMOS spectra from the first two nights and the DEIMOS data cover key nebular emission lines. We measured the fluxes of these lines using the latter data, but for the ones also detected in the GMOS spectra we obtain similar results (Table 1). We apply standard emission-line analysis to derive intrinsic galaxy properties. We have estimated the optical extinction using the Balmer lines decrement and assuming case-B recombination Calzetti et al. (1994), Kennicutt (1998): we derived E(B − V) = 0.43 (A_V = 1.3 mag, assuming a Milky-Way extinction curve). Using the calibration of (Kennicut 1998), the [O ii] line luminosity gives a SFR([O ii]) =1.7 M_☉ yr⁻¹. Similarly, from the Hα luminosity we derive SFR(Hα) = 1.84 M_☉ yr⁻¹ . Together with the B-band luminosity, we derive a specific SFR: sSFR = SFR$/L^*_B\gtrsim 2.1\ M_{\odot }$ yr⁻¹ L*⁻¹. As in other many short GRB hosts, this value is consistent with a star-forming galaxy (see Figure 5; Berger 2009).

Also, collisionally-excited oxygen and the Hα and Hβ Balmer series recombination lines provide an estimate of the gas-phase oxygen abundances of the host galaxy. We adopt the $R_{23}=(F_{{\rm [O\,\scriptsize{II}]}\, \lambda 3727}+F_{{\rm [O\,\scriptsize{III}]}\, \lambda \lambda 4959,\,5007}$)/F_Hβ metallicity indicator (Kobulnicky & Kewley 2004; Pagel et al. 1979), which depends on both the metallicity and the ionization state of the gas. To help disentangle a degeneracy in the values, we use the O₃₂ indicator ($O_{32}=F_{{\rm [O\,\scriptsize{III}]}\, \lambda \lambda 4959,\,5007}$/$F_{{\rm [O\,\scriptsize{II}]}\, \lambda 3727}$).

However, our measured fluxes still allow for two solutions: ≈8.7 for the upper R₂₃ branch and ≈7.8–8.3 for the lower branch. The typical error in this measurement, due to systematic uncertainty in the calibration of these metallicity relations is typically ∼0.2 dex. No field galaxy at z ∼ 0.4 with a similar brightness to GRB 130603B exhibits metallicities consistent with the lower-branch of our R₂₃ analysis. Furthermore, based on the detection of Hα and N ii λ6583 lines in the DEIMOS spectrum we place a lower limit on the metallicity of 12 + log(O/H) ≳ 8.5. So we can conclude the galaxy has approximately solar metallicity (Asplund et al. 2005).

4.3. The Afterglow Spectrum of GRB 130603B

We present our Gemini rapid follow-up campaign on the afterglow of the short GRB 130603B. Despite the intrinsic faintness of these events, the low-energy emission of GRB 130603B, in particular in the optical afterglow, was still detectable several hours after the explosion. We triggered our approved ToO program at the Gemini South telescope and obtained a series of images of the GRB field as well as spectroscopic observations with the GMOS camera starting 8 hr after the burst. We repeated similar observations the following night.

Based on our absorption lines analysis and the very small probability of a random association with such a bright galaxy (P(⩽ δr) = 0.00064) we conclude that this is the host of GRB 130603B. Our optical images provide firm evidence that this event occurred in the outskirt of a star-forming galaxy (M_B = −20.96), around 4.8 kpc from its center (at the GRB redshift of z_grb = 0.3568). While this offset is not unusual for long duration GRBs (e.g., Bloom et al. 2002), the lack of blue light at the location is very unlike long-duration GRB locations (Fruchter et al. 2006). Deep HST observations also confirm this findings (Berger et al. 2013; Tanvir et al. 2013).

The early-time spectra present a bright afterglow continuum superimposed upon a fainter galaxy. This represents one of the few cases where an afterglow-dominated spectrum for a short GRB has been recorded. The host is an ∼L* galaxy, and the spectrum shows strong nebular emission lines as well as recombination features. We measure a dust-corrected SFR = 1.84 M_☉ yr⁻¹ and solar metallicity.

Nevertheless, we were able to extract the GRB spectrum and constrain the properties of the host galaxy at its location: the GRB spectrum present a smooth continuum showing almost no sign of emission lines throughout the wavelength coverage. Also, at the same redshift we identify Ca ii absorption lines, which place a strong constraint to the GRB redshift, despite the lack of fine-structure transitions. Therefore, at the GRB location, the star-formation activity is almost negligible (SFR(Hβ) ≲ 0.4 M_☉ yr⁻¹).

This last result, once more, emphasizes the importance of rapid follow-up observations with large aperture facilities in order to firmly identify the faint afterglows of these short-lived events. Only with a larger sample of data similar to those presented in this work we will be able not only to identify a larger number of short GRBs and measure their redshifts, but also to characterize, via absorption spectroscopy, their explosion site environment and move forward in our understanding of the nature of their progenitors.

A.C. thanks the anonymous referee for the valuable comments and suggestions which have helped significantly to improve the manuscript. A.C., also, thanks D. A. Kann and S. Savaglio for the valuable discussions and comments. Gemini results are based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciência, Tecnologia e Inovação (Brazil), and Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina). Partial support for this work was provided by NASA through Hubble Fellowship grant HST-HF-51296.01-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555.